What We Don't Know About Diet-Breadth Evolution in Herbivorous Insects

Half a million species of herbivorous insects have been described. Most of them are diet specialists, using only a few plant species as hosts. Biologists suspect that their specificity is key to their diversity. But why do herbivorous insects tend to be diet specialists? In this review, we catalog a broad range of explanations. We review the evidence for each and suggest lines of research to obtain the evidence we lack. We then draw attention to a second major question, namely how changes in diet breadth affect the rest of a species’ biology. In particular, we know little about how changes in diet breadth feed back on genetic architecture, the population genetic environment, and other aspects of a species’ ecology. Knowing more about how generalists and specialists differ should go a long way toward sorting out potential explanations of specificity, and yield a deeper understanding of herbivorous insect diversity.