Interference of host plant morphology and phenology and their correlation with abundance patterns of the leaf galling sawfly <Emphasis Type="Italic">Pontania proxima</Emphasis>

The host taxon preference of Pontania proxima (Serville 1823) (Tenthredinidae, Hymenoptera) was investigated by observing densities of galls on previously genotypically and phenotypically characterized clones representing three taxa of a hybrid complex, i.e., Salix alba, Salix × rubens, and Salix fragilis. Gall densities among these three taxa were observed by using two experimental designs: (1) an indoor experiment in a greenhouse flight cage, and (2) an outdoor experiment on a plantation of cuttings. Subsequently, observed gall densities were related with selected, characteristic phenotypical properties of the host plants. In the indoor experiment, S. fragilis was clearly preferred by P. proxima, while in the outdoor experiment, S. × rubens revealed the highest gall densities. The factor “foliation start” was excluded in the indoor experiment, but best explained the preference of S. × rubens in the outdoor experiment. The results of a linear regression model affirmed on the one hand that morphological properties should be expected as relevant signals during the host plant selection process of P. proxima. On the other hand, it was shown that other phenotypical factors such as foliation start in spring contribute to an interference of factors being relevant for host plant selectivity. The earlier foliation start of the morphological intermediate S. × rubens in the outdoor experiment shifts resource availability towards that taxon at an early oviposition phase of P. proxima and therefore entails a different gall distribution pattern with a preference for the hybrid under natural conditions.