Evolution of resistance to pesticides

Chemical pesticides lose their role in sustainable agriculture if the pests evolve resistance. The evolution of pesticide resistance is simply natural selection in action. It is almost cer tain to occur when vast numbers of individuals in a genetically variable population are killed in a systematic way by the pesticide. One or a few individuals may be unusually resistant (perhaps because they possess an enzyme that can detoxify the pesticide). If the pesticide is applied repeatedly, each successive generation of the pest will contain a larger proportion of resistant individuals. Pests typically have a high intrinsic rate of reproduction, and so a few individuals in one generation may give rise to hundreds or thousands in the next, and resistance spreads very rapidly in a population.

This problem was often ignored in the past, even though the first case of DDT resistance was reported as early as 1946

evolved resistance: a widespread problem

4 Insects and mites □ Plant pathogens 1 Weeds

T TTTT

1930

1940

1950

1960

1970

1980

1990

Year

(in house-flies, Musca domestica, in Sweden). The scale of the problem is illustrated in Figure 15.4, which shows the exponential increases in the number of invertebrates, weeds and plant pathogens resistant to insecticides. The cotton pest study described earlier also provides evidence of the evolution of resistance to a pesticide (see Figure 15.2d). Even rodents and rabbits (Oryctolagus cuniculus) have evolved resistance to certain pesticides (Twigg et al., 2002).

The evolution of pesticide resistance can be slowed, though, by changing from one pesticide to another, in a repeated sequence that is rapid enough that resistance does not have time to emerge (Roush & McKenzie, 1987). River blindness, a devastating disease that has now been effectively eradicated over large areas of Africa, is transmitted by the biting blackfly Simulium damnosum, whose larvae live in rivers. A massive helicopter pesticide spraying effort in several African countries (50,000 km of river were being treated weekly by 1999; Yameogo et al., 2001) began with Temephos, but resistance appeared within 5 years (Table 15.2). Temephos was then replaced by another organophosphate, Chlorphoxim, but resistance rapidly evolved to this too. The strategy of using a range of pesticides on a rotational basis has prevented further evolution of resistance and by 1994 there were few populations that were still resistant to Temephos (Davies, 1994).

If chemical pesticides brought nothing but problems, however - if their use was intrinsically and acutely unsustainable - then they would already have fallen out of widespread use. This has not happened. Instead, their rate of production has increased rapidly. The ratio of cost to benefit for the individual producer has generally remained in favor of pesticide use. Moreover, in many poorer countries, the prospect of imminent mass starvation, or

Figure 15.4 The increase in the number of arthropod (insects and mites), plant pathogens and weed species reported to be resistant to at least one pesticide. (After Gould, 1991.)

Table 15.2 History of pesticide use against the aquatic larvae of blackflies, the vectors of river blindness in Africa. After early concentration on Temephos and Chlorphoxim, to which the insects became resistant, pesticides were used on a rotational basis to prevent the evolution of resistance. (After Davies, 1994.)

Table 15.2 History of pesticide use against the aquatic larvae of blackflies, the vectors of river blindness in Africa. After early concentration on Temephos and Chlorphoxim, to which the insects became resistant, pesticides were used on a rotational basis to prevent the evolution of resistance. (After Davies, 1994.)

Name of pesticide

Class of chemical

History of use

Temephos

Organophosphate

1975 to present

Chlorphoxim

Organophosphate

1980-90

Bacillus thuringiensis H14

Biological insecticide

1980 to present

Permethrin

Pyrethroid

1985 to present

Carbosulfan

Carbamate

1985 to present

Pyraclofos

Organic phosphate

1991 to present

Phoxim

Organophosphate

1991 to present

Etofenprox

Pyrethroid

1994 to present

of an epidemic disease, are so frightening that the social and health costs of using pesticides have to be ignored. In general the use of pesticides is justified by objective measures such as 'lives saved', 'economic efficiency of food production' and 'total food produced'. In these very fundamental senses, their use may be described as sustainable. In practice, sustainability depends on continually developing new pesticides that keep at least one step ahead of the pests: pesticides that are less persistent, biodegradable and more accurately targeted at the pests.

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  • grimalda
    Why is the evolution of resistance to insecticides so rapid?
    1 year ago

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