Page 274 - Environment: The Science Behind the Stories

We have developed thousands insects, weeds, and microbes can occur in huge numbers, it is
of chemical pesticides likely that a small fraction of individuals may by chance already
have genes that enable them to metabolize and detoxify a given
The highly modified ecosystems of industrial farming limit pesticide (p. 389). These individuals will survive exposure to the
the ability of natural mechanisms to control pest populations, pesticide, while individuals without these genes will not.
so farmers need to introduce some type of pest control in order Let’s say an insecticide application kills 99.99% of the
to produce food economically on an industrial scale. In the insects in a field. That sounds successful, but it means that 1 in
past half-century, most farmers have turned to chemicals to 10,000 survives. If an insect that is genetically resistant to an
suppress pests and weeds. In that time we have developed insecticide survives and mates with other resistant individuals,
thousands of chemicals to kill insects (insecticides), plants the genes for insecticide resistance will be passed on to their
(herbicides), and fungi (fungicides). Such poisons are collec- offspring. As resistant individuals become more prevalent in
tively termed pesticides. the pest population, insecticide applications will cease to be
All told, roughly 400 million kg (900 million lb) of active effective and the population will grow (Figure 10.18).
ingredients from conventional pesticides are applied in the In many cases, industrial chemists are caught up in an
United States each year. Three-quarters of this total is applied evolutionary arms race (p. 97) with the pests they battle, rac-
on agricultural land. Since 1960, pesticide use has risen fourfold ing to intensify or retarget the toxicity of their chemicals while
worldwide. Usage in industrialized nations has leveled off in the the armies of pests evolve ever-stronger resistance to their
past two decades, but it continues to rise in the developing world. efforts. Because we seem to be stuck in this cyclical process,
Today more than $32 billion is expended annually on pesticides, it has been nicknamed the “pesticide treadmill.” As of 2011,
with one-third of that total spent in the United States. Exposure among arthropods (insects and their relatives) alone, there
to synthetic pesticides can have health consequences for people were more than 10,000 known cases of resistance by 586 spe-
and other organisms under some circumstances (Chapter 14), so cies to over 330 insecticides. Hundreds more weed species
their use in food production can have far-reaching effects. and plant diseases have evolved resistance to herbicides and
other pesticides. Many species, including insects such as the
Pests evolve resistance to pesticides green peach aphid, Colorado potato beetle, and diamondback
moth, have evolved resistance to multiple chemicals.
Despite the toxicity of chemical pesticides, their effectiveness An additional problem is that pesticides often kill non-
tends to decline with time as pests evolve resistance to them. target organisms, including the predators and parasites of the
Recall from our discussion of natural selection (pp. 68–71) that pests. When these valuable natural enemies are eliminated,
organisms within populations vary in their traits. Because most pest populations become harder to control.

1 Pests attack crops 2 Pesticide is applied 3 Most pests are killed. A few CHAPTER 10 • A g R i C ulT u RE , Bi o TECH nology, A nd THE Fu T u RE o F Food
with innate resistance survive

4 Survivors breed and produce a 5 Pesticide is applied again 6 Pesticide has little effect.
pesticide-resistant population New, more toxic, pesticides
are developed
Figure 10.18 Through the process of natural selection, crop pests often evolve resistance
to the poisons we apply to kill them. 273

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