Plagues of caterpillars and other plant-feeding insects tend to occur where trees and other plants grow as man-made monocultures. Herbivorous insects rarely cause extensive damage to mature trees in tropical forests; but even in the tropics, outbreaks are frequent wherever trees of the same species are grown together—as for example along roadways and in built-up areas. In Africa, in particular, peasant cultivators growing their own food tend to plant a variety of species in the same plot, and it is suggested that these people have found by trial and error that polycultures are more resistant to insect damage than are monocultures. English flower-gardeners maintain polycultures in their herbaceous borders and are rarely troubled by insects, but vegetables are now grown as pure stands. This probably leads to a build-up of pests and a need to use pesticides, which of course is encouraged by the gardening and pesticide industries.

Although there remains considerable controversy over the precise relationship between species diversity and population stability, there is much circumstantial and some direct evidence that plagues of plant-feeding insects are much more likely in pure stands of vegetation than in diverse communities. What might be called the community ecology of peasant cultivation is little understood, and there is an urgent need for research before more and more land is converted to monocultures.