pests will survive. As a result, the proportion of resistant individuals in the population
increases. Repeated selection of resistant pests will ensure that every succeeding gener-
ation will have a higher proportion of resistant individuals than the original popula-
tion. Eventually, after repeated and more intensive use of the same pesticide to the
same pest population, the pesticide becomes ineffective. Unfortunately, even under
ideal conditions, pests can become resistant to pesticides.
Cross-Resistance
Cross-resistance may occur where a pest develops resistance to two or more pesti-
cides after exposure to just one. For example, resistance to dieldrin confers cross-
resistance to other related compounds such as chlordane and heptachlor.
In the last decade, the number of weed species known to be resistant to herbicides
rose from forty-eight to 270, and the number of plant diseases resistant to fungicides
grew from 100 to 150. Resistance to insecticides is so common—more than 500
species—that nobody is really keeping score. Unfortunately, pesticides often kill off
pests’ natural enemies along with them. With their natural enemies eliminated, there is
little to prevent recovered pest populations from exploding to larger, more damaging
numbers than existed before pesticides were applied. Additional chemical pesticide
treatments only repeat this cycle.
Secondary Pests
Some potential pests that are normally kept under firm control by their natural
enemies become actual pests after their natural enemies are killed by pesticides. Mite
outbreaks after pesticide applications are a classic example.
Adding to this scenario the intractable problem of pesticide resistance by insects
and disease organisms, it becomes clear why so much attention is being paid to the
development and adoption of pest management approaches aimed at reduction in
pesticide use. There is one basic rule with pesticides in relation to resistance:avoid
unnecessary use. If resistance does occur the simplest procedure is to use an alternative
registered pesticide to which there is no cross-resistance. Proven strategies either to
prevent or delay the development of resistance do not exist at present, either for
plant disease agents or insect pests. Theoretically, the rotation of different chemical
groups should delay resistance, but long-term experimentation under practical condi-
tions is almost impossible to carry out. The problems of implementation are formi-
dable.^44 As of 1999, pest resistance to pesticides was estimated to cost U.S.
agriculture about $1.5 billion per year in increased pesticide costs and decreased crop
yields. On average developing a new pesticide costs $80 million, while a pest typi-
cally develops resistance in only ten to twenty-five years, after which the pesticide’s
utility decreases.
Continued dependence on chemicals as the sole method of pest control is a sure
recipe for the ongoing development of pest resistance.^45
Pesticides in Agriculture | 47