The role of natural enemies in determining the relationship between gall size and emergence success of a host-specific cynipid
Reynolds, Richard J.
Large gall size is considered an adaptation to avoid parasitism for a number of insect gall-formers when gall size is positively correlated with emergence success and negatively correlated with the probability of parasitism. This interpretation is warranted only if gall size at maturity is not an artifact of parasitism before the potential benefit of this structural defense is fully realized. Here I investigate the significance of gall size variation for the parthenogenetic generation of the host-specific gall-former, Belonocnema treatae (Hymenoptera: Cynipidae), which induces galls on the leaves of Quercus fusiformis, and is subject to high levels of mortality from a diverse community of parasitoids. Employing a field study and a parasitoid exclusion experiment I tested two primary hypotheses: (1) gall-former emergence success increases with gall size and (2) the distribution of gall sizes at maturity is independent of parasitism. To test hypothesis one, I collected mature galls in 1998, prior to gall-former emergence, from 5 trees in each of 3 natural populations (n = 21,000 galls total). Galls were measured, housed individually, and scored for gall-former emergence. Additional galls sampled from the same trees in 1998 and 1999 were used to investigate the relationship between mean gall size per tree and emergence success per tree. In all populations the probability of gall-former emergence increased exponentially with gall size. Moreover, mean gall size varied significantly among trees (p = 0.0001) and significantly non-uniform emergence success per tree (p < 0.05) was positively correlated with mean gall size per tree (r = 0.72, DF = 13, p < 0.01) in 1998 but not in 1999 (r = 0.39, DF = 13, p > 0.10). To test hypothesis two, I manipulated levels of parasitism in the field in 1999. On each of 6 Q. fusiformis trees, 15 branches were bagged prior to oviposition and coinicident with the advent of the oviposition period bags were stocked with equal numbers of newly emerged and freshly mated female B. treatae. Following oviposition, parasitoid exclusion, bag control, and open treatments were created. When parasitoids were removed from the system emergence success was independent of gall size. Parasitoid attack significantly reduced mean gall size by 18% relative to unparasitized galls and generated bimodal gall size distributions which are significantly different (p = 0.0001) from the positively skewed gall size distributions of unparasitized treatments. Thus, a substantial portion of galls are parasitized before they reach full size and the interpretation of increasing gall size as an adaptation to thwart parasitoids is not fully supported in this study system. In contrast to other studies purporting the adaptive significance of large gall size in avoiding insect parasitism, this investigation highlights the difficulty in establishing clear links between gall size dependent selection by parasitoids and evolutionary response to this selection in gall former systems associated with complex parasitoid communities which attack galls during all stages of gall development.
gall wasps, gall insects, plant-pathogen relationships, parasitism, evolution
Reynolds, R. J. (2000). The role of natural enemies in determining the relationship between gall size and emergence success of a host-specific cynipid (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.