What makes a generalist? Defining the level of generalism of an extreme generalist herbivore (fall webworm, Hyphantria cunea, Erebidae)

Over 90% of insect herbivores are considered specialists. Even though host plant use and diet breadth of insect herbivores have been studied for decades, it is still unclear why there are so many specialists and so few generalists.  Traditionally we use specialists to study dietary specialization, but this prevents us from understanding what causes generalists to specialize or maintain generalism. Furthermore, there is often an implicit assumption that each individual from generalist species has a broad diet range. However, generalist species may simply be populations of specialized individuals, with few individuals considered true generalists at the population level. If that is the case, predictions about generalists’ ecology and evolution should reflect individual specialization. Thus, the first step to understanding diet evolution is to properly define the level of generalism of generalist species.

To define the level of generalism of the fall webworm, an herbivore considered an extreme generalist at the species level, we conducted a performance experiment with populations across a large geographic range. In Colorado, Massachusetts, and Maryland, we split maternal lines onto four different host plant species. We measured the performance as survival rate, pupal mass, and development time from egg to pupae. By comparing the variation in performance within and across maternal lines, we will tease apart if generalism occurs at the individual or population level. Results from this experiment will inform us about the classification of diet breadth of insect herbivores, which has important implications for their ecology and evolution.