Chapter 17 - Host Suitability, Predation, and Bark Beetle Population Dynamics

Abstract

This chapter discusses the hypothesis that outbreaks in the southern pine beetle (SPB), Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae) are generated by environmental factors that affect host-tree suitability, especially water availability. Many species of bark beetles (Coleoptera: Scolytidae) undergo dramatic population fluctuations that lead to extensive damage of coniferous forests. Several general models have been proposed to explain population variation in herbivorous insects based on the physiological condition of host plants. An alternative physiological model has been tested for the effects of water availability on the suitability of southern pines for bark beetles. In the first attempt of testing these predictions, Dunn and Lorio manipulated the water balance of 11-year-old loblolly pine using a combination of rain-exclusion shelters and irrigation. Once the attack of the host tree by SPB is under way, a large assemblage of natural enemies prey upon or parasitize various stages of SPB, both inside and outside the tree. To estimate the mortality inflicted by adult T. dubius on SPB, laboratory experiments were conducted in which SPB were exposed to different predator densities. Laboratory studies of T. dubius have indicated a development time from egg to adult of about 110–240 days, depending on temperature. SPB population fluctuations show pattern in both time and space. Even infrequent climatic events may be adequate to synchronize population cycles across broad geographic areas. Thanasimus dubius has a long development time, which makes it a good candidate for producing outbreak cycles, whereas climatic effects on host-tree suitability may influence the amplitude of outbreaks and synchronize population cycles on a regional scale.