The effects of changes in region-wide cropping patterns on insect pest populations have not been well studied. This paper reviews the empirical literature and develops a theoretical framework for these effects. Changes in regional cropping patterns do affect insect populations, and in at least one case, the presence of fence rows and thickets provide a net benefit to the nearby agro-ecosystem. Of the 17 studies reviewed here, 15 discuss cotton or wheat. With cotton, increased monoculture leads to increases in 13 pest populations and a decrease in two. With wheat, increases occurred in only three populations, while seven were not affected or decreased. Most of the insect populations that were not affected or decreased are multivoltine and oligophagous or polyphagous. All of the populations that increased are monophagous on the crop in the region in which they were studied. A higher proportion of the wheat fauna is multivoltine and oligophagous or polyphagous than the cotton fauna. This may explain the difference between the response of the wheat fauna and the cotton fauna to changes in the extent of monoculture. Additional factors may be insect vagility, host-finding ability, and the response of the predators and parasitoids.