SummaryHemlock woolly adelgid (HWA) is an exotic insect pest causing severe decimation of native hemlock trees. Extensive research has been conducted on the ecological impacts of HWA, but the exact physiological mechanisms that cause mortality are not known.Water relations, anatomy and gas exchange measurements were assessed on healthy and infested eastern (Tsuga canadensis) and Carolina (Tsuga caroliniana) hemlock trees. These data were then used in a mechanistic model to test whether the physiological responses to HWA infestation were sufficiently significant to induce changes in whole-plant water use and carbon uptake.The results indicated coordinated responses of functional traits governing water relations in infested relative to healthy trees. In response to HWA, leaf water potential, carbon isotope ratios, plant hydraulic properties and stomatal conductance were affected, inducing a reduction in tree water use by > 40% and gross primary productivity by 25%. Anatomical changes also appeared, including the activation of traumatic cells.HWA infestation had a direct effect on plant water relations. Despite some leaf compensatory mechanisms, such as an increase in leaf hydraulic conductance and nitrogen content, tree water use and carbon assimilation were diminished significantly in infested trees, which could contribute to tree mortality.

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Abstract:In southern pines of the United States, resistance to attack by southern pine beetle, Dendroctonus frontalis Zimmermann, is believed to principally involve flow of oleoresin to beetle attack sites. Both environmental and genetic factors are known to affect the quantity of oleoresin flow in loblolly pine, Pinus taeda L., but little is known about the genetic contribution to phenotypic variation in this trait. Here we report estimates of genetic variation in oleoresin flow and growth traits for a population of this species. Oleoresin yield, total height, and diameter were measured on 10-and 11-year-old trees from an experimental test in Santa Rosa County, Florida. Trees were from 72 full-sib families produced by mating 48 parents according to a disconnected partial diallel mating design. Resin yield was determined from breast-height samples collected at two times: once in the summer of 1999 when latewood was being produced (summer resin flow), and once in the spring of 2000 during earlywood formation (spring resin flow). All traits studied were found to be highly genetically variable and to have much greater additive than dominance variance. Estimates of narrow-sense heritability for spring and summer resin flow were in the moderate range and are comparable to values obtained for the growth traits. Additive genetic correlations between oleoresin yield and the growth traits were positive and moderately high, suggesting that directional selection to improve growth in loblolly pine will also result in increased production of oleoresin.

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Under laboratory conditions, Dendroctonus frontalis Zimmermann and Ips grandicollis (Eichhoff) adults became self-marked with fluorescent powders upon emergence from treated logs. The technique provided a reliable procedure for marking adult bark beetles without having to handle them. When the powder remained dry, nearly all of the emerged beetles were marked. However, the percentage of marked beetles decreased dramatically following simulated rain events. Once marked, the mark remains intact on beetles stored dry or in ethylene glycol or ethanol (95%). Marking did decrease the adult life span of both D. frontalis and I. grandicollis, but had no significant effect on flight initiation by either species or semiochemical perception by I. grandicollis. No semiochemical tests were conducted with D. frontalis. If beetles are recaptured shortly following emergence and self-marking, the technique should be useful for examining scolytid dispersal in the field.

The toxicity of four monoterpenes present in loblolly (Pinus taeda L.) and shortleaf (P. echinata Mill.) pine to Dendroctonus frontalis Zimmermann and Ips calligraphus (Germar) (Coleoptera: Scolytidae) was examined in the laboratory. The descending order of toxicity of the monoterpenes to D. frontalis was: limonene, β - pinene, α - pinene, camphene. No clearcut ordering in toxicity to I. calligraphus was observed.

Oleoresin flow is an important factor in the resistance of pines to attack by southern pine beetle, Dendroctonus frontalis Zimm., and its associated fungi. Abiotic factors, such as nutrient supply and water relations, have the potential to modify this plant-insect-fungus interaction; however, little is known of the effects of inoculation with beetle-associated fungi on oleoresin flow. We observed that constitutive and induced resin yield in loblolly pine, Pinus taeda L., were affected by either fungal inoculation (with the southern pine beetle-associated fungus Ophiostoma minus (Hedgcock) H. & P. Sydow) or silvicultural treatment. The effects of mass wounding (400 wounds m(-2)) and mass wounding and inoculation with O. minus were assessed by comparison with untreated (control) trees. The treatments were applied to trees in a 2 x 2 factorial combination of fertilizer and irrigation treatments. Fertilization did not significantly affect constitutive resin yield. Even as long as 105 days post-treatment, however, mass-inoculated trees produced higher induced resin yields than control or wounded-only trees, indicating a localized induced response to fungal inoculation. We noted no systemic induction of host defenses against fungal colonization. Although beetles attacking previously attacked trees face a greater resinous response from their host than beetles attacking trees that had not been previously attacked, the effect of an earlier attack may not last more than one flight season. Despite mass inoculations, O. minus did not kill the host trees, suggesting that this fungus is not a virulent plant pathogen.

The process of resource utilization by Dendroctonus frontalis Zimmerman attacking loblolly pine, Pinus taeda L., was investigated. The quantitative relationship of attacking parent adult D. frontalis as a function of the normalized infested bole height is described by the model y = Ax(1−x)eBx. Greatest attack density occurs at the mid-bole of the tree and tapers toward the top and bottom. Gallery length (and hence eggs)/100 cm2 was independent of attack density. The relationship between gallery length (or eggs) per parent adult and parent adult density is described by the exponential decay curve y = AeBx, indicating that gallery length and egg population density are controlled by a density dependent compensatory feedback process operating instantaneously. Further support for the mechanism was obtained by analyzing the gallery length per parent adult at different locations on the infested bole. The relationship is described by the model y = [AeBx]/[x(1−x)] and indicates that gallery construction and egg population per attacking beetle increase in the upper and basal portion of the bole. The result is a uniform amount of food and space per individual of the developing population.