Generalising about generalists? A perspective on the role of pattern and process in investigating herbivorous insects that use multiple host species

Generalist insect herbivores, those recorded as using numerous hosts (tens or even hundreds of species), are not well understood ecologically. We suggest ways to investigate the ecology of these species beyond the practice of accumulating host records. We present reasons why multiple host use by herbivorous insects needs to be quantified in the field, both locally and geographically, and also through time. Further, the host use patterns generated must be based on certain knowledge that cryptic species are not conflated in the results as one species. Such results reveal a lot about the ecology of the species concerned and about generalist–host relationships more widely. This provides a sound basis for the functional significance of multiple host use to be interpreted and tested further. Structured sampling programs conducted previously in the field indicate that generalist insect herbivores are strongly associated with only a relatively small subset of their recorded host species, their primary host plants. A focus on these particular host plants is fundamental to (i) understanding the ecology of the herbivore species in question and (ii) investigating the sensory and behavioural mechanisms associated with host location. We conclude that the principal influence on patterns of multiple host use in the field is the mechanism by which herbivorous insects recognise cues from potential hosts and use them to localise plants. These mechanisms represent species-specific and species-wide adaptations to the usual environment of the species in question. Working from this perspective should contribute substantially to developing a strong and realistic interpretation of the origins and functional significance of multiple host use in herbivorous arthropods.