ENVIRONMENT AND ONTOGENY MODIFY LOBLOLLY-PINE RESPONSE TO INDUCED ACUTE WATER DEFICITS AND BARK BEETLE ATTACK

We evaluated the impact of tree resistance on within-tree population dynamics of southern pine beetle, Dendroctunus frontalis Zimm. (Coleoptera: Scolytidae) in loblolly pine, Pinus taeda L., as affected by prevailing water regimes, acute water deficits imposed by applying dry-ice (solid CO2) collars to tree boles, and by the seasonal ontogeny of pines. We conducted the study in the spring of 1986, when bole cambial growth had not yet shifted from earlywood to latewood formation, and in the summer of 1987, when latewood formation was well advanced. In each year there were five treated and five control trees. In 1986, under relatively well-watered conditions, results did not support the hypothesis that induced acute water deficit will enhance success of southern pine beetle attack and brood production. All trees were readily attacked and overcome with no apparent beneficial effects of the dry-ice treatment on within-tree population dynamics. In 1987, under drier conditions, results supported the hypothesis. Treated trees again were readily colonized, but three of the five control trees resisted attack to the extent that all attacks eventually failed. In 1986, all study trees maintained relatively high water potentials for more than 3 weeks following beetle attack. However, resin yields from bark wounds decreased rapidly. In contrast, water potentials of controls decreased gradually in 1987 and resin yields increased as controls resisted beetle attacks. Water potential of treated trees dropped rapidly 2 weeks after initial beetle attack, and resin yields decreased rapidly, as they did in 1986. Consideration of environmental conditions and ontogenetic stage of host trees is extremely important in attempts to assess the effects of manipulative treatments on tree susceptibility to beetle attacks. Further, they illustrate the dynamic nature of tree resistance to beetle attack in the absence of abnormal stresses. Simultaneous study of environmental conditions, as well as physiological changes associated with ontogeny of trees, can effectively support research on interactions between bark beetles and host trees.