Prolonged Cross-Resistance
Insects that are resistant to
dichlorodiphenyltrichloroethane (DDT), which was banned in many parts of the world in the 1970's, have a genetic advantage over their rivals that has helped them spread across the globe ever since, according to recent research published in Current Biology. Researchers at the University of Bath in France have demonstrated that the resistance in fruit flies does not carry any ‘costs.’ Scientists had previously believed that the genetic ‘cost’ of resistance would mean that DDT resistance would dwindle once the pesticide was taken out of use and DDT-susceptible insects would regain dominance. The lead author believes previous work may not have looked at genetically related strains and that 'costs' may therefore be associated with the differing genetic backgrounds of insects examined, and not the resistance genes themselves. Using DDT-resistant fruit flies (Drosophila melanogaster) in state-of-the-art controlled temperature rooms, the researchers went to great lengths to make sure that DDT resistant and susceptible strains differed only by the resistance gene itself.
The World Health Organization estimates that during the period of DDT use, approximately 25 million human lives have been saved. Today, pyrethroids are most commonly used in mosquito control but they act on the same target in the nervous system as DDT, and ironically, spraying with DDT may therefore pre-select for resistance to pyrethroids. Use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. Resistant mosquitoes were first detected in India in 1959, and the resistance is so recalcitrant that when a local spray program is begun now, most mosquitoes become resistant in a matter of months rather than years. Worryingly, some resistant strains also show 'cross-resistance' to a number of different compounds, so spraying with one insecticide can unexpectedly increase resistance to newer compounds subsequently introduced to try and overcome resistance. (University of Bath, 8/5/05).
Editor’s note: This is why registrants should be required to determine mode of action before registration is complete. Some pesticides still have unknown modes of action.
