A Native Lepidopteran is Impacted by Host Defenses Induced by Hemlock Woolly Adelgid (Adelges tsugae)

Abstract

Eastern hemlock, Tsuga canadensis, is currently experiencing widespread mortality due to the invasive hemlock woolly adelgid (HWA; Adelges tsugae). Eastern hemlock looper (Lambdina fiscellaria), another hemlock pest, is a native lepidopteran that has reached outbreak levels in the past. While these insects share a host and overlap in range, little is known about their interactions. Research has shown that HWA infestation increases methyl salicylate concentrations (an indicator of the induction of the salicylic acid pathway) in hemlock tissue while hemlock looper, a chewing insect, likely elicits the jasmonic acid pathway. While plants are capable of inducing both of these defensive pathways, they have been found to be mutually antagonistic. We tested the hypothesis that looper performance is affected by prior HWA infestation and plant defense. Specifically, we hypothesized that looper larvae would perform better on HWA infested hemlock foliage. Looper were reared to pupation on hemlock foliage that was either infested or uninfested with HWA. Within those treatments, groups of foliage were sprayed with defensive elicitors to induce either the jasmonic- or salicylic-acid pathway. We also analyzed total phenolic content and terpenoid levels in host needle tissue in all treatments. Application of the chemical elicitors negatively affected looper survival, and their impact was altered by HWA presence. Larval survival was significantly lower in treatments where both HWA and elicitor were present. In contrast, larval survival was higher in the HWA-infested control treatment in comparison to the uninfested treatment. This demonstrates a strong interactive effect between HWA and elicitor. The combined effect of elicitor and HWA infestation led to increases in both phenolics and terpenoids, suggesting a major role for these compounds in plant defense. This study demonstrates the complex host-mediated interactions occurring between a native and an invasive insect which may aid in the further study of insect interactions.