As with synthetic insecticides, natural selection can throw up a strain of resistant insects.

The idea is to keep susceptible individuals alive and prevent the population from being dominated by resistant insects.

Dr. Croft explained that the surviving resistant insects go on to reproduce.

The number of insects resistant to cyclodienes and lindane approached 300 by 1989.

Farmers, in other words, will have to sacrifice some of their crops if they don't want resistant insects.

Some studies have shown that continuous exposure to Bt quickly creates resistant insects, thus destroying the effectiveness of the pesticide.

This strategy also increases the selection pressure for genetically resistant insects - hastening the inevitable switch to a newer, possibly more dangerous, and certainly more expensive insecticide.

Is Monsanto going to be held legally responsible when one of its transgenes creates a superweed or resistant insect?

The tests work by detecting enzymes present in the resistant insects' nervous systems.

But as the history of DDT showed, heavy pesticide use actually encourages the selective breeding of the most resistant insects.