The Manzanita Leaf-gall Aphid, Tamalia coweni (Cockerell) (Homoptera: Aphididae) as a Model System for Studies in Elementary Social Behavior, Sex Allocation and Life History Evolution

Abstract

DOCTORATE DISSERTATION: I present life history details for a California population of the gall-forming aphid, Tamalia coweni, on its host plants Arctostaphylos patula and Arctostaphylos viscida (Ericaceae). I document communal gall occupation by multiple aphid foundresses and place it into a sociobiological context. Relatedness among experimental groups of foundresses had no measurable effect on the rate of communal gall induction; therefore, I conclude that T. coweni foundresses do not actively discriminate among kin during gall initiation. Communal gall occupation was found to be closely correlated with foundress density, and individual aphid fitness was inversely related to the mean number of foundresses per gall. These results suggest that communal gall occupation does not represent mutual cooperation but is a function of competition for gall sites on the host plant. I measured sex ratio and sex allocation for T. coweni. Among broods with a sex ratio greater than zero (proportion male) there is a highly significant inverse relationship between brood size and sex ratio. Males are more likely to be produced towards the end of the growing season as resource quality deteriorates. The Trivers-Willard hypothesis of condition-dependent sex allocation bet fits these data for T. coweni. I outline the development of a microsatellite genetic marker and its application toward the goal of deciphering kin structure in galls of T. coweni. In all five singly-founded galls screened, genotypes were identical among foundresses and offspring within each gall. Identical genotypes among co-foundresses were found in four of ten communal galls; these preliminary data are consistent with the notion that communal gall occupation represents a form of passive cooperation and mutual tolerance among T. coweni foundresses, which is facilitated by kin selection. A novel wingless morph of T. coweni is described which resembles but does not behave like a soldier aphid. This morph is incapable of inducing galls; instead, it appears to occupy mature and abandoned galls. It is therefore termed a squatter. The occurrence of such a squatter morph in the population can be understood in the context of evolutionarily stable strategy analysis