The high profile reporting of non-target impacts of a few classical weed biocontrol agents over the past few years highlighted one shortcoming of many, if not most, biocontrol programmes - related to not inadequate testing, but inadequate monitoring. In trying to answer the critics of biocontrol, its defenders could only point to the absence of reported problems for the overwhelming majority of the 1150 or more planned releases of weed biocontrol agents that have been made worldwide. An equally overwhelming absence of post-release surveys means that there is little concrete evidence about the non-target impact of most of these releases.
In New Zealand, Landcare Research has been dusting off old files and investigating the safety record of weed biocontrol there since the first biocontrol agent, cinnabar moth (Tyria jacobaeae), was released against ragwort (Senecio jacobaea) in 1929. This audit will alert the country's biocontrol community to any lurking dangers, and also identify past flaws that will help tighten up testing procedures for the future.
Landcare has been looking at some agents that are now common in New Zealand, checking what testing was done before each was released, and whether non-target attack has occurred - or might yet. Reporting results at the 11th Symposium on the Biological Control of Weeds in Canberra this year, Simon Fowler said that, overall, the reliability of host specificity testing in New Zealand in the past has been good although a few gaps have been identified.
The largest gap in testing probably occurred in some projects conducted between 1943 and 1982. Although early biocontrol projects included native species in specificity tests, during this period 13 introductions of natural enemies were made which relied heavily on testing in other countries, and New Zealand natives were not tested. In one case, thistles, the rationale was that there are no native thistles in New Zealand so there were no closely related native species to test. While there could have been problems where introductions were made against other targets with New Zealand relatives, no non-target impacts have been recorded on New Zealand native plants, and only one potential serious impact has emerged so far. Three agents were released against St John's wort (Hypericum perforatum) but they had not been tested against native Hypericum species. Hypericum japonicum and H. gramineum are uncommon plants and, although none of the agents has been recorded on them, they could be at risk. In many other projects during that period, and in all projects since 1990, it has been standard practice to test native plants so the omission of indigenous species from test plant lists should not occur now. However, this does not imply that selecting what plants to test is now always straightforward.
The first agent to be released in New Zealand, cinnabar moth, was tested against eight native species of Senecio before permission was given for its release against ragwort. However, when ragwort is defoliated the moth occasionally attacks two species of native Senecio (S. minimus and S. biserratus) which were not tested. Why were these two not included in the test plant list? They were in a different genus at the time. This is the only recorded instance of an introduced weed biocontrol agent attacking a native non-target species in New Zealand, but could such a testing omission happen again? It is unlikely. Plant systematics have clearly progressed since 1929 and if the testing were being conducted now, these two species would be included. However, even with the taxonomic relationships clarified, there is another source of uncertainty.
Alternanthera sessilis, a close relative of alligator weed (A. philoxeroides) was not included in host specificity tests in that biocontrol programme. Both are exotic species and indeed there are no native New Zealand plants in the family. But what was not foreseen was that A. sessilis would subsequently attain cultural importance as a new vegetable crop for some sectors of the community. Short of including a fortune-teller in the biocontrol project, this would have been hard to predict but Toni Withers (Forest Research), who has been working with Fowler and others on this project, believes that the current rigorous process for drawing up test plant lists means that a similar case would not now be overlooked. Of two agents introduced against alligator weed, one (the moth Arcola malloi) attacks other Alteranthera species, and although damage to A. sessilis has not been observed it remains a possibility.
The above rare instances of the failure of New Zealand's past testing procedures occurred because plants that might have been tested were not. There are just two recorded cases where plants were tested, but the testing failed to predict the non-target attacks that subsequently occurred. The broom seed beetle Bruchidius villosus and the gorse pod moth Cydia succedana have unexpectedly attacked seed of other exotic Fabaceae, although pre-release testing suggested that this would not happen. Investigations into both are continuing, but Fowler noted that a common link is that both use seasonally ephemeral resources (young pods) whose phenology in comparison to that of the agent differs slightly between Europe and New Zealand, potentially offering novel no-choice situations to agents in the field after release. This gives us a new concept to consider: Do agents introduced for discrete seasonal resources need more careful assessment? Should more rigorous no-choice testing be considered in these circumstances? Withers believes that the answer lies in our ability to accurately interpret the results of host specificity testing, something that retrospective analysis of the methods used in the past, combined with post release field assessment, will help us with.
The message is that if weed biocontrol is to win the confidence of its often vociferous sceptics, it is likely to require more sophisticated yet transparent interpretation of the host range testing carried out, and potentially will become more time-consuming and (therefore) more costly as a result. At a time when time and budgets are being cut this fills the biocontrol community with dismay. As a direct consequence of the greater cost some projects will not be undertaken, while the increased scrutiny is likely to raise the bar so high that potentially useful and harmless agents will be rejected.
Contact: Simon Fowler,
An Old World birch leaf-mining sawfly, accidentally introduced to Alaska, is the subject of a classical biocontrol programme employing a natural enemy from neighbouring Canada. This programme, capitalizing on the successful biocontrol of birch leaf-mining sawflies in Canada, will also mark the first instance of a natural enemy being released as part of a major programme against an invasive alien species in the wild in Alaska.
Birches have a worldwide distribution in the North Temperate Zone, and some 12 native species are found in North America. They are an important component of the boreal forests that extend north to the treeline. A number of native and exotic species are highly valued and widely planted as ornamental and shade trees in town and cities. The wood from some birch species is used for wood pulp and timber and for a large variety of products, from furniture to canoes to ornaments. Birch sap is starting to find a niche as a non-timber forest product. In the harsh climate of the northern forests, birch buds, twigs, pollen catkins (flowers) and nutlets (fruits) are an important food source for birds, mammals and insects.
Since the early 1900s, North American birches have suffered the invasion of five leaf-mining sawflies, all of European origin. By mining the leaves, their larvae cause extensive discoloration that is unsightly and reduces amenity values. The annual destruction of the trees' photosynthetic capacity has long-term impact on their health, and severe attacks over several years can weaken them and make them susceptible to attack by other insects, diseases and drought. Application of systemic pesticides (dimethoate), the most common means of control for these pests in urban centres, has had significant monetary costs to the public, and an undoubtedly significant environmental cost.
The spread of the leaf miners across the continent was slow but unremitting. Two species, the birch leaf miner (Fenusa pusilla) and the amber marked leaf miner (Profenusa thomsoni), became particularly widespread and destructive. From their point of introduction on the east coast of North America, these two species spread across the intervening ca. 4000 km to the western Canadian province of Alberta by the 1960s and were soon causing alarm. Once populations were well established, most birch trees in urban centres had turned brown by midsummer, and this level of infestation continued year after year. The additive action of the species heightens the problem: attack begins with first generation F. pusilla in early- to mid-May, as the trees begin to break bud. This is followed by new waves of attack in June-July from the univoltine species, P. thomsoni , together with a second generation of F. pusilla. Although a third species, the late birch leaf edge miner, Heterarthrus nemoratus, is present in Alberta, it is very rare and causes little damage.
In the early 1990s, scientists were surprised to notice a dramatic drop in sawfly damage to birch trees in Edmonton, Alberta and even more surprised to find that a native parasitoid was responsible by reducing populations of one of the species, P. thomsoni. The parasitoid responsible, Lathrolestes luteolator, appears to be native to both Old and New Worlds. It had not previously been recorded from P. thomsoni, although it attacks sawflies in the genus Caliroa in Europe and North America, and Profenusa alumna on northern red oaks (Quercus rubra) in eastern Canada and the USA. It is not unusual to find native parasitoids switching to an exotic host, but they are generally not sufficiently efficient to exert effective control - indeed, it is an axiom of classical biological control that co-evolved natural enemies from the area of origin of the pest exert the most effective control because their life histories and population dynamics are most closely linked. However, there are a number of examples of such `new associations' providing useful levels of control (e.g. citrus leaf miner, Phyllocnistis citrella).
The control of F. pusilla followed more conventional classical biological control lines, utilizing two parasitoids, Lathrolestes nigricollis and Grypocentrus albipes, which are highly specific to F. pusilla in Europe. These parasitoids had previously been released against F. pusilla in eastern Canada in the 1970s and the northeastern USA in the 1980s, with excellent results, at least for L. nigricollis (G. albipes did not establish). Following on earlier successes, scientists from the Canadian Forest Service - Northern Forestry Centre (CFS-NoFC) teamed up with colleagues from the University of Alberta, and contracted CABI Bioscience (then the International Institute of Biological Control, IIBC) in Delémont, Switzerland to supply natural enemies from Austria. Lathrolestes nigricollis and G. albipes were shipped and released between 1994 and 1996, and both species became established although L. nigricollis was most successful. Since then, the wasp populations have increased rapidly and spread throughout the Edmonton area. In 2003, L. nigricollis was recovered from F. pusilla approximately 300 km from the release sites. It is now difficult to find the birch leaf-mining sawflies in Edmonton. As a result of the impacts of the parasitoids, the city curtailed its expensive (and sometimes controversial) policy of spraying its birch trees on public lands with dimethoate, and initiated a successful communication campaign to dissuade the public from applying insecticide on the grounds that it would disrupt biocontrol. Edmonton's success has been noted by authorities in areas still troubled by the sawflies, and NoFC is currently working with Northwest Territories and Alaska on control programmes.
Alaska remained free of alien birch leaf-mining sawfly pests until about the 1990s when the first damage to birch was noticed. Subsequently, birches in Anchorage began to sustain the greatest damage, which now extends over more than 14,000 ha in and around the city. The pest has also spread as far north as Fairbanks, east to Glennallen and south to Haines and Skagway. It was not until 2002 that the major culprit was positively identified as P. thomsoni; however, F. pusilla and H. nemoratus are also present in Anchorage but very rare. Birch is one of Anchorage's most common shade tree species, and the impact of them turning brown at the peak of the short summer is particularly dramatic. The rapid rate of spread of P. thomsoni, coupled with the failure of weekly trapping to find any evidence of the parasitoid, suggested that the sawfly has left behind the natural enemies that now control it farther east. A 3-year cooperative multi-agency programme has been initiated between CFS-NoFC and the US Department of Agriculture Forest Service (USFS) to study populations of P. thomsoni in Anchorage, locate populations of the parasitoid L. luteolator in Canada and introduce parasitoids to Alaska, in the expectation that it may re-enact its Canadian success and suppress P. thomsoni again. Of the known hosts of L. luteolator, only P. thomsoni and Caliroa cerasi occur in Alaska. No other members of these genera occur there so there is low risk of host switching. In 2003, baseline data were obtained on the ecology and mortality of P. thomsoni in Alaska, sources of L. luteolator were located in the Northwest Territories, parasitoids have been collected and are overwintering in Edmonton before being transported to Anchorage in the spring of 2004, and import permits have been secured to allow the releases to proceed.
While scientists are gaining the upper hand with P. thomsoni and F. pusilla, they are also keeping a close watch on two more recently introduced species that mine leaves of birch, Scolioneura betuleti and Messa nana. Both species are currently distributed only in eastern North America; S. betuleti has a localized distribution in Ontario and M. nana is widely distributed from Ontario to New Brunswick and in the northeastern USA. With its large range and propensity to cause high levels of damage, M. nana is a growing concern and warrants increased vigilance.
Sources: Anon. (2003)
Science and nature give sawflies a one-two punch. Solutions
(newsletter of the Canadian Forest Service) Fall/Winter 2002
Contact: David W. Langor,
Ed Holsten, Research
Cane toad (Bufo marinus), biocontrol's best-known disaster story, may become a model for developing a new form of biocontrol. According to a recent report*, the technical feasibility of producing a viral agent that can disrupt the development of cane toad looks extremely promising, but a number of other issues need to be addressed if the overall objective of producing an effective self-disseminating viral vector is to be achieved.
The report was the result of a review, funded by Department of the Environment and Heritage (DEH) through the National Heritage Trust, of the government-funded `Development of a cane toad biological control' project. The Australian Government is continuing its support for the approach through the DEH. In September it also announced that an additional A$ 200,000 of funds is being released by the DEH for new projects on short- and medium-term control techniques for cane toads in Australia.
Cane toads were introduced to Queensland in 1935 to control beetle pests in sugar cane, a job to which they proved singularly ill suited. Instead they became a pest themselves, preying on small animals and poisoning larger predators (including household pets) that try to eat them, and out-competing native reptiles and amphibians for habitat and food resources. They have spread beyond Queensland, with a range currently extending from northern New South Wales into the Northern Territory. They threaten the World Heritage Site of Kakadu National Park and are continuing to spread through the tropical north towards Western Australia. There are currently no control measures effective for anything but small, restricted areas.
Investigations into possible biocontrol solutions for cane toad began in 1990, with surveys for pathogens in the toad's area of origin in Venezuela and ecological studies of the pest there and in Australia. More recently, research has focused on investigating potential viral biocontrol agents. A stimulus for this work was rapid advances made in gene technology during the 1990s together with the progress made by CSIRO in genetic manipulation of viruses to interrupt animal development. While this approach fits with cultural pressure for `humane' control methods, it also needs to address public concerns about the safety of genetic modification. A project aimed at producing a recombinant viral agent was initiated in 2000, and was the subject of the recent review.
The report notes that, although there have been hitches in maintaining a healthy captive colony of cane toads for research purposes, excellent technical progress has been made and objectives have been met on time and on budget. It draws attention to the project's achievements and is encouraging about its prospects for success, while also pointing out areas that need to be addressed: