Read more: 鈥�Unnatural selection: How humans are driving evolution鈥�
Had any strange itchy bites or rashes recently? You might have fallen victim to bedbugs. The little bloodsuckers are back in a big way, thanks in part to the fact that, like head lice and human fleas, to many common pesticides.
Whatever their drawbacks, there is no doubt that pesticides have made a huge difference to our lives. They have helped eliminate diseases like malaria from some areas and made possible the switch to intensive farming. As soon as we started using them, though, resistance began to evolve.
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鈥淚nsects that succumb readily to kerosene in the Atlantic states defy it absolutely in Colorado [and] washes that easily destroy the San Jos茅 scale [insect] in California are ridiculously ineffective in the Atlantic states,鈥� wrote entomologist John Smith in 1897 鈥� the first known report of insecticide resistance.
The use of synthetic pesticides like DDT took off in the 1940s. Resistant houseflies were discovered in 1946. By 1948, resistance had been reported in 12 insect species. In 1966, James Crow of the University of Wisconsin-Madison reported that the count had exceeded 165 species. 鈥淣o more convincing examples of Darwinian evolution could be found than those provided by the development of resistance in one species after another,鈥� he at the time.
It鈥檚 not just bugs. Rats and mice around the world have become resistant to the poison warfarin, and in Europe some have even become resistant to warfarin鈥檚 replacement, superwarfarin (). In Australia, meanwhile, rabbits are becoming resistant to the poison used to control their numbers, called Compound 1080.
Because of its economic importance, pesticide resistance has been studied far more closely than other kinds of ongoing evolution. In many cases we know which mutations are involved, how they make organisms resistant and sometimes even how the mutations spread through populations.
Resistance often arises due to mutations that alter the shape of proteins and thus prevent insecticides binding to their targets. For instance, DDT and pyrethroid compounds kill insects by opening sodium ion channels in nerve cells, but in the malaria-carrying mosquito Anopheles gambiae, variants of the channels that cannot be opened this way have evolved on at least four separate occasions ().
The other main mechanism of resistance involves enzymes that inactivate pesticides before they can kill. Some resistant strains of A. gambiae, for instance, of an enzyme called CYP6Z1 that can inactivate DDT.
Pesticide resistance is becoming such a serious problem that strategies for preventing it evolving in the first place are taken increasingly seriously. One approach is to alternate the type of pesticide applied, to try to avoid applying sustained selective pressure in one direction.
At present, though, the pests seem to be evolving faster than we can develop new pesticides. In one region of Burkina Faso, A. gambiae to all four classes of insecticides used for malaria control.
Read next article: 鈥�Unnatural selection: Introducing invaders鈥�
