Generalized Response Sequence

Conifer resistance to stem-invading insects has received much attention and involves a generalized, three-step sequence of wound cleansing, infection containment, and wound healing (Berryman 1972, Hain and others 1983). The first step of this response, wound cleansing, is characterized by the production and flow of constitutive resins. The second step of the resistance sequence, infection containment, can be described as a rapid necrosis of cells surrounding the infection site that is accompanied by the development of traumatic resin ducts and an increased concentration of monoterpenes and phenolics in the reaction zone (Cook and Hain 1986, Raffa and Berryman 1982, Reid and others 1967). The accumulation of terpenes and phenolics in the reaction zone is also accompanied by a decrease in the level of soluble sugars in that zone (Cook and Hain 1986, Wong and Berryman 1977). Wound healing, or formation of wound periderm, is the final step of the resistance sequence. This isolates the wound from the rest of the tree. Wound periderm is located adjacent to the necrotic tissue and protects living tissue from the adverse effects of the dead cells in the necrotic zone surrounding the attack site(s) (Mullick 1977). The three-step resistance sequence requires an expenditure of energy by the tree, and there is typically a resulting change of color (fading) within the tree’s foliage.

Fir Response to Stem Attack by the Balsam Woolly Adelgid

The impact of balsam woolly adelgid infestation on North American firs has been studied extensively over the past several decades. Infestation by the adelgid results in anatomical and structural changes within host tissues that may be the result of salivary excretions from the insect’s stylet during feeding. Physically, the xylem tissue of infested trees has higher concentrations of ray tissue (Mitchell 1967, Smith 1967), thickened cell walls, and shorter tracheids (Doerksen and Mitchell 1965). The tracheids have encrusted pit membranes that more closely resemble the pit membranes associated with heartwood (Puritch and Johnson 1971). There is a corresponding reduction in water flow in infested trees (Mitchell 1967) that puts the tree into a state of physiological drought; this, in turn, reduces photosynthesis and respiration (Puritch 1973) and can ultimately result in tree death.

The damage to the host tree is related to both the size of the tree and the intensity of the infestation. Balsam woolly adelgid infestations in the crown of a tree usually result in gouting of the outer branches (characterized by node or bud swelling or both with a decrease in new growth of the stem and foliage) (Mitchell 1966). Over time, the crown thins, and the foliage fades in color. Balsam woolly adelgid infestations also occur on the stems of trees. In North America, these stem infestations usually kill native firs within 6 years (Hain 1988).

Hemlock Resistance to Attack by Hemlock Woolly Adelgid

Once hemlock woolly adelgid (HWA) settles onto a twig, the tree usually suffers needle loss and bud mortality, followed by branch and whole tree mortality (usually within 6 years) (McClure 1991, Shields and others 1995). Foliar chemistry appears to play some role in host susceptibility/resistance to hemlock woolly adelgid, with resistance being related to foliar levels of calcium, potassium, nitrogen, and phosphorous (Pontius and others 2006). These authors suggest that higher levels of N and K in the foliage enhance host palatability and, thus, result in increases in the population levels of hemlock woolly adelgid. In addition, soil and foliar chemistry along with landscape position can be used to model hemlock susceptibility to HWA (Pontius and others 2007). These hypothesized relationships between foliar chemistry and infestation could be important for early detection of hemlock woolly adelgid infestations because some foliar constituents such as chlorophyll, nitrogen, cellulose, and sugar can be accurately estimated using spectral data (Curran and others 2001).

As with other conifers, monoterpenes are major constituents of tree chemistry of hemlocks, (i.e., Li and others 2001). These compounds may function in several ways to mediate the interaction between trees and herbivores, but one impact is that they are frequently toxic to attacking insects such as bark beetles, (i.e., Cook and Hain 1988) or other arthropods such as spider mites, (i.e., Cook 1992). It has been suggested that the monoterpene content of western hemlocks may function as a deterrent to hemlock woolly adelgid (Lagalante and Montgomery 2003). The authors suggest that elevated levels α-pinene, β-caryophyllene, and α-humulene may act as feeding deterrents against hemlock woolly adelgid, and that elevated levels of isobornyl acetate may attract the adelgid.