The Environmental and Social Costs of Improvement 21natural enemies that control pests. They can produce new pests, by killing off the
natural enemies of species which hitherto were not pests. And they can induce
resistance in pests to pesticides.
Resistance can develop in a pest population if some individuals possess genes
which give them a behavioural, biochemical or physiological resistance mechanism
to one or more pesticides. These individuals survive applications of the pesticide,
passing their genes to their offspring so that with repeated applications the whole
population becomes resistant. High and frequent applications of pesticides exert
the greatest selection pressure on populations. Resistance has now developed in all
insecticide groups and at least 480 species of insect, mite or tick have been recorded
as resistant to one or more compounds (Georghiou, 1986). Resistance has also
developed in weeds and pathogens. Before 1970, few weeds were resistant to her-
bicides but now at least 113 withstand one or more products. Some 150 fungi and
bacteria are also resistant (WRI, 1994).
Unfortunately, natural enemies appear to evolve resistance to pesticides more
slowly than herbivores, mainly because of the smaller size of the natural enemy
populations relative to pests and their different evolutionary history (Risch, 1987).
The co-evolution of many herbivores with host plants that contain toxic secondary
compounds means they have metabolic pathways easily adjusted to produce resist-
ance (Croft and Strickler, 1983). In Sudan, the increasing application of pesticides
to cotton over the past 50 years has steadily reduced the number of predator spe-
cies. One insect, the whitefly, which was formerly kept in check by predators, is
now an economically very important pest (Kiss and Meerman, 1991; PT, 1990).
Outbreaks and resurgences are more likely to occur when the landscape has
been simplified to contain just a single crop. This may be of cereals, such as wheat
or rice, or of plantation crops, such as bananas, cotton or coffee. In Costa Rica, 30
per cent of imported pesticides are used in the production of bananas for export.
Bananas are grown in huge plantations, which are highly susceptible to pests and
diseases, and there have been repeated cycles since the 1950s of heavy applications
of one product, closely followed by pest outbreaks caused by the rapid develop-
ment of resistance (Thrupp, 1990). When decisions were taken to stop spraying
because of inefficiency and growing costs, insect pests rapidly declined: ‘two years
after insecticides were halted, the previous [predator] species became established
again’ (C Stephens in Thrupp, 1990). Today’s integrated approach to pest manage-
ment requires greater technical expertise and labour of managers and operators,
and now incorporates cultural methods, minimal and selective use of insecticides,
and threshold monitoring. Insects rarely now present any problems, though spray-
ing against disease, nematodes and weeds is still heavy.
In Asia, where 90 per cent of the world’s rice is produced and consumed,
reports of disease and insect outbreaks are numerous (Khush, 1990; Kenmore,
1991; Winarto, 1994). Brown planthopper (Nilaparvata lugens, BPH) outbreaks
have at various times destroyed hundreds of thousands of ha of rice in countries
from India in the west to the Solomon Islands in the east. In Indonesia, the first
problems started occurring in 1974. Losses jumped in 1975, after the government