Competitor avoidance drives within‐host feeding site selection in a passively dispersed herbivore

Abstract

Evolutionary theory predicts that ovipositing females that can actively disperse should select hosts that increase offspring performance. However, for organisms that are exclusively passively dispersed, feeding site selection is possible only at the within‐host level. This is particularly important for their offspring, which have strong temporal and spatial dispersal constraints within a host. Such constraints are probably magnified by tissue quality heterogeneity caused by competing herbivores. We investigated within‐host feeding site selection of passively dispersed neonates of a sessile herbivore, the hemlock woolly adelgid (Adelges tsugae Annand), when it shares its eastern hemlock (Tsuga canadensis Carrière) host with another sessile herbivore, the elongate hemlock scale (Fiorinia externa Ferris). Within‐host feeding site selection was studied (a) at the shoot level (with or without F. externa) using choice tests, and (b) at the needle level by scoring insect presence in field surveys. Adelges tsugae avoided F. externa‐colonised foliage in both the choice tests and field surveys. As A. tsugae has no efficient predators or parasitoids in the study area, we conclude that our results are due to the presence of the competing herbivore. Although A. tsugae cannot actively disperse among hosts, we showed that within‐host feeding site selection is an important mechanism to minimise the co‐occurrence with a competing herbivore that has known negative impacts on A. tsugae population densities. Studying within‐host feeding site selection in a multiple‐species context could assist in understanding and predicting the impact of destructive pests such as A. tsugae and the interactions with their novel hosts.