The Window of Vulnerability of a Shoot‐Galling Sawfly to Attack by a Parasitoid

We studied the susceptibility of a shoot—galling sawfly, Euura lasiolepis, to attack by the parasitoid Lathrostizus euurae. Each Euura had a window of vulnerability: a limited time period when the parasitoid attacked it. Lathrostizus attacked a limited range of gall sizes and ages during each sample period. Gall age was related to its position on a shoot, and Lathrostizus shifted the gall position it attacked as it tracked vulnerable—aged galls. The parasitoid emergence period matched the window of vulnerability of its host. Lathrostizus attacked and successfully parasitized a low proportion of hosts encountered. A low interval—parasitism rate uniformly distributed among Euura galls formed at different times is consistent with the pattern expected if the window of vulnerability influenced attack. The window of vulnerability was formed by the interaction of gall and sawfly development rates. Two hypothesis that defined the window were supported: (1) the host development hypothesis, that Lathrostizus is a larval parasite and does not attack galls containing eggs and (2) the gall toughness hypothesis, that large, old galls that are tough are not attacked by Lathrostizus. Our data did not support (3) the ovipositor limitation hypothesis, that some galls were too large for the larva inside to be reached by the Lathrostizus ovipositor. We observed that if these galls were soft, they were attacked. Lathrostizus parasitism rates on Euura varied widely among willow clones. Willows differed in the rate of gall toughening, which produced different—sized windows and thus contributed to variation in parasitism rates among Euura populations on different willow clones.