September 2000, Volume 21 No. 3
USA Welcomes Settlers
Two classical biological control programmes in the USA have reported progress with establishment of introduced agents this year: one is for an invasive tree threatening an important wetland habitat, the other against an insect causing significant economic damage in key grain crops.
The broad-leaved paperbark tree, Melaleuca quinquenervia, was introduced to Florida at the beginning of the last century. Hopes of using the tree for timber were not fulfilled, although it did prove economical to grow for ornamental production. Unfortunately it proved an even bigger success as an invasive [See: BNI 18(3), 67N `Melaleuca Swamping Florida' September 1997], and now infests some 200,000 ha of the Everglades and is threatening native habitat. It thrived in Florida - growing fast, flowering up to five times a year from 2 years old, and producing copious amounts of seed. A single tree can hold over 60 million seeds in the canopy, releasing them in times of stress (fire, frost, herbicide or death).
The trees were planted as a means of `reclaiming' the Everglades. They grow densely, forming impenetrable thickets, and spread partly by adventitious roots. The thick mats of roots at the water surface cause soil accretion, and this leads to an increase in the elevation of the infested area. A few centimetres in elevation can cause big differences in the composition of plant communities here, and the spread of melaleuca is threatening to transform the Everglades from a wet prairie into a closed-canopy swamp.
A USDA-ARS (US Department of Agriculture - Agricultural Research Service) project based at its Australian Biological Control Laboratory identified some 400 species attacking this and other related Melaleuca species in its native range in Australia. It identified, amongst others, a leaf-feeding weevil, Oxyops vitiosa, as a promising potential biocontrol agent. The weevils were first released in Florida in March 1997, and between then and June 1998, some 1500 adults and 6700 larvae were released at 13 sites in six counties.
Earlier this year it was reported that populations of the weevils are now well-established, increasing, and beginning to disperse to other melaleuca infestations. The promising results show that the weevils are highly effective at defoliating existing stands of melaleuca saplings, and it seems likely that they will be able to prevent further spread of the invasive tree. Melaleuca trees shed 25-33% of their leaves each year, but O. vitiosa has a preference for new leaves, and attack these as they are produced. The weevils in Florida are already consuming enough new foliage that some trees are showing evidence of die-back. In time, trees may become completely defoliated. Ultimately, the weevils will contribute to stopping the problem at its source, as stressed trees no longer seem to be producing flowers or seeds. This will facilitate control by conventional means, as it overcomes the problem that has complicated herbicidal or mechanical control measures so far: the millions of seeds that trees can shed when they die.
Additional agents from Australia are in the pipeline which target the mature foliage (Lophyrotoma zonalis, melaleuca sawfly), saplings (Boreioglycaspis melaleucae, melaleuca psyllid), and bud growth (Fergusonina sp., melaleuca bud gall fly). Host range studies have been completed with B. melaleucae, and a proposal for field release has been submitted to federal authorities. Psyllids feed on the phloem and heavy nymphal feeding kills saplings. A proposal for release of L. zonalis is still pending. Concern about possible vertebrate toxins might prevent eventual release of this species. Australian host range studies have demonstrated the specificity of Fergusonina sp. This fly will be tested soon in the Florida quarantine facility with the eight native species of Myrtaceae. Both leaf bud and flower bud galls are produced by the fly and its symbiotic nematode, Fergusobia sp. This will further help stem flower production and enhance traditional control measures.
Contact: Ted Center,
Russian wheat aphid (Diuraphis noxia) (RWA) arrived in the USA in 1986, and since then has cost more than US$1 billion in insecticide costs and related losses. Its North American range now extends through 16 US states and two Canadian provinces. It spends the winter primarily on wheat and barley, while several native and introduced grass species harbour populations during the summer. The most important non-cultivated grass hosts of D. noxia in North America are the wheatgrasses (genus Agropyron; particularly crested wheatgrass, A. cristatum) and wildryes (genus Elymus; particularly Canada wildrye, E. canadensis).
The US government is developing an arsenal of weapons against the aphid: conventional breeding has produced aphid-resistant varieties, which are now available to producers; microbial agents are being tested [see BNI 21(1), 8N-9N, `Fungal solution for crop pests?' March 2000]; and insect parasitoids have also been introduced from the area of the origin of the aphid.
In 1988, USDA-ARS (US Department of Agriculture - Agricultural Research Service) began work with a consortium of federal and state scientists to release 11 species of exotic RWA parasitoids in the wheat and barley growing areas of the western USA. They released a staggering 11.8 million parasitic wasps, representing more than 80 geographic strains collected from 25 different Eurasian countries where the aphid originated.
Key to the success of the project was the collection of these exotic enemies by staff of the ARS European Biological Control Laboratory in Montpellier, France. Over 60 exploration trips (throughout the endemic range of D. noxia) were made between 1988 and 1994. Seventeen different countries were visited by more than 40 federal and state scientists and technicians, and that resulted in the collection (mostly from D. noxia-infested wheat and barley fields) of at least 29 species of natural enemies for release in North America to combat the pest.
From 1991 to 1993, ARS scientists at the Plant Sciences and Water Conservation Research Laboratory in Stillwater, Oklahoma released seven parasitoid species in wheatfields in eastern Colorado, with the aim of establishing natural enemies in the grain-growing areas there. Seven years later, John Burd (ARS) reports that four of the species have become established throughout a six-state area: Colorado, Kansas, Montana, Nebraska, Oklahoma and Wyoming. Three of the species were also found parasitizing related greenbugs (Schizaphis graminum) on sorghum, and two species successfully parasitized RWA on summer wild grass hosts.
Contact: John D. Burd, USDA,
Water hyacinth (Eichhornia crassipes), which entered the country as an ornamental, is the most serious aquatic weed in India, creating a large number of problems in the management and utilization of freshwater resources. Attempts made from 1969 by the Indian Station of the Commonwealth Institute of Biological Control (CIBC) to import exotic natural enemies were unsuccessful owing to an ongoing debate on the potential of this weed as a resource. With the inception of the All India Coordinated Research Project on Biological Control of Crop Pests and Weeds in 1976, renewed efforts were made by the Indian Council of Agricultural Research to import host-specific natural enemies. Water hyacinth creates special problems in India, because drought and associated problems are chronic features. Water hyacinth reduces the volume of available fresh water by increasing losses through evapotranspiration (up to 9.84% reduction reported). It impedes the flow of water in irrigation systems (40-90% reduction reported) and impairs the quality of water, making it unfit for human consumption. After taking into consideration these and other major disadvantages of the weed, it was finally agreed by the Indian authorities in July 1981 that some biocontrol agents should be obtained for trials.
With the assistance of CIBC (now part of CABI Bioscience) a shipment consisting of the two weevil species, Neochetina eichhorniae and N. bruchi, and the mite Orthogalumna terebrantis was received from the US Department of Agriculture, Fort Lauderdale, Florida. Neochetina eichhorniae was also imported from Long Pocket Laboratories, CSIRO, Indooroopilly, Queensland, Australia (although this culture was subsequently discarded owing to microsporidian infection).
The weevils and the mite were reared in a glasshouse under quarantine conditions. Detailed host-specificity tests, involving 76 species of plants belonging to 42 families for the weevils and 88 species of plants belonging to 42 families for the mite, confirmed their safety to cultivated plants in India. Limited field trials were then conducted with permits issued by the Plant Protection Adviser to the Government of India (PPA). Based on the results obtained during these studies the PPA gave permission for open field releases throughout the country.
Neochetina eichhorniae and N. bruchi established readily under field conditions in India after large-scale releases were initiated in 1983. Field studies indicated that release of a breeding population of about 1000-5000 adults, depending on the area of weed coverage, invariably resulted in establishment, irrespective of whether the initial releases were made at one spot or over a wide area. During the first year the insects were observed to multiply and spread throughout the weed mat on which they had been released. Thereafter, an increase was seen in the number of adults per plant, which in turn caused a reduction in the number of leaves and petiole lengths. Once the population of adults exceeded five individuals per plant, the leaves of the weed turned brown, starting from the tip, and ultimately the whole plant collapsed and sank to the bottom of the water body.
Successful biological control ranging from 90-98% was achieved in six tanks, with a combined total surface area of over 1000 ha, in Bangalore within 3-4 years of the release of a total of about 25,000 weevils. The tanks are manmade lakes created some four centuries ago by damming natural rainwater channels to provide water for use during the summer months, mainly for irrigation but also as a source of drinking water. The proliferation of water hyacinth in them is a symptom of the pollution afflicting these once pristine water bodies. Some have been breached, but sewage has polluted others as the city has grown. Both N. eichhorniae and N. bruchi, separately and in combination, were found to be effective in suppressing water hyacinth. However, the rate of weed suppression was found to be very slow in a partially sedimented tank, where the water hyacinth plants were anchored. This was probably due to silt coverage of root hairs of the weed, on which the insects pupated.
Interestingly, there were fluctuations in weed cover, ranging from 5-30%, in all the tanks in which the weevils had been released for 3-5 years after the initial collapse. However, residual weevil populations were found to be capable of reducing weed populations every time such an increase was observed, and no additional releases were necessary.
Studies in Bangalore indicated that N. eichhorniae and N. bruchi could be successful even in tanks that dry during the summer. In the laboratory they survived without food for as long as 48 and 28 days, respectively, in 95% humidity, and much longer in the presence of free water. They can probably survive periods of drought by remaining below plant debris or within crevices in the soil and water may be available to them in the form of dew.
More than 150,000 weevils were released at different locations in 15 states elsewhere in India. Spectacular biological control of water hyacinth was achieved in the 286 km2 Loktak Lake in Manipur, 75% of which was covered by the weed. Control was achieved within 3 years of release of some 18,500 adult weevils during 1987-88. Besides the lake's importance to the local community for fishing, a hydroelectric power project set up at Loktak Lake in 1983 has an installed capacity to generate 105 MW of power and irrigate 23,000 ha of land. Control of water hyacinth by release of weevils has brought about a permanent solution to the weed problem, which in turn has benefited the economy of the entire region.
Results with O. terebrantis are so far more equivocal. Although it established under field conditions after releases were initiated in 1986, the mite does not appear to be capable of suppressing water hyacinth on its own. Heavy population build up was observed in all the tanks where releases had been carried out, leading to browning of leaf laminae, but promising results are yet to be achieved.
The results of the studies carried out to date clearly show that N. eichhorniae and N. bruchi are effective biological control agents, capable of bringing about a permanent solution to the problems posed by this weed in India. Hence, there is an urgent need to spread them to regions where releases are yet to be made. This objective can easily be achieved by releases of field-collected natural enemies into selected weed-infested water bodies in different parts of the country and redistribution from these spots after establishment and population build up.
By: K. P. Jayanth, Bio-Control Research Laboratories, A
division of Pest Control (India) Limited, P. O. Box 3228,
This article also appears in Water Hyacinth News No. 2 (October 2000) with agreement of the author and the publishers.
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