The successful introduction of Cactoblastis cactorum from Argentina to Australia in the 1920s for the biological control of Opuntia cactus is the flagship programme for weed biological control, and its success has since been repeated in South Africa and elsewhere. It was introduced with great success in various Caribbean islands, but has since reached the American mainland from there. The threat it is now posing to native Opuntia species in the Americas has economic, ecological and social dimensions.
During the 1950s, the Commonwealth Institute of Biological Control (now part of CABI Bioscience) facilitated the introduction of C. cactorum to the Caribbean island of Nevis, and achieved spectacular success at a cost of a mere US$1300 with year-on-year savings of $24,000. Subsequently the moth crossed 3 km of sea to the nearby island of St Kitts. CABI also facilitated its introduction to the Cayman Islands, and it was imported to Antigua and Montserrat by the ministries of agriculture of these islands.
More recently C. cactorum has spread more widely in the Caribbean, and in 1989 it reached the mainland USA in Florida, most probably on an infested Opuntia cactus, since several interceptions of this type had been made previously. By 1999, alarm was growing for endemic species in the USA; six were affected and notably there were fears that the semaphore cactus, Opuntia spinosissima, already brought to the brink of extinction by habitat destruction, might be pushed to extinction in the wild. More recently, C. cactorum has been reported from Georgia and South Carolina. In addition, with more-or-less continuously available host plants and no major barrier to C. cactorum spreading along the coast around the Gulf of Mexico, there is consternation that it could reach Mexico, the centre of endemism of Opuntia, where Opuntia is both a key member of major ecosystems, and a significant local crop used for its pads and fruits.
A total of 107 species of Opuntia have been recorded in Mexico; 56 of these (and 38 native species) are in the subgenus attacked by C. cactorum, and eight have been recorded attacked by C. cactorum in other countries. As well as being a major component of the Chihuahuan and Sonoran desert flora and providing nesting sites and food for wildlife, Opuntia is an important agent in the fight against desertification and in soil regeneration, and is recognized as the most important plant for maintaining the ecological balance in large areas of desert and semi-desert. Opuntia has a wide range of uses; for example, in a range food products (varying from green vegetables and fruit to juices, jams and alcoholic beverages), medicines and cosmetics; in agriculture as hedging, fodder or fertilizer; and in energy production. In addition, in a process dating from time immemorial, scale insects feeding on Opuntia are 'farmed' for the dye cochineal.
With some 360,000 ha of Opuntia under cultivation and 3 million hectares of wild Opuntia also utilized in Mexico, and with 310,000 people engaged in the production chain, the sector is estimated to be worth some US$80 million per annum, with exports valued at $30 million. Opuntia has similar importance elsewhere in the region. For example, Peru currently supplies 90% of the world's cochineal, while Chile has important and high-earning cochineal and fruit exports, and the crop is growing in importance in Bolivia. Brazil, with 400,000 ha under cultivation (and expanding), uses Opuntia mainly for fodder. In Curacao there is great concern because Opuntia is a major part of the island's ecosystem.
It is not straightforward to identify responsibility for the threat C. cactorum now poses to these countries. As with the Rhinocyllus/thistle debate, there is likely to be criticism of the decision-making process that allowed the moth to be introduced although it was known to feed on a range of Opuntia species. However, the cases differ because C. cactorum reached the American mainland, not by approved introduction (as was the case for Rhinocyllus), but through a probable breakdown of quarantine. Although the USA has one of the best quarantine services in the world, its effectiveness in this case appears to have wavered (and such occasional lapses are universally recognized to be almost inevitable).
This case does highlight the irreversibility of classical biological control while the values and concerns of society may change over time, so that yesterday's correct decision can look questionable today (which is true of any irreversible decision that a government takes). Although there is an argument for including remedial action plans for what to do should things go wrong (or priorities change) in dossiers supporting applications for introduction, those involved in the process would in all probability feel that such information might jeopardize approval because of the negative interpretation that could be placed on it.
Moreover, in St Kitts & Nevis and other Caribbean islands the introduction of C. cactorum is still considered an outstanding success, and the authorities would probably assert that their decisions to introduce the agent had been correct. This illustrates the complexity of risk assessment and perhaps the greater need for regional co-operation in the implementation of biological control. It should always be borne in mind that biological control agents will spread, perhaps to unanticipated areas. The USA, Mexico and the Caribbean island nations do not consult each other about what biological control agents will be released, reflecting the common global situation.
In the case of C. cactorum, what can be done now? The Mexican Plant Protection Authorities are understandably very worried about the C. cactorum threat to Opuntia in Mexico, and are in the process of implementing an Action Plan, firstly to prevent entry of C. cactorum into Mexico, and secondly to monitor for it in at least 20 states (the most vulnerable to entry of C. cactorum ) to assess the situation. Mexico is also undertaking a process of training people to recognize the pest and the damage it causes.
But what if C. cactorum does reach Mexico? Biological control of the moth itself might be considered. However, C. cactorum is common in its natural range in spite of a suite of natural enemies. Furthermore, there are indigenous cactus-feeding pyralids in Mexico, and the impact on these of any natural enemies found in Argentina would need to be evaluated. It is also possible that these Mexican species may have natural enemies that may attack the exotic species and may be able to prevent C. cactorum reaching pest status on crops in Mexico; this only surveys and testing will resolve.
Alternatively, in South Africa, a conflict of interest between the invasive weed status and commercial potential of Opuntia has been resolved by developing a pest management programme for the introduced C. cactorum. Synchronized generations of the moth mean that manual removal of the moth eggs from Opuntia crops twice a year is highly effective, and this can be followed up with an insecticide spray if necessary.
A new option, however, has been discussed at a meeting coordinated by the International Atomic Energy Agency (IAEA) and the Food and Agriculture Organization of the UN (FAO) and held in Vienna, Austria on 15-19 July 2002. The meeting on the Study and Control of Insect Pest of Ecological and Agricultural Importance in North America and the Caribbean using the Sterile Insect Technique (SIT) looked at whether this technique had anything to offer Cactoblastis control.
Sources: GISP (2001)
Invasive alien species:
Contacts: Moses Kairo,
Ana Lilia Alfaro Lemus,
Héctor Sánchez Anguiano,
The following six articles, which give news from some of the many biological control programmes against climbing weeds around the world, were precipitated by the latest issue of Landcare Research's excellent biological control of weeds newsletter1. The newsletter is available by post, by email or on-line at Landcare Research's award-winning website.
Damage from weeds is not always obvious, but climbers are an exception. A blanket of climbing weeds growing over groundcover, shrubs and even trees spectacularly illustrates how they can quickly place a stranglehold on an ecosystem by smothering native vegetation. Some of the attributes that have led to climbing plants becoming popular for ornamental purposes (fast growth, screening vegetation) are precisely the ones that make them disastrous as weeds - and could things be getting worse?
A recent publication in Nature2 by 18 authors from Bolivia, Ecuador, Peru, the UK and the USA reported that non-fragmented Amazon forests are experiencing a concerted increase in the density, basal area and mean size of woody climbing plants (lianas), possibly in response to the increasing atmospheric concentrations of carbon dioxide. A study conducted over the last two decades of the twentieth century found that the dominance of large lianas relative to trees increased by 1.7-4.6% a year in this ecosystem. The research was concerned with changes in community composition in pristine rainforest in relation to raised atmospheric CO2 levels. The authors suggest that the additional carbon appears to benefit resource-hungry vines more than slower-growing trees. However, the results raise the possibility that climbing weeds could be in the ascendant in other ecosystems.
1Hayes, L. (ed)
Two agents widely established against old man's beard (Clematis vitalba) in New Zealand are showing promise for reducing infestations. The leaf miner Phytomyza vitalbae and the leaf fungus Phoma clematidina established readily and have been spreading exceedingly well since they were introduced in 1996 [see BNI 20(4) (December 1999), 108N, Bearded wonder] but news of any substantial impact has been eagerly awaited.
It is now normal to find one or two mines per leaf in almost any C. vitalba infestation, but whether numbers would increase further was open to question. Laboratory studies of the leaf miners suggested that just one mine per leaf can reduce growth of small plants by some 17%, and extrapolation of these results indicated that as much as a 50% reduction could occur with 2-3 mines per leaf. This suggested that existing levels of attack might reduce the weed's vigour in the field, particularly with small plants invading cleared areas. However, the question of how the promising test results might translate into field impact on larger plants remained unanswered until this year, when heavy infestations of 10-15 mines per leaf were found throughout the Marlborough District of northern South Island.
The biocontrol team are waiting to see what happens next year, both at this site and at others, to see if the success is repeated and/or replicated elsewhere, and whether this signals the beginning of the end for old man's beard. They also have another agent, the sawfly Monophadnus spinolae, still under investigation and are renewing studies into the potential of a fourth, the bark beetle Xylocleptes bispinus.
By: Lynley Hayes,
Banana passionfruit (Passiflora spp.) is an ornamental escapee that has now become a serious climbing weed in New Zealand and Hawaii. Banana poka (Passiflora tarminiana) is listed as the number one weedy threat to native forests in Hawaii. New Zealand has seven species of weedy Passifloraceae, and at least half of these are beginning to cause concern in many areas of the country.
The United States Forest Service (USFS) in Hawaii has host tested and released a foliage-feeding moth (Cyanotricha necyria), and another moth that mainly attacks the buds but also feeds on young shoot tips (Pyrausta perelegans). They have also tested and released a leaf spot fungus, (Septoria passiflorae). Cyanotricha necyria is not believed to have established and P. perelegans appears to be struggling, but S. passiflorae is doing well. Widespread disease epidemics of the fungus occurred in 1999 causing biomass reductions of 50-95%. Biomass reductions of 80-95% were recorded over more than 2000 ha. Other insect agents may be released in Hawaii in future if they are needed and funding permits.
Landcare Research (funded by Regional Councils in New Zealand) has recently begun a collaborative project with the USFS to develop biological control for banana passionfruit in New Zealand. The two moths, a drosophilid fly (Zapriothrica nr. nudiseta) that feeds on the flower buds causing them to abort, and a lonchaeid fly (Dasiops caustonae) that feeds on the fruit are all being considered for release. Recently the USFS has helped Landcare Research to conduct some preliminary host testing on the drosophilid fly. In the coming year Landcare Research hopes to import all four insects into containment in New Zealand for host testing, and to also begin testing the fungus in Hawaii. The USFS has contacts in the native range of banana passionfruit, in South America, which will be useful for collecting and shipping agents to New Zealand. Jointly funded surveys in this region in the near future may also yield further useful species for both countries.
By: Hugh Gourlay,
Cape ivy (also known as German ivy), a native of South Africa, has recently become one of the most pervasive and alarming non-native plants to invade the coastal areas of the western USA. Introduced in the late 19th century as an ornamental, this plant is a member of the sunflower family (Asteraceae), and, in the USA, is still frequently referred to by its old name, Senecio mikanioides. However, its accepted scientific name in most other countries is Delairea odorata. A recent survey found Cape ivy infestations from San Diego in California to southern coastal Oregon. Cape ivy is spreading in riparian forests, coastal scrubland, grassland, Monterey pine forest, coastal bluff communities, and seasonal wetlands. Although the species prefers moist, shady environments along the coast, there are increasing reports of infestations from inland riparian locations. This vine has the potential to cause serious environmental problems by overgrowing riparian and coastal vegetation, including endangered plant species, and is potentially poisonous to aquatic organisms. In Hawaii, where it was introduced in the early 20th century it is a serious weed in a variety of upland habitats between 200 and 3000 m above sea level in Big Island. Cape ivy has also become a pest in other countries, including Australia, England, Italy, Portugal, and Spain.
A collaborative programme between biocontrol scientists from the US Department of Agriculture - Agricultural Research Service (USDA-ARS) Western Weeds Quarantine Facility in Albany, California and the Plant Protection Research Institute in Pretoria, South Africa (PPRI), which began in 1996, initially identified six potential agents in Cape ivy's native range along the east coast of South Africa. Now three have been prioritized for testing, and the team is hopeful that a request for release for one of them will be submitted by the end of 2003. The work has benefited from financial support from the California Exotic Pest Plant Council (CalEPPC) and the California Native Plant Society (CNPS), who have gathered funds for the Cape Ivy Project from an array of government and environmental agencies, as well as private individuals.
The programme began in 1996 with an exhaustive search of South African Cape ivy herbarium records to identify promising areas for exploration in the native range of the weed. The records were used both to locate Cape ivy sites for future surveys and to develop a distribution map of Cape ivy in South Africa. Beginning in 1998, extensive surveys visited most of the Cape ivy sites in the country, and collected over 230 species of natural enemy attacking it.
Subsequent searches for these six insect species on relatives of Cape ivy growing at the same sites were almost entirely fruitless. More than a dozen close relatives of Cape ivy were repeatedly examined, but only one of the six, Diota rostrata, was collected on anything other than Cape ivy, and so it appears that at least five insects are very host-specific to Cape ivy.
Since then, work has focused on further scrutiny of the host range of these promising insects. In South Africa, laboratory colonies of P. regalis and Digitivalva n. sp. were established, and laboratory studies were also conducted on Diota rostrata. Information on the biology and life history of these three insects was gathered and rearing techniques developed. In January 2001, two of the potential biocontrol agents, P. regalis and Digitivalva n. sp., were imported into the USA for evaluation in quarantine.
Female P. regalis gall flies, which are about the size of a housefly, generally lay eggs inside the growing tip of Cape ivy. The larvae that hatch within the tip cause Cape ivy to grow a spherical gall, some 2 cm in diameter, within which the larvae complete their life cycle. The galls seem to inhibit further elongation of that stem, although side shoots are usually produced. The weight of the gall causes the stem to droop, and most galls are beneath a 'mat' of Cape ivy. Thus, it seems likely that 'galled' Cape ivy plants would be less aggressive in clambering over native trees and shrubs.
Safety is the primary concern for those involved in introducing herbivorous insects with the aim of controlling an exotic weed. The degree of host specificity of candidate insects is usually determined by exposing them, characteristically in laboratory cages, to an array of potential host plants (including closely related species, economically important species and rare or endangered species in the proposed area of introduction) and noting which, if any, are fed or oviposited on/in, and which, if any, can support insect development through to the viable adult stage. Traditionally, these evaluations include both 'no-choice tests' (or starvation tests), in which a single species of test plant is present but the known host is not, and 'choice tests', which present both test and known host plants together. Because Parafreutreta regalis adults are short-lived (averaging about 7 days) and the plants chosen for testing were limited in number and usually seasonal, the US team developed a new protocol to maximize data collection from whatever flies and plants were available. Their 'no-choice/host added tests' are multi-plant tests. Each cage contains plants of four different species, one in each corner and four pairs of flies are released within 2 days of emergence (females seem to be ovipositional within 48 h of emergence). After 3 days, a Cape ivy plant is added to the cage. After 3 more days, the plants and flies are removed from the cage, and the plants are observed for gall formation; if none appears, they are then dissected for signs of damage. A similar protocol has been used in South Africa. Host-specificity assessments of P. regalis have been carried out in this manner in California and South Africa on over 40 relatives of Cape ivy. So far galls have only been produced on the target, Cape ivy.
Digitivalva n. sp. (previously identified as Acrolepia n. sp.) is, despite being previously undescribed, one of the most widely distributed of Cape ivy's natural enemies. It was collected at nearly all Cape ivy sites surveyed in South Africa. This tiny moth (< 5 mm in length) lays eggs within Cape ivy leaves. Minute caterpillars hatch out and tunnel within the leaves, leaving distinctive, narrow 'mines'. Some of the caterpillars bore down through the leaf petiole and into the stem. In the laboratory, most mined leaves and many bored stems die, and sometimes the entire plant is killed. Since the stem moth has a longer life cycle than the gall fly (approximately 3 months from egg to adult moth, versus 2 months for the gall fly) testing is less complete. The testing has also been slowed because, for unknown reasons, in almost 50% of host specificity tests, the stem moths fail to oviposit on Cape ivy control plants, meaning these tests have to be repeated. Encouragingly, testing has so far not found any plant, other than Cape ivy, that appears to be acceptable for oviposition by female Digitivalva n. sp. Moreover, preliminary results suggest that the stem moth prefers cooler winter temperatures, thus implying a promising synchronicity with its host, Cape ivy, which also flowers and flourishes best through winter and spring.
Further laboratory and field testing of the host range of both P. regalis and Digitivalva n. sp. is continuing in South Africa and California, before an application for release is made. In addition, testing was instigated on a third species, the defoliating moth Diota rostrata. The hairy caterpillars of this moth are voracious feeders, and in South Africa the team frequently encountered patches of Cape ivy where most of the leaves were either totally missing or reduced to only small tatters. Over three dozen species of plants have been tested in South Africa, but while D. rostrata caterpillars fed on less than a handful, this included one of the three California native Senecio plants tested. The degree of attack on Senecio flaccidus will be investigated further, but the introduction of this moth into the USA is starting to appear less likely.
While P. regalis has cleared the most host-specificity testing hurdles of the three potential agents tested so far, there are still questions to be answered. Although P. regalis females readily oviposit and the larvae develop well on Cape ivy, its likely impact on Cape ivy in the wild is not yet clear. Even a 'safe' biological control agent that is entirely restricted to its target weed host could have unforeseen consequences in an ecosystem, especially if it were to have little impact on the weed while its numbers built up to high levels. Application for introduction will not be made, therefore, unless the team is confident that the agent could reduce the invasiveness of the weed. Pre-release impact assessments were instigated at the Albany quarantine facility. These demonstrated that for small Cape ivy plants caged with ten pairs of gall flies, attack and gall formation led ultimately to shorter plants with fewer nodes that had more small leaves, at the expense of full-sized leaves. This was encouraging, but impact assessments on larger plants were still needed. The team is now examining whether a smaller number (two pairs) of flies can significantly alter the growth of larger Cape ivy plants.
In anticipation of good outcomes from tests on P. regalis and Digitivalva n. sp., however, several sites in California have been selected as suitable for their release. Studies on Cape ivy insect fauna have already been initiated at two of these sites. By gaining a good understanding of the current insect fauna of Cape ivy, the team plan to be able to assess more accurately the damage caused by the biocontrol agents after release. A year-long field study of the insects attacking Cape ivy at two sites in Monterey County found little sign of natural enemy activity, and almost all samples scrutinized were nearly devoid of insects or insect damage. This confirms that the California insects are not utilizing this now abundant plant, and that the biocontrol project should proceed.
By: Joe Balciunas,
Old World climbing fern (Lygodium microphyllum) is another horticultural escapee, and one possessing some of the attributes of a trapeze artist. Currently wreaking havoc in Florida's natural wetland habitats, it may be about to be brought down to earth by a troupe of moths.
First found naturalized in Florida in 1965, L. microphyllum has proved to be an aggressive invasive weed of moist habitats in the southern parts of the state and has become one of the state's most dangerous weeds. It is a long-lived perennial vine with underground rhizomes, and growth and sporulation occur throughout the year. It spreads rapidly by vegetative growth, aided by its ability to colonize undisturbed habitats, and this frequently leads to complete dominance of native vegetation as it grows up and over trees and runs horizontally over the ground forming mats up to 1 m thick. It is common in bald cypress (Taxodium distichum) stands, but also infests pine flatwoods, wet prairies, saw grass (Cladium jamaicense) marshes, mangrove communities and Everglade tree islands, and has been identified as an imminent threat to rare bromeliads such as Tillandsia utriculata. In addition to posing a threat to native vegetation, the blanket of weed disrupts the pattern and progression of fire, which is a key natural management event in the Florida wetlands. Fire normally limited to the understorey can use the fern as a ladder to leap into the canopy and thus kill trees, and flaming fern can be carried some distance by wind to start new fires.
In the USA, L. microphyllum is a subtropical and tropical plant requiring shallow aquatic habitats or moist soils, and is currently limited to the southern third of the Florida peninsula. However, it produces large numbers of spores (< 800 spores/m3/h have been recorded in Florida), and this aids long-distance dispersal. Climatic analysis suggests it could eventually extend up the eastern coast to the Georgia border, and above Tampa on the west coast. Spores could also potentially be carried across the Gulf of Mexico to southern Louisiana, Texas and Mexico, and the potential for onward spread means it presents a threat to much of wet tropical America. (It is naturalized to some extent already in Jamaica and Guyana through separate introductions.)
Chemical control is expensive (< US$3750/ha in 2000) and is inappropriate and often impractical for natural habitats. Mechanical control suffers the same drawbacks, with the additional problem that broken pieces quickly regenerate. Classical biological control was therefore quickly identified as providing the best long-term solution, but this approach is not without obstacles either. Foremost, L. microphyllum has an exceptionally large native range, occurring in much of the Old World tropics and subtropics. It spans more than half the world's circumference, occurring from South Africa and Australia to Assam (India) and the Ryukyu Islands (Japan), and from Tahiti to Senegal. Such a vast native range makes a search for natural enemies challenging, as pinpointing the area of origin of the weed, and the origin of the population now in Florida (which would be expected to provide the best source for effective co-evolved natural enemies) is difficult. Comparisons of DNA analyses of Florida material with material from around the world are currently underway.
Surveys for natural enemies have been carried out in Australia, Benin, Cameroon, China, Ghana, India, Indonesia, Japan, Malaysia, New Caledonia, Singapore, South Africa, Taiwan, Thailand and Vietnam by a team from the USDA-ARS (US Department of Agriculture - Agricultural Research Service) Australian Biological Control Laboratory (ABCL) and CSIRO (Australia's Commonwealth Scientific and Industrial Research Organisation) at Indooroopilly in Queensland and the Invasive Plant Research Laboratory in Fort Lauderdale, Florida.
Most efforts so far have concentrated on examining the natural enemies of the above-ground parts of the plant using a variety of collection methods. Some 18 species of insects have been found, of which a pyralid moth, Neomusotima conspurcatalis, was the most widely distributed, followed by an eriophyid mite, Floracarus perrepae. These were found in the Australian and Asian range. Few damaging agents were found in South Africa, while the most damaging insect in West Africa was a mite, Tenuipalpis sp., but its occurrence on ferns in other genera suggests a wide host range. The paucity of the African fauna may be related to the season in which the survey was conducted, or to the low diversity of Lygodium species there (only two species occur in Africa compared with some dozen in Southeast Asia).
A rust fungus, Puccinia lygodii, which is native to South America and naturalized in the USA, has recently been found infecting L. japonicum in northern Florida. It is not known to occur in L. microphyllum's America range and its capacity to infect this species is not yet known, although it is a glasshouse pest of ornamental Lygodium spp.
A second obstacle faced by this biological control programme was that host testing schemes based on plant family affiliations, which are used for flowering weeds, are not appropriate for ferns because of lack of agreement on fern families. Although recent molecular work has helped to identify groupings, fern genera are generally still viewed as the most reliable taxa on which to base host specificity testing. Fortunately, there is a relatively modest number of ferns in Florida and the southeastern USA, and because of the relative isolation of the genus Lygodium, the team expected to be able to find natural enemies restricted to it. The most closely related species in the region (USA, the West Indies and Central and South America) include the temperate species L. palmatum, which is indigenous in the eastern USA, and four West Indian Lygodium species. Another Lygodium species, L. japonicum, is also an invasive weed in the southeastern USA. In addition, there are three Anemia species, one Actinostachys species and one Schizaea species that are considered to be sufficiently closely related to L. microphyllum to warrant inclusion in host specificity testing. The 40 spp. of ferns considered threatened or endangered in Florida, most of which are neotropical species at the northern edge of their range here, were most difficult to evaluate, with permits required to collect small numbers of each. In addition, specimens of ferns native to nearby countries in the Caribbean and Central and South America were obtained for testing.
A pyralid moth collected in subtropical Queensland from L. microphyllum and L. reticulatum was prioritized for testing. Despite heavy predation and parasitism, Cataclysta camptozonale was observed to cause heavy damage by skeletonizing the leaves of L. microphyllum, sometimes destroying much of the new growth. It also has several generations per year (with a generation time around 44 days), which adds to its promise as a biological control agent as it would exert pressure on climbing fern throughout the year. In the laboratory, larvae consumed all the foliage and scarified the stems, which killed the plant. The moth was subjected to preliminary screening against 28 fern species in Queensland, before being imported into quarantine in Florida where further screening was conducted. No-choice tests were used to measure oviposition by the adult female, and feeding by neonate naïve larvae and their subsequent development, while choice tests with older larvae assessed preferences for cut foliage from a number of potential hosts presented simultaneously. The results showed that C. camptozonale oviposits and feeds on not only L. microphyllum and L. reticulatum, but also L. japonicum which is invasive in the USA and the US indigenous species L. palmatum. The team thought that the moth would be unlikely to survive in the temperate zone where L. palmatum occurs. To verify this prediction they used cold tolerance tests to determine the lower limit for survival. Host specificity testing in Gainesville, Florida indicated that none of the three tested Caribbean Lygodium species supported full development of the moth. The moth will be petitioned for release within a year.
Next in the priority list is Neomusotima conspurcatalis, distributed from Hong Kong to Australia. This species is highly damaging in the wild, defoliating and skeletonizing leaves in a similar fashion to C. camptozonale, and proved to have a similar generation time. Having passed preliminary screening in Queensland (where it was found to have a similar host range to C. camptozonale), it is now being tested further in quarantine in Florida. Preliminary testing suggests that it is also a narrow specialist. So far it has only been collected from the tropical range of L. microphyllum, which suggests that it may not be tolerant of the cooler winters of the subtropics, but this will be verified by temperature tolerance tests.
A third pyralid moth, Musotima sp., which has been collected in Thailand, Malaysia and Singapore, is also exciting interest as it has proved to be restricted to Lygodium and to be more geographically restricted to the tropics than either of the above species. It has been found not to survive temperatures below 0°C, which means it is unlikely to survive winters in the eastern and northeastern USA where the native L. palmatum occurs. The larvae are again vigorous defoliators (although the effect in the field appears to be mitigated by predation and parasitism) and the generation time is about 18 days.
There are other agents to follow these moths. Currently, research is also underway at Indooroopilly to assess the efficacy of the mite Floracarus perrepae against L. microphyllum. Results so far indicate that this previously undescribed species is yet another potential agent with a short generation time (around 12 days) and that it may reduce L. microphyllum growth by up to 75%. Feeding causes the leaflet margins to curl, but the mite may also transmit disease as feeding has been found to be associated with black streaking and necrosis in both Australia and Southeast Asia. A fungus, Botryospheria sp., isolated from the necrotic patches associated with mite damage is thought to be a secondary pathogen. The studies will continue for 2 years to gauge the mite's long-term effects. In addition, DNA analyses will be conducted to compare populations from Australia and Southeast Asia to determine whether there is one or more species feeding on L. microphyllum in different parts of its range. Host range testing so far indicates that F. perrepae is highly specific, and field studies will continue to assess this further and elucidate its life cycle. Recent studies indicate that the Florida L. microphyllum is less acceptable to the mite than the Australian plants. Searches are underway in Asia to find a form or race of the mite that performs well on the Florida plants.
The latest to be added to the potential agents under study is another pyralid moth, Ambia sp., a stem-borer collected in Thailand and Singapore. The stem-borer feeds at the crown of the plant causing whole leaves (vines) to die. Interestingly, its wing pattern mimics that of a spider. Efforts are underway in Indooroopilly and Chiang Mai (Thailand) to rear the moth on artificial diet which will enable host testing of this potential agent in Queensland.
Recently a new pyralid moth was discovered to damage L. microphyllum in New Caledonia. This species may represent an entirely new subfamily of Pyralidae. It is now in culture at ABCL and preliminary host range testing has begun. In Noumea (New Caledonia), cooperators at the French IRD (Institut de Recherche pour la Développement) are assisting in monthly field studies to determine the seasonal phenology of the moth and the associated F. perrepae mite.
This programme is combining traditional wide-ranging exhaustive (and sometimes exhausting!) field surveys with DNA analysis and cold tolerance studies to select the agents likely to be most effective against the target weed, while posing least threat to non-target plants. In addition, because of the potential future spread of the fern through the region, it is taking great care to assess vulnerable nontarget species in other countries. Although the herbivore fauna associated with L. microphyllum is not as large as is typical of flowering plant species, the reduced suite of herbivores on the fern appears to be fairly host specific and therefore useful in the biological control programme. We believe the suite of leaf defoliating moths, plant stunting mites and stem-borers could be effective against the fern in south Florida where it is invasive.
Pemberton, R.W.; Goolsby, J.; Wright, T. (2002) Old World climbing fern (Lygodium microphyllum (Cav.) R. Br.). In: Van Driesche, R.; Blossey, B.; Hoddle, M.; Lyon, S.; Reardon, R. (eds) Biological control of invasive plants in the eastern United States. Morgantown, West Virginia; US Forest Service Forest Health Technology Enterprise Team - 2002-04, pp.139-147.
Plant Research Laboratory, USDA-Agricultural Research Service,
Australian Biological Control Laboratory,
Introduced vines or creepers have become very serious weeds throughout the world, and South Africa is no exception. Three vines, Barbados gooseberry (Pereskia aculeata), Mauritius thorn (Caesalpinia decapetala) and cat's claw creeper (Macfadyena unguis-cati) have been targeted for biological control.
Barbados gooseberry is a primitive, leaf-bearing cactus that clambers over indigenous and commercially planted forest trees in the tropical and subtropical regions of the country, eventually killing them. It is native to Mexico, the West Indies and Central and South America. Biological control is considered the only solution for this plant because of the habitat that it invades. Two species from Brazil have been evaluated for its biological control, a stem-boring pyralid moth, Epipagis cambogialis, which was rejected as it was not host specific, and the flea beetle Phenrica guérini, which was released in 1991.
Releases of the flea beetle were concentrated along the KwaZulu-Natal coast, but it appears that they have not established in that region, although very few follow-up surveys were made. One release was made on a small patch of the weed in the town of Port Alfred in the Eastern Cape Province in the southernmost extent of the weed's distribution. Numbers of the flea beetle have slowly increased over the last 10 years and the beetles are now abundant during the summer months. The impact of the beetle has not yet been evaluated, but the larval damage to the growth tips is very obvious and appears to be curbing the spread of the weed. Additional agents are being considered for release on this serious weed.
Mauritius thorn is a clambering tree species that was introduced to South Africa in the late 1800s, probably originating in the foothills of the Indian Himalayas. It invades the tropical and subtropical eastern portion of South Africa and is particularly invasive in the Limpopo and Mpumalanga provinces. Several surveys have been conducted in India and Malaysia for phytophagous insects, and a fairly extensive list exists. Two species have been evaluated for its biological control, a leaf-mining gracillariid moth, Acrocercops hyphantica, which was rejected because it was not host specific, and the seed-feeding weevil, Sulcobruchus subsuturalis, which was released in 1999.
The female weevils lay their eggs on the mature seeds that have already fallen from the pods, and the larvae develop in the seeds. This biological control agent is extremely easy to mass rear, and well over 100,000 have been released by dispersing seeds containing larvae in infested areas. Most of the releases have been made in the Limpopo Province, and establishment has been confirmed. It is anticipated that the weevils will reduce the number of seeds available for the regeneration of Mauritius thorn after clearing operations, although the impact of the weevil has not yet been evaluated.
Cat's claw creeper is an aggressive climber, smothering indigenous and commercially planted forest trees, in the tropical and subtropical regions of the country, eventually killing them. This former commercially sold plant is present in gardens all over South Africa and has been reported causing problems in Limpopo Province, Mpumalanga, Gauteng, the North West Province and KwaZulu-Natal. A survey in its home range in South and Central America has identified several promising potential biocontrol agents for this invasive weed.
The golden-spotted tortoise beetle (Charidotis auroguttata) was cleared for release in 1999 [See BNI 20(4) (December 1999),109N, Tortoise beetle to tame a wild cat]. Successful establishment has been confirmed in at least three areas (Nelspruit in Mpumalanga, Groot Marico in North West Province, and in Pretoria, Gauteng) where releases were made. Follow-up surveys are planned to determine further establishment and impact of the insect.
During an overseas survey to Brazil in 2002 several other insect species were collected and imported to South Africa, and among these were promising species in the families Tingidae, Buprestidae and Curculionidae, and a leaf-tying moth. Studies on their biology and host specificity are planned.
By: Martin Hill,
We finish this round-up of climbing weeds news in this issue with an update on the article in the March 2002 issue on community involvement in bridal creeper (Asparagus asparagoides) control agent distribution [BNI 23(1), 16N-17N]. Since that article was written, the Australian Quarantine and Inspection Service (AQIS) and Environment Australia have given approval for release of the chrysomelid foliage-feeding beetle, Crioseris sp. The beetle has now been released in Western Australia. It is active from March/April until August then hibernates as a pupa through spring and summer. Its larvae can cause major damage to bridal creeper by stripping the shoots and leaves that enable it to climb. By stopping the weed from climbing, it can be stopped from fruiting and spreading to new areas.
Once the beetle has established it will become part of the major redistribution project described in the last article, which has already facilitated the widespread release of the leafhopper Zygina sp. in June 1999, and the rust fungus Puccinia myrsiphylli in June 2000.
Kate Smith, CSIRO
The cherry bark tortrix, Enarmonia formosana, is an exotic lepidopteran pest that poses a threat to nursery and orchard industries in the Pacific Northwest of North America. First detected in Canada in 1989, this insect has since spread east into the interior of British Columbia, Canada, and south through Washington State into Oregon State, USA. The cherry bark tortrix causes direct and indirect damage to Prunus, Malus, and other economically important rosaceous plant species. The larvae feed just beneath the coarse outer bark, making irregular tunnels and causing the bark to loosen and crack over successive generations. This activity may girdle a tree and make it more vulnerable to disease, insect, and freezing mortality factors. Pesticides have not been strongly promoted to combat this invasion because of the concealed nature of the pest and its extended adult emergence period. Parasitism by indigenous larval and pupal parasitoids was investigated separately in Washington State and British Columbia, but found to be contributing to less than 3% of pest mortality in both regions. This tortricid is known to exist throughout Eurasia, but typically at low-to-moderate densities. The benign levels of infestation occurring in Eurasia are suspected to be the result of top-down control by natural enemies; however, virtually no attention has been given to these entomophagous species.
Based on experiences from other classical biological control programmes it has been observed that the USA and Canada often both spend research money and time on independent foreign explorations, which result in the selection of the same parasitoid species for classical biological control (e.g. the apple ermine moth). Therefore an international team consisting of state governmental, national, university and international institutions was established to develop an integrated pest management strategy for cherry bark tortrix management with a strong biological control base. Dr L. Tanigoshi (Washington State University), Dr B. Bai (Oregon Department of Agriculture), and Dr J. Cossentine (Agriculture and Agri-Food Canada) have been making a joint effort to explore the potential for a classical biological control programme against cherry bark tortrix in North America. In 1999, a research project was launched in Europe, under the guidance of Dr U. Kuhlmann at CABI Bioscience Centre Switzerland, to survey the native distribution and status of parasitism of cherry bark tortrix.
The initial field studies in Europe in 1999, 2000 and 2001 were important for discovering reliable collection sites, improving collection techniques, and identifying the structure of the parasitoid community. Larval and pupal parasitoids were obtained by collecting host material from orchard and ornamental cherry trees in Germany, Switzerland, and France. Collected host specimens were then reared on a pinto-bean meridic diet in the laboratory and all emerging parasitoids were ultimately preserved and prepared for identification. The following hymenopteran species were found in association with the cherry bark tortrix: Campoplex cf. dubitator, Tycherus vagus, Pimpla turionellae, Pimpla spuria, Liotryphon crassisetus, Theroscopus hemipterus, Dibrachys affinis, Gelis longicauda, Cyclogastrella simplex, Phaeogenini sp., Phygadeuontini sp., and Isadelphus inimicus.
Cherry bark tortrix infestations were ubiquitous in the surveyed areas of central Europe, but never appeared to reach severe densities. Parasitoid activity was also evident from all substantial host samples, despite an extremely patchy distribution of cherry bark tortrix hosts. Field parasitism was independent of host densities (per tree), and in certain regions reached rates of up to 64%. This suggests that parasitoids may be important in controlling cherry bark tortrix populations in Europe and that the introduction of specific exotic parasitoids may provide a long-term control alternative to the aforementioned chemical control for North America.
During the current (2002) field season, two major orchard sites were selected for repeated sampling to study the temporal and within-tree spatial patterns of cherry bark tortrix infestation and parasitism. Weekly collections at each site and subsequent host and parasitoid rearing will give an indication of pest and parasitoid phenology and parasitoid foraging behaviour. It is already very apparent that, in Europe, cherry bark tortrix is most abundant at the base of a tree trunk and pest densities increase when the surrounding vegetation (typically grasses) is denser. This differs notably from the situation in North America where cherry bark tortrix larvae regularly mine the bark higher up at the tree crotch where the limbs segregate from the main stem. This difference is most likely an artefact of the immediate environment around the trees as well as the presence of major wounds on the trees. The ornamental cherry trees of North American cities often bear obvious graft wounds at the crotch, which allow easy access for larvae into the bark. Also, the vegetation at the bases of these trees is routinely kept short, eliminating any benefit provided by these herbaceous plants.
The most promising candidate biological control agent, the ichneumonid Campoplex cf. dubitator, is a larval-prepupal parasitoid which actively forages throughout the summer months. These large parasitoids are responsible for a significant majority of cherry bark tortrix parasitism. The species is common throughout central Europe but has never been identified in North America. While difficult to maintain in culture, progress is being made to develop a system for reliable laboratory rearing. Controlled studies and observations are ongoing to elucidate the mating, host finding, and oviposition behaviour of this parasitoid. This research direction will provide the basis for natural enemy propagation on a scale that is adequate for further non-target testing and, ultimately, release.
By: Wade Jenner,
And: U. Kuhlmann,
Lygus lineolaris is an important mirid pest in a number of North American crops such as alfalfa, many vegetables, tree fruits, strawberries, greenhouse crops and occasionally canola. Since 1996 collaboration between Agriculture and Agri-Food Canada and CABI Bioscience Switzerland Centre has been ongoing to determine possible biological control agents for control of L. lineolaris in eastern Canada. Initial efforts were made to identify the native pest and parasitoid host ranges of Lygus and other mirid pests as well as the nymphal parasitoids in both Europe and in Canada. Several egg parasitoids and nymphal parasitoids have been identified in Canada although the nymphal parasitoid complex consisted primarily of univoltine species of Peristenus. Both bivoltine and multivoltine species were identified from mirid pests in Europe that could be useful in a Canadian biological control strategy. Controlled studies of non-target and competition effects were examined for native and candidate species and a decision was made to apply for release of Peristenus digoneutis into non-cropping areas or unsprayed alfalfa fields. In addition, it was decided to develop a mass rearing protocol for this parasitoid to increase the number of wasps that would be available for releases. Mass-production would also eliminate the presence of hyperparasitoids that accompany exotic field-collected material.
Mass collections of late season diapausing generations of P. digoneutis and P. stygicus have been made annually in the southern German Rhine Valley and Switzerland to supply material for both laboratory studies and mass-production methods development in Canada. US Department of Agriculture (USDA) personnel had previously released P. digoneutis in the northeastern USA. These had eventually established in the Hudson River Valley of New York State and were also a source of mass collection material for this project. In 1998, we first discovered that P. digoneutis had moved into southern Quebec.
The primary candidate species, P. digoneutis, has been successfully mass-reared, with circa 10-fold increases per generation, using the intended host, L. lineolaris, for parasitism, and also using diapause induction conditions to enable stockpiling of material at 2°C. Prior to release, wasps emerged and were checked for identity and gender, and were combined for mating in a 1:2 ratio of males to females for 1-2 days with a water source and natural honey. Wasps were then released at the Southern Crop Protection and Food Research Centre, Canada, into field cages (replicated study) or in larger numbers (3748 mated females over two generations in 2001) directly into alfalfa and weedy plots in 2001 and 2002. The caged releases were made into 2 ×1 × 1 m screened cages. Seventy-five mated female wasps and several hundred laboratory-reared L. lineolaris nymphs were released into each cage during June and July to approximate the timing of the second and third generation of the pest.
Successful recoveries of over-wintered P. digoneutis wasps have been made from all three 2001 release cages and from mass-collected first generation L. lineolaris nymphs from the release area during June 2002. A dissection sample determined 11.1% nymphal and 10% adult parasitism rates for the release plot prior to mass collection. The field collection of host nymphs was made on 20 June 2002, with emergence commencing on 15 July.
We expect that releases of P. digoneutis will continue for a third year and monitoring protocols will follow in southern Ontario to check the rate of establishment and expansion of the parasitoid range. If establishment of the parasitoid proves successful at our research farm, this method of large-scale release of laboratory-reared material could prove beneficial for rapid establishment of biological control species for future projects.
By: Jay Whistlecraft and
And: Ulrich Kuhlmann,
Buddleia, Buddleja davidii, is admired and nurtured by gardeners for its dramatic purple flower spikes in late summer, and as a nectar source for Lepidoptera in autumn, the latter explaining how it acquired its common name, the butterfly bush. Native to China, B. davidii was one of many ornamental plant species transported to other parts of the world by an enthusiastic band of amateur plantsmen in the 18th and 19th centuries. Their collective eye for the exotic left a legacy of colourful and varied gardens in their homelands, marred by the occasional intractable exotic weed problem. They also germinated a horticultural industry which became accustomed to introducing new plants with thought for little but their novelty and beauty.
There are some hundred species of Buddleja, all native to Asia except for a handful that have their origins in the southern regions of Africa and America. Buddleja davidii is the most popular species in gardens, but outside this protected environment, the weediness of this and other Buddleja species is causing concern in many parts of the world. Unfortunately, buddleia produces numerous small seeds that can quickly colonize bare or disturbed ground such as riverine alluvial deposits, forestry sites, roads, railway embankments, eroded slopes or even structures. First introduced into the British Isles at the end of the 19th century, by the middle of the 20th century it had thoroughly naturalized the wastelands of southern England. It is now listed among the top 20 invasive weeds in England and is also naturalized in New Zealand and many parts of the USA.
Here, we report on approaches being taken to manage buddleia on opposite sides of the world that differ, not because the nature of the problem varies, but because of stakeholder priorities and attitudes. New Zealanders are familiar with the concept of classical biological control as a weed management tool; Landcare Research, for example, actively includes landowners and other members of the public in weed biocontrol implementation, engaging them in redistribution of biocontrol agents, and monitoring of their spread and impact. New Zealand is hoping to adopt this same approach for buddleia control. In the UK, however, classical weed biocontrol in something of a novelty and it has only recently begun its first weed biological control programme (against Japanese knotweed Fallopia japonica). This conservatism, combined with a dense population of avid gardeners and armchair naturalists, makes it unlikely that permission would be given for the importation of buddleia natural enemies to the UK. Therefore, CABI Bioscience, funded by Railtrack, is pursuing the development of a stump treatment approach based on naturally-occurring fungi already present in the wild.
Buddleia is now abundant in the North Island of New Zealand, and common in the north of the South Island. It ranks as one of the worst weeds threatening new pine plantations. A number of studies demonstrated the competitive nature of buddleia in relation to the key exotic forestry species, Pinus radiata. Buddleia out-competes the pine seedlings for light, resulting in reduced pine growth rate or even tree death. Growth benefits from weed control typically may be equivalent to 1-4 years of extra growth. Effective control of buddleia would have other economic benefits besides increasing growth and therefore stand value, for example in reducing costs associated with herbicide application. Buddleia also takes over forestry roads and firebreaks, increasing the costs of tending forests.
A range of possible growth benefit scenarios was used to calculate the economic benefits of biological control of buddleia to plantation forestry. The overall gain in net present value of the crop ranged from $NZ 1352/ha (at the most optimistic scenario) to $NZ 242 (for the most pessimistic) [1NZ$ = 0.4US$]. While benefits from removing buddleia might appear likely to be at the high end of this range, consideration of the possible side-effects of biological control showed the importance of including these in the design of a solution. In central North Island forests, buddleia normally dominates other weeds as successfully as it does pine trees, but biological control of buddleia might change which weed achieves dominance. Heavy destruction of buddleia in the first years of forest establishment could allow invasion by equally damaging weeds such as broom (Cytisus scoparius) or pampas grass (Cortaderia sp.). This would be no more damaging to pine production than buddleia domination, but it could mean that there would be no economic benefits from biological control of buddleia in plantations. Subsequent assessment of natural enemies took account of this potential constraint.
Buddleia efficiently colonizes and stabilizes disturbed ground in protected natural areas as well. Flood events dump fresh alluvial deposits (silt and gravel beds) in streams and riverbeds which are naturally colonized by a range of native herbs, grasses and woody shrubs. Buddleia can be particularly aggressive in such places, and quickly displaces all other species. Where disturbance is frequent, succession can be initiated again and again, prolonging buddleia dominance. Buddleia can also seriously affect access along stream and river beds, and change the hydrology of natural waterways.
Working with staff of Nanjing Agricultural University, China, Forest Research scientist Nod Kay studied the insects feeding on buddleia in China, and selected three that appeared to reduce growth, vigour and seed production. Two have been reared and imported into quarantine in New Zealand.
Cleopus japonicus is a weevil that skeletonizes buddleia leaves both as a larva and as an adult. A laboratory study has shown that feeding damage significantly reduces all measures of plant size and performance and that as little as 25 larvae can destroy a 1m-tall plant. Newly emerged adults and larvae are voracious feeders and this, coupled with a mobile adult stage, high fecundity, short generation time and several generations per year, make it a promising agent. In addition, host specificity testing has shown it is probably restricted to Buddleia species. There are no related native species in the family Buddlejaceae in New Zealand. In other taxonomically related families, a native Loganiaceae, Geniostoma ligustrifolium, was not fed upon to any extent, while minimal feeding but no oviposition was recorded on native Scrophulariaceae. Forest Research will soon seek permission to release this weevil in New Zealand.
Laboratory studies indicated that moderate levels of C. japonicus infestation would result in suppression of buddleia growth in young plants rather than massive plant mortality. With growth and biomass accumulation suppressed by biological control, buddleia could continue to dominate other weeds, especially if biological control was most active in older plants (after other weeds were suppressed). In this scenario, there would be benefits from successful control, probably of the order of 1-2 years extra growth, but competitor weeds would be kept in check so control would bring no nasty new surprises.
The second species under consideration is Mecysolobus erro, a stem weevil. Adults feed on the tender terminal shoots causing the tip to wither and die. Larvae bore 10-15 cm through the centre of young stems, often causing the stem to break or die back. Host-range testing of this species is underway.
The area of pine plantation in Central North Island is approaching 600,000 ha, and it is conservatively estimated buddleia is a problem in 10% of this region. Buddleia control could give a total return of between approximately $15 and $81 million here and this translates to an annual economic benefit of control in the Central North Island of between approximately NZ$0.5 million and $2.9 million.
Contact: Nod Kay,
In the UK, the mere suggestion of introducing another alien species to control one that has got out of hand sends sections of society into indignant rage. However, alien species certainly form the majority of most garden plants and a chorus of superlatives from the numerous media sources dedicated to gardening greets each new arrival. It is due to this lack of understanding of the process and lack of guidance from respectable sources that has convinced staff at CABI Bioscience to pick the battles that can be won.
However before any armour can even be pulled on, the spectre of conflict of interest raises its head. This problem is particularly acute when the target species has an attractive name and is valued by many. The widescale decimation of the Butterfly bush population using a classical biological control strategy would clearly be an inappropriate first 'success' story for biological control in the UK especially when 'public enemy' weeds such as Japanese knotweed are waiting for such an approach.
What is required is a more conservative strategy by which only problem plants are targeted. The pioneering work carried out in the Netherlands and South Africa producing products such as Biochon® and Stumpout? paves the way for the selective treatment of woody weeds such as buddleia and work is already underway at CABI Bioscience to develop a similar product to control Rhododendron ponticum.
Problems caused by buddleia are most severely felt by those with responsibility for properties and structures, since the wind-blown seed of B. davidii can find its way into the most inaccessible places. Old brickwork, roofs and guttering provide a perfect if not extreme environment for this hardy plant and once established it is capable of displacing many courses of bricks and even pushing over walls. The situation is particularly challenging on the railways where buddleia teams have to abseil down embankments and scale buildings to cut plants back. Clearly this activity is highly expensive given the 33,000 miles (some 60,000 km) of track in the UK. Equally important is the problem of sight lines for drivers and interference with signalling and safety equipment which makes any regrowth unacceptable.
The challenge is to find a native fungus that is amenable to bulking up, formulation and application as a stump treatment. If this can be achieved, then an active non-chemical treatment is feasible. This treatment would have to be more effective than the current chemicals available and could have the advantage of killing the plant irrespective of the seasonal direction of sap flow.
The Mascarene islands of La Réunion and Mauritius have long been the recipients of non-native species and their native flora and fauna have suffered as a consequence. Indeed the situation in Mauritius has deteriorated so much since European colonization in the 17th Century that only 1% of its surface area retains its natural vegetation. In light of the Mauritian experience, the Réunionnaise became concerned about one of the more recent arrivals, the privet Ligustrum robustum ssp. walkeri. This species, along with Psidium cattleanium, are the two main weeds threatening the remaining vegetation in Mauritius. It was clear that if nothing were done to control the privet species in La Réunion then it would continue its spread unchecked into the pristine forests, which cover the upper slopes of the mountainous 'cirques' of the island.
Given the problems encountered with controlling such a robust plant and the wealth of ecological knowledge on the invasion, biological control was suggested. In November 1997 funding from the European Union via the Conseil Regional Réunion allowed work by CABI Bioscience to begin on a classical biocontrol project. A review of the literature soon revealed a considerable number of natural enemies on the genus Ligustrum around the world including more than 70 arthropods and over 120 fungi. However it emerged that there was a lack of highly specific co-evolved species, probably because the plants that had been studied were ornamental hedge/amenity trees outside the species' original range.
Ligustrum is a member of the Oleaceae, none of whose members has ever been the target of classical biocontrol. However, commercial species do suffer from pests and the experience of olive growers suggested that fruit production could be significantly reduced by just one species of natural enemy. It is the prolific fruit production coupled with the activity of an introduced bird, the bulbul, which is speeding up the rate of spread in La Réunion, potentially into forest niches.
This project is one of the first to break with tradition and look at the arthropod and fungal suite of natural enemies from the very beginning. Such an approach means real cost savings in the long term but the different approaches of the entomologist and plant pathologist necessitated some changes in procedure, not to mention reconciling the equally curious but differing habits of exploratory entomologists and pathologists: one will spend 2 hours observing the feeding and oviposition habits of a single insect whilst the other needs to cut down the branch and get the prized parts into a plant press or onto agar before swiftly moving on.
Early visits coupled with later DNA work revealed Sri Lanka to be the area of origin of the target privet, yet surveys appeared to reveal little in the way of suitable biological control agents (BCAs), in particular no co-evolved obligate biotrophic fungi. A student from the Post Graduate Institute of Agriculture, University of Peradeniya in Sri Lanka was appointed after a Memorandum of Understanding had been signed to ensure that repeated field observations were made.
The failure to turn up agents on the target species in Sri Lanka meant a re-evaluation was needed and it was agreed that other hosts within the genus should be surveyed. Given the restricted number of representative Oleaceae in La Réunion, natural enemies that could be shown to be restricted to Ligustrum would still have been suitable for introduction. The decision was made to broaden the search to India where L. robustum ssp. robustum also occurs and brief surveys were made to the Western Ghats in southern India and Meghalaya in the northeast in search of genus-specific agents.
Despite the fact that Sri Lanka appeared to be the source of the invasion, the relatively low numbers of suitable agents, in particular a lack of biotrophic pathogens, led the team to consider even more distant regions where L. robustum occurs and indeed where the 'species' possibly originated. Consequently, a survey visit was made to Vietnam which was disappointing because of deforestation. This situation also hindered a final survey in China, although in this case the target plants themselves had been selectively removed because of a belief that their leaves provide a medicinal product. A commercial tea (Kou-ding-sha) is marketed there, and it was even suggested that the people of La Réunion, who have been blessed with pest-free privet, should harvest it as a crop for export to China where the species (based on our bitter experience) appears to be in danger of extinction!
What did become apparent was that the niche normally filled by the obligate biotrophs had been filled in each region by representatives of the hemibiotrophic order Dothideales. Very little work has been carried out on these fungi and pathogenicity studies were difficult to say the least. One Thedgonia sp. was collected from various Ligustrum spp. in Sri Lanka, India and even the UK and this was capable of infecting the target species and causing heavy defoliation.
In the meantime 16 arthropods were selected for consideration and ultimately three of these were deemed worthy of full host range studies given their life histories and initial evidence on host specificity. The only species to pass host range testing was the moth Epiplema albida after exposure to 89 species of test plant.
This species has shown a high level of feeding specificity and an even more restricted range of acceptable plants for oviposition. It is highly damaging in the lab and easily capable of killing the small plants available and, without the pressure of the parasitoids that keep it in check in Sri Lanka, may well be capable of reaching very high populations. This will be helped by its very short development period allowing multiple generations per year. The next step will be to take the moth to the new quarantine facility on La Réunion for final testing against native species and crops that were not available to the team.
This project has involved more than 30 scientists from eight countries and allowed considerable interaction and new associations. It is hoped that this has established a framework for the consideration of other potential targets in Indian Ocean Islands and more opportunities for biological control of invasive species, even though this privet has proven to be a more elusive target than most.
Contact: Dick Shaw,