Quantitative assay demonstrates continuous variation in aggregative tendency within and between pine sawfly species (genus Neodiprion)

Aggregative behavior is one of the most common social behaviors in the animal kingdom.  However, a comprehensive understanding of why some species aggregate and others do not, including closely related species, has not yet been achieved.  Because they vary in larval gregariousness and are experimentally tractable, pine sawflies in the genus Neodiprion (Hymenoptera: Diprionidae), a Holarctic group of approximately 50 species, provide an excellent model for understanding the evolution of feeding aggregations.  To date, larval behaviors have been described qualitatively, with species categorized as either gregarious or solitary.  However, our preliminary observations do not support this discrete categorization; instead, larval behavior appears to vary continuously from highly solitary to highly gregarious. To describe this variation, we developed a simple and effective assay to quantify the tendency for sawfly larvae to aggregate. We then applied this assay to: (1) different larval instars from a single population, (2) different populations from a single species, and (3) different species. Using this assay, we were able to detect significant differences within and between species that have all been classified as “gregarious”. When coupled with additional trait and environmental data and analyzed with phylogenetic comparative methods, these quantitative behavioral measures will enable us to test competing hypotheses about the evolution of feeding aggregations. More generally, our results to date suggest that this assay could be useful for quantifying differences in larval gregariousness for a wide range of species.