Long‐term population dynamics of a sawfly show strong bottom‐up effects

Abstract

Summary 1 Population dynamics of the arroyo willow stem-galling sawfly Euura lasiolepis Smith (Hymenoptera: Tenthredinidae) on its host plant, Salix lasiolepis Bentham (Salicaceae), was studied for 22 years in the vicinity of Flagstaff, Arizona, USA. 2 Comparatively low levels of population change in drier and wetter sites ranged over two orders of magnitude. 3 Time-series analysis revealed best fit models of population change that included total winter precipitation from October to May and/or cumulative Pearson Drought Severity Index (PDSI) for the same period, plus a 1-year delay in precipitation or PDSI in some cases. 4 For populations on willow clones in drier sites, the model accounted for 73% of sawfly population change, and in wetter sites, with high mean populations, the model predicted 67% of the variation in gall density. 5 The mechanistic link between predictors of water availability to host plants and sawfly populations was better growth of willow after high winter precipitation, producing more and longer shoots. Also, willow clones with younger ramets, with more vigorous growth, were more favourable to sawflies than clones with older ramets. 6 Strong female preference for longer shoots resulted in more eggs laid, and this was coupled with higher larval survival on longer shoots. The major component of loss in a cohort was aborted ovipositions, with significant and negative influence on survival through the immature stages. Survival in one generation was correlated with gall density in the following generation. 7 Dynamics were entirely driven through resource supply. 8 Combining the evolutionary Phylogenetic Constraints Hypothesis with detailed mechanistic studies reveals an explanation of population dynamics which is more broadly applicable than purely ecological studies.