Eastern Hemlock, Tsuga canadensis, is experiencing widespread mortality due to the invasive hemlock woolly adelgid (HWA; Adelges tsugae). Eastern hemlock looper (EHL; 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. Past research demonstrates that HWA infestation results in increase methyl-salicylate emissions and the likely associated defense responses are activated. However, EHL likely elicits jasmonic acid (JA)-associated responses in hemlock and these two defense responses are known to be antagonistic. We investigated indirect effects of HWA infestation, the direct effects of induced defenses, and their interactive effects on the survival, performance, and sub-lethal physiological stress of EHL. Caterpillars were fed HWA-infested or uninfested hemlock foliage that was either constitutive, JA-induced (via methyl jasmonate [MeJA] application), or SA-induced (via SA application), and EHL survival and performance through time were tracked. Larvae were harvested once they reached the fourth instar and the activity of detoxification, digestive, and antioxidant enzyme activities were quantified. Additionally, host defenses and nutritional quality were assessed.
Results/Conclusions
Preliminary trials show that HL larvae survival is higher on constitutive HWA-infested foliage, but lower on MeJA- and SA-induced if infested. MeJA-induced needle tissue has significantly greater levels of soluble phenolics relative to constitutive and SA-induced needles, while HWA-infestation had a notable but insignificant influence. A significant interactive effect of HWA-infestation and induction was detected where the terpenoid content of SA-induced and HWA-infested foliage was highly variable but significantly greater than other treatment combinations. Further experiments will reveal sub-lethal physiological effects and stress in these insects as well as the interactive effects of defense and nutrition. Our results partly suggest that eastern hemlock cannot successfully defend itself against looper herbivory when it has been previously infested by HWA due to defensive signaling cross-talk.