Preference and performance of a gall-inducing sawfly: a test of the plant vigor hypothesis

Abstract

To test the Plant Vigor Hypothesis, we determined female oviposition preference of Phyllocolpa leavitii (Hymenoptera: Tenthredinidae) for shoot lengths on 20 clones of Salix discolor (Salicaceae) and examined larval performance by determining larval survival based on clone, shoot length, and leaf length for each Phyllocolpa gall observed. Sawfly galls were found on significantly longer shoots compared to shoots without galls, and shoots with 2, 3, 4, and 5 galls were successively longer than shoots with fewer galls. Furthermore, a much higher proportion of longer shoots had galls that did shorter shoots. These data demonstrate strong sawfly oviposition preference for long shoots. However, when shoot length was adjusted for numbers of available leaves per shoot, the number of galls per available leaf showed a weaker, but still positive relationship with shoot size. Using a logistic regression on survival of Phyllocolpa larvae, we detected highly significant effects of clone, shoot length, leaf length, and the square of leaf length on survival. Additionally, much higher sawfly survival on the long shoots of one clone caused a significant clone by shoot-length interaction. Survival was positively correlated with shoot length, indicating directional selection favoring oviposition on longer shoots; still, shoot length explained only 11.8% of the variation in survival. The directional component of selection favored oviposition on smaller leaves as survival was inversely related to leaf length; however, disruptive selection for leaf length oviposition preference was also detected, with survival of Phyllocolpa galls lowest on intermediate leaf lengths. This study provided evidence supporting the preference prediction of the Plant Vigor Hypothesis. However, much of the data contradicted the performance prediction of the Plant Vigor Hypothesis, with only a modest amount of evidence supporting the performance prediction.