Colony size of pine sawflies (genus Neodiprion) is driven by female oviposition behavior rather than larval aggregative tendency

Aggregative behavior is one of the most common social behaviors in the animal kingdom.  However, we still lack a comprehensive understanding of why some species aggregate and others do not.  Because they vary in larval gregariousness and are experimentally tractable, pine sawflies in the genus Neodiprion (Hymenoptera: Diprionidae), provide an excellent model for understanding the proximate and ultimate mechanisms underlying feeding aggregations.  Historically, larval behavior has been described qualitatively, with species categorized as either gregarious or solitary, presumably on the basis of typical wild colony size.  However, the extent to which this categorization is driven by larval or adult female behavior is unknown.  To address this question, we quantified larval aggregative tendency for 19 species from the monophyletic eastern North American Neodiprion clade. For each species, we also compiled data on colony size and female oviposition behavior from the literature, field observations, and laboratory assays. We then used comparative methods to assess correlations between colony size and larval or adult female behavior. These data revealed substantial interspecific variation in larval aggregative tendency that did not correspond with published solitary/gregarious designations for these species. Moreover, comparative analyses suggested that variation in colony size is driven not by larval group cohesion, but by the tendency of ovipositing females to cluster eggs or not.  In addition to revealing the proximate basis of larval feeding aggregations, these data set the stage for future comparative work that will test competing hypotheses regarding their ecological function.