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We performed an experiment on preference and performance of the leaf galling sawfly, Phyllocolpa leavitti (Hymenoptera: Tenthredinidae), to test predictions of the plant vigor hypothesis, the sex-biased herbivory hypothesis, and the enemy-free space hypothesis. We cut ramets on 8 male and 6 female genets of Salix discolor to induce large, vigorous shoots. Shoots on an undamaged ramet of the same genet served as a control. We measured shoot length, length of galled and ungalled leaves, gaps along the margin of the leaf fold, and node position of each leaf fold. We collected and determined the survival and stage of mortality of each leaf fold on 50 treatment and 100 control shoots per ramet. Cutting ramets induced longer shoots and leaves on treatment compared to control ramets. The density of P. leavitti was significantly greater on the treatment ramets, supporting the preference prediction of the plant vigor hypothesis. Density of P. leavitti did not differ between the sexes, which failed to support one prediction of sex-biased herbivory. Survival of P. leavitti did not differ between the treatments, contrary to the performance prediction of the plant vigor hypothesis. There was, however, a large effect of plant sex on sawfly survival - the probability of survival was more than twice as great on female plants than on male plants. This supports the prediction of sex-biased herbivory hypothesis that herbivore fitness should vary between plant sexes, but is opposite to the expected direction of higher survival on male plants. Gaps that form along the margin of the leaf fold explained a significant amount of variation in survival. An increase in gap number by 1 decreased sawfly survival by 50%. Since gap number is positively related to leaf length, this implies selection to oviposit on shorter leaves, which is also contrary to the performance prediction of the plant vigor hypothesis. Finally, leaf folds formed later in the oviposition period had a higher probability of survival, suggesting selection favoring later phenology of the sawfly. This study refuted the performance prediction of the plant vigor hypothesis, but it provided insights into the traits that should be under natural selection to provide enemy-free space in this insect-plant interaction. |
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