Competitor avoidance drives within-host selection in hemlock woolly adelgid (Adelges tsugae), a passively-dispersed herbivore

The hemlock woolly adelgid (HWA; Adelges tsugae), a lethal exotic hemipteran pest on eastern hemlock (Tsuga canadensis) frequently shares its host with another exotic but non-lethal hemipteran, the elongate hemlock scale (EHS; Fiorinia externa). HWA colonization on EHS-infested trees results in a 40% decrease in HWA population densities, suggesting that EHS-induced changes in foliage quality negatively affect HWA performance and survival. Thus, there may be a selective advantage for HWA crawlers that are capable of detecting and avoiding EHS-infested foliage within a host. The objective of this study was to determine if HWA actively demonstrate preference for uninfested vs. EHS-infested foliage. We studied HWA preference at the needle and shoot level by using a combination of laboratory choice tests and observational field studies.

In the dual choice tests, a higher percentage of HWA crawlers were present on uninfested-shoots than on EHS-infested shoots from the same tree (P = 0.03, n = 48). In the field, HWA and EHS co-occurred on the same needle less frequently than predicted by chance (P < 0.001, n = 75). This pattern was consistent among the three different field sites studied. Although HWA crawlers are passively dispersed between trees, these results suggest that HWA can actively avoid EHS-infested foliage at small, within-host scales. This avoidance behavior may minimize the impact of EHS on HWA by reducing resource competition, and in the long-term it could result in HWA adaptations that might exacerbate the negative impact of this pest on the eastern hemlock.