Studies Find How Bugs Beat Leading Safe Pesticide

Will Dunham / Reuters 3aug01

Researchers have discovered how some pests including a type of a roundworm and a type of a moth develop resistance to a leading environmentally friendly called Bt pesticide. Resistant roundworms fed Bt toxin show no damage to internal structures (top) unlike the susceptible form (bottom). (/Joel Griffitts/UCSD via Reuters )

WASHINGTON  -- Researchers have discovered how some pests develop resistance to a leading environmentally friendly pesticide in a finding that could inspire ways to keep bugs in check without resorting to synthetic chemicals.

Two separate research teams, whose findings appear in Friday's issue of the journal Science, pinpointed the genetic mechanisms used by the tobacco budworm moth and a type of roundworm to elude the toxic effects of the most widely used biologically produced pesticide.

The Bt pesticide, made of toxins created naturally by the bacterium Bacillus thuringiensis, is used widely by organic farmers as an alternative to potentially dangerous chemical pesticides. It does not harm people or any vertebrate animals, but inflicts intestinal damage on key crop pests.

Crops such as cotton and corn that have been genetically engineered to produce Bt insecticidal proteins to control caterpillars and beetles have been planted for the past five years. But this practice has prompted worry that extensive use of these altered crops could enable pests rapidly to develop resistance to Bt toxins, leaving farmers without a reliable organic pest-control agent.

Bt toxins also have played an important role in Africa in controlling insects that carry disease.

Insects and worms adapt to Bt just as they do to man-made chemical pesticides. In fact, a type of moth that wreaks havoc on cabbage crops no longer is harmed by it.

``By actually understanding how resistance takes place, we can develop strategies to combat it,'' biologist Raffi Aroian of the University of California at San Diego, who led one of the research teams, said in an interview. ``But the first step toward that is understanding at the molecular level how this happens.''

TARGETING A MOTH AND A WORM

Researchers from Clemson University in South Carolina, North Carolina State University and the University of Melbourne in Australia figured out how the tobacco budworm moth beats the pesticide. They identified a gene that confers high levels of Bt resistance in the moth.

The tobacco budworm moth feeds on a variety of crops and makes cotton a host plant for its larvae. It has developed resistance to most conventional chemical insecticides. The researchers examined a laboratory strain of the moth that resists a Bt toxin present in a genetically modified cotton produced by Monsanto Co.

``This is a first step to understanding the development of Bt resistance by insect pests,'' said Clemson biologist Linda Gahan, who led the study. ``This discovery will open the door for other researchers and monitoring governmental agencies to assess resistance mechanisms and management strategies in insect pests.''

Aroian's team examined mutant genes they discovered in the roundworm Caenorhabditis elegans that confer resistance to a particular Bt toxin. The researchers cloned one of these five mutant genes, then compared differences in the proteins produced by the mutant gene and the corresponding normal gene.

This comparison allowed the researchers to determine that the roundworm's Bt toxin resistance stemmed from the loss of an enzyme that adds carbohydrates to proteins and fats.

The findings by both teams could pave the way for DNA tests that could be used to detect emerging resistance in pests.

Aroian emphasized the importance of developing ways to keep Bt pesticides useful.

``They have very minimal environmental impact while providing good protection against a given insect pest that you might be looking to kill,'' Aroian said.

Aroian labeled as ``fantastic'' the advent of crops genetically engineered to produce a Bt toxin.

``The farmer does not have to spray his field with as many chemical pesticides,'' Aroian said. ``The people working the field aren't going to be exposed to those pesticides. You and I as consumers aren't going to have to deal with those chemical pesticides.''