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Table 1. Introduction of pesticides in the tropics
for common agricultural use.
Family Year of introduction
Organochlorines 1940
Organophosphates 1960
Herbicides 1960
Carbamates 1970
Pyrethroids 1975
Bacillus thuringiensis 1980
Source: World Resources (1986).
treated with fungicides compared with control plots (Zweig and Aspelin
1983). The use of pesticides can increase rice yield by 50% compared to an
untreated test plot (Zweig and Aspelin 1983), and treating tomatoes with
pesticides increased production by 141% compared with untreated plants
(Decker 1974). Similar treatment with fertilizers alone increased the yield by
18 1%, while combining fertilizers with pesticides increased the yield by 253%
(McNews 1967).
The common wisdom is that pests (insects, weeds, and pathogens) are responsible
for a loss of one-third of preharvest yields (Cramer 1967). Surprisingly,
this ratio changed little between 1942-45 (31.4%) and 1974 (33.0%) when
pesticide use was common (Fig. 1). In fact, losses due to insects increased
steadily from 7.1% to 13.0% during this period. The loss of cotton crops to
insects increased from 10% in 1900 to 19% in the 1950s. For maize, the loss
increased from 8% to 12% over the same period (Plucknett and Smith 1986).
In 1986, the percentage of all US crops lost to insects was twice that of 1940
(Metcalf 1980). This has led many to conclude that crop damage from insect
infestation has not decreased as a result of pesticide usage, at least in the USA,
and that it has actually increased (Pimentel 1976; Odhiambo 1984; Plucknett
and Smith 1986).
Overall, pesticides have had a positive effect on crop yields that is not offset
by the poor record of certain insecticides. Herbicides, for example, have
become central to many types of agriculture and their use has been instrumental
in decreasing preharvest losses in the USA from 13.8% in 1942-51 to 8% in
1974 (Cramer 1967). Headly (1968) estimated that each additional $1 spent on
pesticides in 1968 would increase the value of protected crops by $4. However,
he stressed that these values were based on field trials and did not account for
geographic, climatic, and other real-life variables such as variations in pest
infestations.
Modern, high-yield cultivars depend on higher levels of inputs that must be
applied at the right time. However, the monocultures that are now the mainstay
of world agricultural production have made this system unstable. This
may explain why the use of pesticides has not increased world food production
to the levels predicted in experimental trials.
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