Can Plants Gain Protection from Herbivory by Association with Unpalatable Neighbours?: A Field Experiment in a Willow-Sawfly System

Abstract

The fact that the spatial association of plants can influence the risk of herbivory has received little interest from ecologists. We tested the prediction, generated by patch use theory, that plants can gain protection from herbivory when associated with plants of lower palatability. Nine clones (referred to as matrix-clones) of the willow Salix lasiolepis were identified in the field. Twenty-seven potted willows (S. lasiolepis) originated from three different clones (9 potted willow from each clone) were selected in such a way that intra-clonal variation in age, size and morphology (number of branches and last year shoots) was minimised. Prior to sawfly egg-laying we placed three potted individuals, one of each clone origin, inside each matrix-clone. The densities of the stem galler Euura lasiolepis, the petiole galler Euura sp. and the leaf folder Phyllocolpa sp. were measured on the matrix-clones and on the potted willows. For stem gallers, but not for petiole gallers and leaf folders, we found positive correlation between sawfly density on the matrix-clones and sawfly density on the potted willow. Thus, as predicted the potted willows seemingly gained protection from stem gallers by association with neighbours (matrix-clones) of low palatability. However, only one of three tests supported our predictions. On balance we should therefore reject our hypothesis. Still, it might be too early to totally discard this hypothesis. More case studies are needed before a final evaluation can be done. There was a strong correlation between clone origin, shoot length and sawfly densities for the potted willows, suggesting an underlying genetic variation among clones in susceptibility to sawfly oviposition.