The immense diversity of life on Earth has been attributed to the partitioning of available resources into ecological niches, but it is not obvious what determines the niche size of each species. For example, most plant-feeding insects consume only one or a few closely-related host-plant species despite the advantages of having a broader diet. Many researchers have therefore suggested that the evolution of broad diets in plant-feeding insects must be constrained by genetic trade-offs between adaptations to alternative host-plants. Despite its intuitive feel, however, little empirical evidence in support of the trade-off hypothesis has emerged from decades of experimental studies comparing individual performance on alternative hosts within insect populations.
Here I use a broader approach to evaluate the role of trade-offs in driving ecological specialization in plant-feeding insects. By collecting host-use data for thousands of insect species and fitting those data into long-term evolutionary models, I investigate whether trade-off constraints have left observable signatures in the present ecological niches of existing species. Chapter 1 focuses on a single family of insects, the armored scales (Hempitera: Diaspididae), revealing that positive correlations between evolutionary changes in host performance best fit the observed patterns of diaspidid presence and absence on nearly all focal host taxa, suggesting that adaptations to particular hosts enhance rather than reduce performance on other hosts. In chapter 2, I uncovered a complex network of evolutionary interactions between caterpillar adaptations to eleven host-plant orders, indicating that different host-use trade-offs act over long- and short-term evolutionary timescales. In contrast, host-use patterns of true bugs revealed a total lack of trade-offs for the same host-plant orders over both timescales. Chapter 3 turns to armored scale insects again, this time those that we collected in systematic surveys across a large diversity of trees in two tropical rainforest habitats. Using each insect species’ abundance on each tree as a proxy for host-plant performance, we found no evidence for performance trade-offs on alternative hosts despite apparent host-use specialization. Overall, these results suggest that the extreme specialization of plant-feeding insects arises from long-term, potentially nonadaptive evolutionary processes rather than simple genetic trade-offs
