Abstract
Eastern hemlock (Tsuga canadensis) is in rapid decline because of infestation by the invasive hemlock woolly adelgid (Adelges tsugae; 'HWA') and, to a lesser extent, the invasive elongate hemlock scale (Fiorinia externa; 'EHS'). For many conifers, induced oleoresin-based defenses play a central role in their response to herbivorous insects; however, it is unknown whether eastern hemlock mobilizes these inducible defenses. We conducted a study to determine if feeding by HWA or EHS induced changes in the volatile resin compounds of eastern hemlock. Young trees were experimentally infested for 3 years with HWA, EHS, or neither insect. Twig and needle resin volatiles were identified and quantified by gas chromatography/mass spectrometry. We observed a suite of changes in eastern hemlock's volatile profile markedly different from the largely terpenoid-based defense response of similar conifers. Overall, both insects produced a similar effect: most twig volatiles decreased slightly, while most needle volatiles increased slightly. Only HWA feeding led to elevated levels of methyl salicylate, a signal for systemic acquired resistance in many plants, and benzyl alcohol, a strong antimicrobial and aphid deterrent. Green leaf volatiles, often induced in wounded plants, were increased by both insects, but more strongly by EHS. The array of phytochemical changes we observed may reflect manipulation of the tree's biochemistry by HWA, or simply the absence of functional defenses against piercing-sucking insects due to the lack of evolutionary contact with these species. Our findings verify that HWA and EHS both induce changes in eastern hemlock's resin chemistry, and represent the first important step toward understanding the effects of inducible chemical defenses on hemlock susceptibility to these exotic pests.
Highlights
Conifers in the family Pinaceae are among the largest and longest-living organisms on earth
We found evidence of an induced response in eastern hemlock during infestation by both HWA and EHS, encompassing a number of feeding-elicited changes in the tree’s resin volatile profile
The modest induction of resin metabolites in twig tissue and the non-significant trend of modest increases in needle tissue produced by both insects, was conspicuously different from the profuse resinosis observed in insect-infested pines, spruces, and firs (Trapp and Croteau 2001)
Summary
Conifers in the family Pinaceae are among the largest and longest-living organisms on earth Their striking longevity means that individual trees face an imposing array of biotic and abiotic challenges. ‘resin,’ is a complex and species-specific mixture of phytochemicals that is usually dominated by volatile monoterpenoids and non-volatile diterpenoid acids and contains smaller amounts of volatile organic chemicals such as sesquiterpenoids, benzenoids (including phenolics), and fatty acid derivatives These compounds are produced in resin-cells of buds, needles and woody tissue, and in some conifers (such as Pinus species) they accumulate in intercellular ducts or canals either constitutively or in response to trauma (Keeling and Bohlmann 2006)
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