September 1997, Volume 18 No. 3

Biorational

Integrated pest panagement (IPM) involves the use of many techniques, including biological control, to provide effective control of crop pests with minimum harmful side-effects. Those techniques which are compat-ible with the use of biological control or have little impact on natural enemies have been described as `biorational'.

Housing Benefits

Adding shelters or `leaf domatia' to cotton plants has been found to be a simple method of increasing natural enemy populations in cotton crops in California, and shows promise as a natural biological control strategy. Naturally occurring domatia, small hair tufts or pockets on the lower surfaces of leaves, occur widely in perennial flowering plants and were first described in the 19th century by the Swedish naturalist Axel Lund-strom. It has been suggested that such structures give mutual benefits to the plants and protective insects and mites; one of their striking features is that they are inhabited only by insects and mites that feed on other arthro-pods and not phytophages.

Anurag Agrawal and Richard Karban of the University of California at Davis investigated the impact of predators on spider mites which are one of the most serious pests of the California cotton industry. Unlike naturally occurring close relatives, commercial cotton varieties have no natural domatia, so the researchers glued dots of cotton fibres to the undersides of cotton leaves in the field. They found that natural enemies of the spider mites - the western flower thrip (Frankliniella occidentalis), the big-eyed bug (Geocoris spp.) and the pirate bug (Orius tristicolor) - all readily took up residence in the simulated domatia. The presence of domatia from early in the growing season led to an increase in cotton yields of 30% in cultivars which were normally tolerant of high levels of herbivory. When domatia were added later in the season, spider mite populations were kept below pest levels but yields were not significantly affected; late-added domatia had the greatest effect on populations of the late-colonizing bugs, but had little effect on the thrip, which arrives early in the season.

Putting out simulated domatia is a time-consuming business, but Agrawal suggests that as domatia are found on wild forms of other crops attacked by spider mites, traditional breeding or transgenic techniques might be used for introducing them into commercial varieties.

For further information see: Anurag A. Agrawal and Richard Karban (1997) Domatia mediate plant-arthro-pod mutualism. Nature 387 (5 June), 562-563.

A Novel Fungal Antibiotic

Carlos A. Neyra of Rutgers' Cook College in New Jersey has discovered a great rarity: a novel natural antibiotic. The substance is produced by a new bacterial strain found on the roots of a plant growing on the College Research and Extension Farm at Adelphia. Neyra has been awarded one patent for Bacillus licheniformis and its applications, and another for the lipopeptide antibiotic it produces which has been shown to destroy plant pathogens affecting maize, grasses and vegetable crops. Singular properties of B. licheniformis include its ability to survive in high tempera-tures, low oxygen levels, high salinity and dry conditions - significant properties which suggest that it may have potential as a natural control agent in semiarid and arid areas of the semitropics and tropics.

Trials are being conducted this summer in collaboration with Bio Integrated Technology of Italy to determine the efficacy of the anti-fungal agent in an agricultural environment.

For more information contact: Michelle Hujber

Tel: +908 932 9003

E-mail: hujber@aesop.rutgers.edu

Trojan Horse for Baculovirus Delivery

A novel method which takes advantage of mechanisms evolved by baculo-viruses to overcome insect immunity may provide a means of more effective delivery of baculovirus biocontrol agents, according to Ping Wang and Robert R. Granados at the Boyce Thompson Institute for Plant Research at Cornell University, USA.

Insects have evolved gut-mediated immunity to ingested pathogens in the form of a nonporous mucosal barrier in the peritrophic matrix (the interior lining of the gut). Wang and Granados cloned a major protein constituent of this barrier, mucin, in the cabbage looper, Trichoplusia ni. They also discovered and cloned a baculovirus-encoded protein, enhan-cin, which binds to the mucin protein and renders it permeable to companion baculoviruses.

By enclosing both enhancin and a pest-specific baculovirus in a `Trojan Horse'-like polyhedron crystal which was ingested by the insect they were able to deliver both components simultaneously to the insect gut. The crystal dissolved in the gut and released its contents; the enhancin broke down the mucinous component of the pertrophic matrix facilitating the passage of the virus and resulting in the rapid death of the insect. Initial experiments were conducted on T. ni, but the success of the method has since been confirmed in six other lepidopteran pests: fall armyworm, beet armyworm, cotton bollworm, American budworm, velvetbean caterpillar and common armyworm.

Research is also being conducted into the possibility of using genetic engin-eering techniques for incorporating the enhancin gene into crop plants.

For further information see: Ping Wang and Robert R. Granados (1997) An intestinal mucin is the target substrate for a baculovirus enhancin. Proceedings of the National Academy of Sciences 94, 6977-6982.

Nematode News

Randy Gaugler of Cook College in New Jersey, USA has developed the World's first genetically engineered nematode by inserting an additional heat-shock protein from another nematode species into Heterorhabditis bacteriophora. It has been hailed as an important break-through for insecticidal nematodes. Trials indicated that the transgenic nematode was 18 times more heat tolerant than untransformed individ-uals, while no other characteristic tested was altered.

Transgenic nematodes that can withstand high temperatures would ease problems with storage and transport of nematodes used for insect control, and Gaugler says that this will be particularly beneficial to small companies who will now be able to improve the quality of the product delivered at the field level.

Elsewhere in the USA, a new nematode species, Psammomermis nitiduesis, has been described from the maize sap beetle, Carpophilus lugubris, in Illinois and shows potential as a biological control agent for this pest (USDA-ARS Agricultural News, April 1997). The beetle can be an economically impor-tant pest in some parts of the US Corn Belt and larvae can cause severe damage to the ears. The adult stage acts as a vector for toxic fungi which it picks up while feeding on plant residues and later spreads to other plants as it feeds on the pollen.

The nematode is thought to enter the beetle's body in late summer during pupation and grows - to about 20 times as long as the beetle. Some 80% of beetles collected from one field in early spring were infected. Studies are continuing on ways of moving infected beetles into areas where the nematode has not yet been found.

Transgenic Crop Research in India

Following the halting at the Indian Council of Agricultural Research (ICAR) of a transgenic trial involving aubergines expressing a Bacillus thuringiensis gene (reported in the last issue, BNI 18(2), Aubergines Out), the Indian Department of Biotechnology (DBT) and ICAR have announced that they have agreed measures to regulate the entry and use of transgenic material in India. It has been agreed that the National Bureau of Plant Genetic Resources will be the sole import agency, and containment facilities for testing transgenic plants in limited field situations will be built to specifications submitted by ICAR, initially at New Delhi and Bangalore. A Review Committee on Genetic Manipulation has been set up by the DBT to examine proposals from industry and goverment research scientists, and first approvals have been granted. A joint committee comprising representatives from the Departments of Biotechnology and Environment and ICAR will oversee all work related to transgenic material, and a protocol for field testing is to be designed.

Bt Resistance and Pest Management in Malaysia

Following the results of research led by Bruce Tabashnik on the genetic basis of the development of Bacillus thuringiensis (Bt) resistance in diamondback moth, Plutella xylostella (BNI 18(2), Plutella resistance to Bt toxins), the results of laboratory and field-based studies in the Cameron Highlands, Malaysia suggest that there the pest is becoming increasingly resistant to Bt and abamectin. The researchers draw attention to the urgent need for an insecticide resis-tance management strategy to prevent these products which have proved compatible with an IPM strategy for diamondback moth in the Cameron Highlands becoming ineffective.

Plutella xylostella, introduced to the Cameron Highlands in the 1920s, became a serious pest of crucifers in the 1940s but was kept under reason-able control by a combination of botanical and inorganic insecticides and cultural methods. However, in the 1950s there was a change in control strategies with the introduc-tion of organic insecticides, and farmers became increasingly reliant on these. The development of insect-icide resistance to successive groups of compounds has bedevilled control ever since, and there is evidence that the intensive use of these insecticides, including those normally regarded as relatively harmless to nontarget organisms, has prevented key hy-menopteran parasitoids from exerting control on P. xylostella.

The replacement of organic insect-icides since 1988 by Bt-based insect-icides and the actinomycete-based compound abamectin as the principal chemical control agents led to a significant improvement in P. xylostella control. This change also facilitated the establishment and population increase of key parasitoids (Cotesia plutellae, Diadegma semiclausum and Diadromus collaris) which in turn contributed to control.

Resistance to Bt var. kurstaki was recognized in the Cameron Highlands in 1992, but resistance to Bt var. aizawai and abamectin resistance now appears to be present and increasing. It seems inevitable that if current usage continues, severe resistance and control failures are inevitable, which will encourage the reintroduction of non-selective insecticides (to which resistance would have temporarily declined) with catastrophic effects on the natural enemy complex of diamondback moth.

For further information see: Verkerk, R. H. J.; Wright, D. J. (1997) Field-based studies with the diamondback moth tritrophic system in Cameron Highlands of Malaysia: implications for pest management. International Journal of Pest Management, 43(1), 27-33. Also: Iqbal, M.; Ververk, R. H. J.; Furlong, M. J.; Ong, P. C.; Rahman, S. A.; Wright, D. J. (1996) Evidence for resistance to Bacillus thuringiensis (Bt) subsp. kurstaki HD-1, Bt subsp. aizawai and abamectin in field populations of Plutella xylostella from Malaysia.