A virus that kills insect pests can't be all bad. Nor can a deadlyscorpion, whose venom is making that good virus even better.The baculoviruses infect only insects, and one of their species,the nuclear polyhedrosis virus (NPV), attacks the larvae of mothsin particular. That specificity qualifies NPV as "organic" pesticides.But by the time they get in their virulent licks on the likes of thealfalfa looper (Autographa californica) or the cabbage looper(Trichoplusia ni), these voracious caterpillars have munched theirway through a lot of leaves.To improve the breed of NPV as a potential crop saver, entomologistJennifer Cory has cloned a subunit gene from the venom of Africanscorpions (Androctonus australis) into a NPV genome, and speededup the larvae-killing process. Her paper in Thursday's Natureis titled "Field trial of a genetically improved baculovirus insecticide."Cory directs the Ecology and Biocontrol project at Oxford University'sInstitute of Virology and Environmental Microbiology. "The virus,an off-the-shelf item isolated from the alfalfa looper, gets intothe insects by the normal infective route," she explained."The scorpion toxin acts by altering sodium-channel conductance tothe larval axons, which leads to paralysis and eventual death."
Scientists Tinker With ToxinAs scorpions live largely on insect prey, which they sting to death,the selectivity of their venom to insects comes naturally. Cory andher team derived a synthetic DNA sequence from the native toxin,added a promoter, and inserted it into a NPV that targets brassica,of which cabbage is an economically prime example.Last August, at the university's experimental farm, the team planted20 cabbage seedlings in 16 one-square-meter plots. Four weeks later,they released 500 cabbage-looper larvae onto the growing plants ineach plot. (Loopers are so-called because they describe loopswhile crawling.)Then they sprayed the leaves selectively with low, medium and highconcentrations of recombinant and wild-type NPV. Final damageassessment, 10 days after spraying, revealed "the key result wasthat we actually got a significant decrease in damage withthe engineered virus in comparison with the wild-type virus," Cory said.Untreated larva reduced leaf area by 22 percent, nearly twice the12.5-percent damage done by virus-infection, wild or recombinant.The latter scored 29 percent less leaf loss at high doses ofvenom-altered NPV. These larvae died 10 to 15 percent earlier than controlinsects infected with the wild-type virus."Toxin expression by the recombinant virus," Nature reported, "induceslarval paralysis in the field. This resulted in many larvae fallingoff the plants before death." Those killed, at a slower rate, bywild-type virus, decomposed on the leaves, thus releasing more virus,whereas the recombinant-felled ones failed to perpetuate the viral spread.This reduced secondary infection, Cory's paper observed, "if confirmed,could have important implications for risk assessment."Her institute is now carrying out risk-assessment studies, she said,"rather than going the immediate commercial route, because nobody hasever done this sort of release before." Back at their square-meterexperimental plots, they have already completed one environmental-risktrial, asking such questions as, "How persistent are the geneticallymodified viruses? How competitive are they, compared to the wild-type?Can they infect non-target hosts in the wild?" n
-- David N. Leff Science Editor
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