An investigation of communal gall sharing by Tamalia coweni under variable conditions of population density on Arctostaphylos manzanita (Ericaceae)

Tamalia coweni is unusual among gall-forming aphids in that it will often share its gall space with other members of its species, even as this has been shown to lower its fitness. Previous research utilizing manipulated densities of T. coweni on inflorescences of its host plant has indicated that population density is a strong predictor of gall-sharing rates, while kinship appears to be less predictive. The current study expands on this finding by investigating gall-sharing rates under natural population density conditions in leaf galls. In addition, it does an initial examination of whether gall-sharing correlates with preferential or optimal galling sites, and in line with the preference-performance aspect of the Plant Vigor Hypothesis (PVH). Cofounding rates were found to strongly align with those from studies using artificial aphid densities, and population density was shown to be a strong predictor of cofounding rates in a natural setting. Although strong patterns showing basal leaf positions were more frequently chosen as galling sites than distal ones, there was no evidence these sites were more likely to be cofounded. Over all, gall and cofounding distributions did not show a strong alignment with the PVH. Instead, other factors, notably timing, may offer more promising explanations, and offer a rich set of questions for further examination.