Associational susceptibility of a native shrub induced by context‐dependent attraction of an invasive herbivore

Abstract

AbstractIndirect interactions among native and invasive species are notoriously difficult to predict. Here, we apply theory from trait‐mediated indirect effects and plant–insect interactions to explain the outcomes of multiple invader interactions. The present study investigates the roles of herbivore preference and plant ontogeny in mediating associational effects among a southern California native shrub, Atriplex canescens, an invasive annual forb, Brassica tournefortii, and an invasive stink bug, Bagrada hilaris. Ba. hilaris have been observed to form dense aggregations on A. canescens in late spring following senescence of Br. tournefortii, with subsequent increased mortality of A. canescens. A manipulative experiment found Ba. hilaris recruited to A. canescens in approximately 70 times greater numbers when neighbored by Br. tournefortii than when alone. Ba. hilaris nymph production, while minimized experimentally, occurred only on Br. tournefortii, suggesting A. canescens is not of sufficient quality for Ba. hilaris reproduction. In greenhouse preference trials, Ba. hilaris exhibited an overall preference for Br. tournefortii over A. canescens; however, the magnitude of preference depended on ontogenetic stage of Br. tournefortii. Overall, our results suggest significant potential for associational susceptibility mediated by a combination of Ba. hilaris preference for reproductive Br. tournefortii, substantial aggregation of Ba. hilaris onto Br. tournefortii, and a marked decline in Ba. hilaris preference for Br. tournefortii with advancing ontogeny—triggering spillover onto neighboring plants. Our results demonstrate an important role for multiple invaders and phenologically driven trait changes in mediating associational susceptibility.