Population density affects propensity to share gall space in a primitively social aphid

In the vast majority of gall insects, solitary establishment of a gall is the rule, but the primitively social aphid Tamalia coweni exhibits communal gall occupation under conditions of increased competition for gall sites. Here we tested the hypothesis that gall-sharing rates are a function of variable population densities in galls and on leaves of the host plant. Current results, observed under natural density and temporal variation, demonstrate that population density is a strong predictor of co-occupation rates. Although basal leaf positions were more frequently chosen as gall-induction sites than distal ones, there was no evidence these sites were more likely to be co-occupied. In addition to spatial constraints, temporal constraints may be important in mediating primitive sociality in T. coweni.