Impact of the hemlock woolly adelgid and plant defenses on a native forest defoliator
Plants are vulnerable to simultaneous or sequential attack by herbivores. The activation of defense pathways can alter foliage quality for subsequent attackers. Multiple insects from distinct feeding guilds attack eastern hemlock (Tsuga canadensis), which is widespread and ecologically important in the eastern United States. Following the introduction of the hemlock woolly adelgid (Adelges tsugae; HWA) from Japan in the 1950s, hemlocks in the eastern United States have rapidly declined. A native pest of hemlock in eastern North America, the looper (Lambdina fiscellaria) is a chewing lepidopteran that was linked to the mid-Holocene hemlock decline. The hemlock looper and woolly adelgid may co-exist on the landscape in the coming years. We assessed the performance of hemlock looper on adelgid-infested eastern and Carolina hemlock (T. caroliniana). We also reared larvae on foliage gathered from a rare tree that appears resistant to the adelgid. To examine preference, we conducted a series of bioassays where larvae were presented with multiple foliage types.
Larvae reared on the HWA-resistant foliage gained 3.7 times more weight, pupated earlier and nearly twice as often as the average larva across the other three treatments. The number of surviving larvae reared on adelgid-infested Carolina hemlock foliage decreased more rapidly over time than larvae reared on other foliage types. In a preference experiment, larvae presented with uninfested and HWA-resistant foliage consumed significantly more HWA-resistant foliage. Differences in looper maturation and preference are likely due to varying foliage quality driven by activation of defense pathways in the plant. The looper and HWA are from distinct feeding guilds and thus activate distinct plant defense pathways. In HWA-resistant trees, the defense pathway that negatively impacts the performance of HWA is likely up-regulated, which would decrease resource allocation toward the pathway that negatively impacts looper. This difference would explain the improved looper development on HWA-resistant foliage. Exploring the interaction between an exotic and native insect herbivore enhances understanding of past and novel pressures faced by the host plant.