Variation in Plant Defense against Invasive Herbivores: Evidence for a Hypersensitive Response in Eastern Hemlocks (Tsuga canadensis) Laura Radville & Arielle Chaves & Evan L. Preisser Received: 15 February 2011 /Revised: 26 April 2011 /Accepted: 28 April 2011 /Published online: 15 May 2011 # Springer Science+Business Media, LLC 2011 Abstract Herbivores can trigger a wide array of mor- phological and chemical changes in their host plants. Feeding by some insects induces a defensive hypersen- sitive response, a defense mechanism consisting of elevated H 2 O 2 levels and tissue death at the site of herbivore feeding. The invasive hemlock woolly adelgid Adelges tsugae (‘HWA ’) and elongate hemlock scale Fiorinia externa (‘EHS’) feed on eastern hemlocks; although both are sessile sap feeders, HWA causes more damage than EHS. The rapid rate of tree death following HWA infestation has led to the suggestion that feeding induces a hypersensitive response in hemlock trees. We assessed the potential for an herbivore-induced hypersen- sitive response in eastern hemlocks by measuring H 2 O 2 levels in foliage from HWA-infested, EHS-infested, and uninfested trees. Needles with settled HWA or EHS had higher H 2 O 2 levels than control needles, suggesting a localized hypersensitive plant response. Needles with no direct contact to settled HWA also had high H 2 O 2 levels, suggesting that HWA infestation may induce a systemic defense response in eastern hemlocks. There was no similar systemic defensive response in the EHS treatment. Our results showed that two herbivores in the same feeding guild had dramatically different outcomes on the health of their shared host. Key Words Adelges tsugae . Fiorinia externa . Tsuga canadensis . Hypersensitive response Introduction The rapid expansion of global trade and transport links has been accompanied by a concomitant increase in the number of invasive species and the threat they pose to native ecosystems (Mooney and Cleland, 2001). As the number of biological invasions increases, the potential for interactions among invasive species will become more likely. Since invasive species tend to have high growth rates and population densities (Sakai et al., 2001), the likelihood and importance of these interactions is great (Denno et al., 1995). Such interactions are especially likely among specialist sessile herbivores that cannot emigrate in response to changes in host quality induced by competing insect species (Karban, 1989). Herbivores can have differing effects on their host plants if one species, but not another, induces changes in plant physiology. Infestation by one herbivore may lead to the systemic induction of plant defenses and reduce the plant’ s susceptibility to subsequent invaders (Karban, 1990; Alborn et al., 1996; Bezemer et al., 2003). Some herbivores also are able to directly manipulate host plant metabolism and nitrogen allocation to their benefit, changes that can lead to the induction of systemic effects (Karban and Agrawal, 2002). Herbivores may be particularly harmful to their host if they are capable of inducing a pronounced hypersensitive response (HR). The HR is a plant defense response that increases reactive oxygen species, including hydrogen peroxide (H 2 O 2 ), superoxide anions, and hydroxyl radical levels, and induces cell death in herbivore-damaged areas that isolates and starves invading organisms (Heath, L. Radville : E. L. Preisser (*) Department of Biological Sciences, University of Rhode Island, Kingston, RI 02881, USA e-mail: preisser@uri.edu A. Chaves Department of Plant Sciences and Entomology, University of Rhode Island, Kingston, RI 02881, USA J Chem Ecol (2011) 37:592–597 DOI 10.1007/s10886-011-9962-z