Densities of Callirhytis cornigera (Osten Sacken), a cynipid with alternating agamic and sexual generations that develop in woody stem galls and blister-like leaf galls, respectively, often vary among neighboring pin oaks, Quercus palustris Muenchhausen. We studied the ovipositional behavior of agamic wasps and tested several hypotheses to explain intraspecific variation in susceptibility to leaf gall induction. Agamic females laid eggs primarily into green-tip buds. Heavily galled trees had proportionately more vulnerable buds than lightly galled trees during peak oviposition in 1998, supporting the phenological synchrony hypothesis, but not in 1999. Contrary to plant vigor hypothesis predictions, rates of shoot or leaf expansion were similar for lightly and heavily galled trees. Agamic wasps were transferred from heavily infested trees onto shoots of heavily or lightly infested novel trees, or back onto their natal trees, to test for inherent differences in susceptibility to leaf galling. Leaf gall induction was greater on natal than novel trees in 1997, supporting the demic adaptation hypothesis, and there was a trend for greater leaf gall induction on heavily galled than lightly galled novel trees. These patterns, however, did not reoccur in 1998. Although synchrony of emergence with budburst is critical to ovipositional success of agamic C. cornigera, other host-related factors, including those affecting stem gall induction or survival of the agamic generation, probably are more important in determining variation in tree infestation levels. Once a suitable host is colonized, limited wasp dispersal may also contribute to buildup of insular populations on particular trees.
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