Tamalia gall aphid parasites are more social than their gall-inducing host aphids

Tamalia gall aphids (Hemiptera: Aphididae) include species inducing galls on host plants in the Ericaceae, as well as a closely-related taxon acting as inquilines within galls caused by the gall-inducing aphids. I have examined data on primitive sociality in these aphids from a long-term study of a population in the Sierra-Cascade foothills. Data from 2003-2024 indicate that, when Tamalia inquilines co-occupy galls with gall-inducers, they do so in larger groups than do the gall-inducing Tamalia host aphids. One hypothesis explaining this is that the inquilines are limited by a narrower window of opportunity to invade galls, with the result that multiple inquiline females enter the host gall simultaneously during the initial stages of gall invasion. An alternative hypothesis may be described as “generation packing,” such that overlapping generations are produced within the gall, resulting in multiple apterous females co-occupying galls. Here I present qualitative evidence to evaluate these hypotheses separately.