In the never-ending war between farmers and insects, the ultimateweapon on the human side is a protein produced by a soil bacterium,Bacillus thuringiensis _ Bt for short.Like DDT in decades past, Bt puts down a very wide range of croppests, but unlike DDT it is environmentally correct, harmless tohumans, and apparently defies anti-Bt resistance developed in theinsect enemy.Apparently, but not really.Some years ago, watercress growers in Hawaii found that thediamondback moth (Plutella xylostella), which feeds exclusively oncrucifer plants _ cress, cabbage, cauliflower and the like, wasmunching on the watercress crop with impunity, even though its leaveshad been sprayed with Bt.Soon, cabbage farmers in Florida, New York and Japan were reportingsimilar insect immunity to the reputedly infallible pesticide.Meanwhile, entomologist Fred Gould at North Carolina StateUniversity has found that another crop marauder, the tobacco budworm(Heliothis) is ready, able and willing to become resistant to Bt. So hasthe Colorado potato beetle.At the University of Hawaii's College of Tropical Agriculture andHuman Resources, entomologist Bruce E. Tabashnik moved intoaction. He and collaborators at the Universities of Georgia andAlabama started to study the mechanisms that conferred resistance toBt on P. zylostella, and the measures that might restore theirvulnerability to the protein.In the current issue of the Proceedings of the National Academy ofSciences (PNAS), Tabashnik's team reports progress to date in a papertitled "Reversal of resistance to Bacillus thuringiensis in Plutellaxylostella."As they explain, the Bt that farmers apply to their crops consists of acrystalline form of the toxin that the kirstaki strain of B. thuringiensismakes, often mixed with its spores. When a hungry insect ingests thisproduct, the protein destroys its intestines _ specifically, the brush-border membrane of its midgut. Resistance develops when naturalselection raises up a cohort of insect survivors with membranes whollyor partially devoid of receptors for the lethal Bt molecule. Insectsinherit this resistance factor as an autosomal recessive trait.Generations Of Hyper-resistant MothsIn the lab, the entomologists raised generations of moths made hyper-resistant by imposed selection pressure. One group of larvae, exposedintensively to Dipel (Abbott Laboratories' version of Bt), had attainedresistance 2,800 times greater than the concentration of Bt needed tokill a susceptible Plutella strain. "This," the PNAS paper reported,"represents one of the highest levels of resistance to Bt ever recorded."Instead of a quasi-military response _ steadily escalating dosage of thetoxin to keep one jump ahead of the naturally increasing resistance _Tabashnik and his co-authors tested a non-violent counter-measure.They found that resistant insects taken off Bt for a few generationswould lose or "forget" their immunity, and revert to susceptibility.This armistice from the toxin caused Plutella to lose biotic fitness. Inthe absence of Bt, survival to adulthood was higher in coloniesreverting to their innate vulnerability than in those persisting in theirresistance.So the entomologists prescribed a strategy of "refuge" from thepesticide, both in time and space. Concretely, as Tabashnik toldBioWorld Today, "The way Bt has been used in the past, it degradesvery rapidly _ which is what makes it environmentally attractive.Even if you sprayed it, say, once a month, it would hang on in thefields only a few days, so lots of individuals in the insect populationwould never be exposed. Thus, planned abstention from spraying forseveral insect generations, which average three weeks, would limitdevelopment of resistance temporally."Spatially, so would selective spraying of only certain fields.Dousing with Bt though, is rapidly being overtaken by transgenic cropplants, which express the gene for Bt, and kill their insect predators oncontact. This may sound like the hyper-ultimate solution, butTabashnik warns, "Putting it in a transgenic plant is equivalent tospraying every day. So growers might need to skip a few crop cycles,enough time for a few generations of the insect to pass by." At the veryleast, he suggests, "alternate planting of transgenics and non-transgenics.""Thus, his PNAS paper concludes, "the usefulness of one of the newestproducts of biotechnology _ transgenic plants _ may be prolongedeffectively by one of the oldest strategies for pest management _ croprotation."Not A Practical Problem, YetEcogen, Inc. in Langhorne, Pa., is a major biotech player in Btdevelopment. The firm's executive vice president of research anddevelopment, Bruce C. Carlton, told BioWorld Today, "From apractical sense, there is really very little resistance to Bt in the fields.So the bottom line," he added, "is that resistance is not a practicalproblem at this point in time." He describes the crop reports fromHawaii, Florida, New York and Japan as "a few isolated cases."Nevertheless, Carlton noted, "In practical terms, a Bt ManagementWorking Group was formed four years ago specifically to address thisissue. It has already funded over $250,000 in basic research grants atuniversities and institutes to address various aspects of insect resistanceto Bt.The group consists of many major agricultural and chemical companies_ DuPont, Sandoz, Ciba-Geigy, NovoNordisk, Monsanto -- andvirtually all small biotech-oriented firms involved in either spray-onproducts or genes in crop plants."Carlton endorses the "refuge" idea of spraying non-transgenic plantsinfrequently "so as not to overwhelm the insect to the point where itdevelops resistance." As for the transgenics, "every plant expressing allseason long, then obviously the insects have a hard time escaping it."These resistance-enhanced pests, he pointed out, are going to bemoving from the transgenic plants to other crops, "so from a practicalpoint of view are likely to create a major problem."He estimates roughly that Bt's present annual market worldwide isbetween $150 million and $200 million, only a fraction of the total saleof conventional, synthetic insecticides, but growing rapidly.Tabashnik said, "Now that Bt has been genetically engineered, its useis going to increase dramatically. It's projected to become a major partof the pest-control market." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.