Variation in plant defense against invasive herbivores: Evidence for a hypersensitive response in eastern hemlocks

Background/Question/Methods:

In the northeastern US, the invasive hemlock woolly adelgid Adelges tsugae (‘HWA’) and elongate hemlock scale Fiorinia externa (‘EHS’) feed on eastern hemlocks (Tsuga canadensis). Although both are sessile sap feeders, HWA causes much more damage than EHS. Due to the disproportionately high mortality rates caused by HWA, it has been suggested that HWA are ‘phytotoxic’ to hemlocks and that their feeding induces a hypersensitive response (HR). The HR is a defense mechanism consisting of elevated H₂O₂ levels and tissue death at the site of herbivore feeding. We hypothesized that both insects would induce a hypersensitive response at the site of feeding, but that HWA settlement would induce a stronger response. Do to the high levels of mortality caused by HWA, we also expected the HWA infested trees to show a systemic response to feeding. We measured H₂O₂ levels, indicative of the HR, and needle size in new and old growth foliage from HWA-infested, EHS-infested, and uninfested trees. The response was analyzed using DAB staining, which stains H₂O₂ an orange-brown color, and image analysis of the stained needles. ANOVA was used to test for the effects of treatment, foliage age (old versus new growth), and their interaction.

Results/Conclusions:

Needles with settled HWA or EHS had higher H₂O₂ levels than control needles, suggesting a localized hypersensitive response in both insect treatments. In the HWA treatment, new growth needles with no direct contact to settled HWA also had high H₂O₂ levels. This was not the case in the EHS treatment, suggesting that HWA feeding may induce a systemic defense response in eastern hemlocks. This may be one of the reasons that HWA causes significantly more damage than EHS. Needle size data also indicated that the herbivores affected the tree differently. Old growth HWA needles were the smallest, while new-growth needles on HWA-infested branches were as large as controls. HWA needle size increased at a much greater rate from old to new growth than did EHS or control needles. This may be due to remobilization of nutrients away from stressed older needles and into new growth. Our results showed that two herbivores in the same feeding guild had dramatically different effects on the health of their shared host.