IS LEAF SIZE IMPORTANT FOR A LEAF-GALLING SAWFLY (HYMENOPTERA, TENTHREDINIDAE)

We evaluated the importance of leaf size in determining oviposition choice (i.e., gall densities), gall (and larval) size, and larval survivorship for a population of leaf-galling sawflies, Pontania sp. nr. pacifica Marlatt, on arroyo willow, Salix lasiolepis Bentham. Our results did not support the hypothesis that female sawflies prefer to oviposit in developing willow leaves that will be larger when they are fully expanded. Although leaves with four to seven galls had bigger leaf areas than leaves with one to three galls, the size of the mature leaf did not appear to be an important factor determining which leaves were selected for oviposition among the leaves available. The data did not support the prediction that larger leaves produce Pontania sp. galls (and larvae) that are substantially bigger than those on smaller leaves; gall size was only weakly correlated with leaf area (r less-than-or-equal-to 0.253). We hypothesized that survivorship of Pontania sp. larvae would be positively related to leaf size, but the result was opposite. In total, larval survival was weakly biased toward smaller leaves, primarily because of lower mortality from natural enemies. Mortality from two species of inquilines was positively related to leaf area, whereas mortality from two ectopamsitoids showed inconsistent relationships to leaf area. The absence of strong directional selection illustrates that it would be unlikely for female sawflies to evolve a leaf size choice strategy based on maximizing survival rates of their offspring.