Natural Controls Urged as Insects, Weeds Rapidly Build Resistance to Pesticides

Insects are building resistance to pesticides at such a furious clip that chemicals will become obsolete unless farmers adopt new tactics for pest control, according to a new study.

The National Research Council, the research arm of the National Academy of Sciences, suggests that chemical pesticides be used only in alternate years and that farmers return to such old-fashioned methods as introducing natural predators into their fields.

“What we need to realize is that we can no longer depend totally on one type of control,” said Homer M. LeBaron, a member of the council’s pesticide committee and a researcher at Ciba-Geigy Corp. in Greensboro, N. C. “We have to educate everyone not to consider chemicals (to be) the only means of control.”

Problem Escalating

The problem of insects’ and weeds’ building natural resistance to pesticides is escalating, members of the committee said in interviews.

Before World War II, seven species of insects and mites were known to be resistant to insecticides. Today, that number has climbed to 447 species, and effective chemical controls no longer exist for such major agricultural pests as the Colorado potato beetle and the diamondback moth.

“The problem of herbicides is not as great as the problem of insecticides,” committee member Bruce R. Levin, a professor of zoology at the University of Massachusetts at Amherst, said.

Some studies suggest that farmers lose the same proportion of their crops to pests as they did before the advent of chemicals, according to committee members.

Pathogens Also Resistant

The resistance is not confined to insects. Rodents, weeds, fungi, parasites and pathogens (any microorganisms or viruses that can cause disease) also have become impervious to toxic chemicals.

According to the report, at least 100 plant pathogens have developed some resistance to pesticides, as have 55 species of weeds, two species of parasitic worms and five species of rodents.

“This would indicate we are losing the race, we are approaching the end of chemical control,” Levin said. “In the 40 years since pesticides first made their appearance, we’ve established a situation for evolution to operate. Forty years represents a lot of generations when it comes to insects.”

The resistance that pests build to chemical pesticides can be explained by Darwinian selection, the committee members said. Some pests have a natural ability to produce an enzyme that detoxifies chemicals, they said. Those pests survive the chemical spray, and their offspring inherit the trait.

Within a few generations, the pests with the inherited ability to detoxify the chemicals become abundant enough and hardy enough to limit the effectiveness of the pesticide. Within decades, the whole population of insects can have the ability to render pesticides useless.

“Anytime you rely on one means of pest control, you pressure that pest to build up a resistance, to find a way around that control element,” LeBaron said.

Sparing Use Urged

The council committee members believe it may be possible to stave off insect resistance to chemicals by using the pesticides only sparingly. This would mean that insects capable of detoxifying the chemicals would be killed by other means, preventing their offspring from multiplying.

“That second method of pest control will eliminate those special insects and break the chain,” LeBaron said. “That second generation able to detoxify the chemicals is really very weak. They are susceptible to natural predators. They wouldn’t be able to withstand a crop rotation.

“Once they are gotten rid of, pesticides can be used effectively again,” he said.