D. frontalis population fluctuations show pattern in both time and space. Time series analyses indicate a cyclical pattern to D. frontalis outbreaks (Turchin et al. 1991). The regular periodicity of outbreaks is difficult to explain with any climatic mechanisms (e.g., effects of precipitation patterns on suitability of host trees) unless there is some periodicity to the climatic patterns; none have yet been discovered (Turchin et al. 1991), although our revised understanding of moisture effects on tree physiology suggest that linear models may be an inadequate test (Fig. 1). It seems more likely that the cyclic tendencies of D. frontalis populations are due to some biological agent(s) that act in a delayed density-dependent fashion (e.g., natural enemies or competitors). D. frontalis population fluctuations are also characterized by large scale spatial synchrony. For example, severe D. frontalis outbreaks occurred in 1985-86 from east Texas through Louisiana, Mississippi, and Alabama (Price et al. 1992). This spatial synchrony is difficult to explain if population dynamics are primarily driven by natural enemies, because most natural enemies of D. frontalis are other arthropods that seem unlikely to move more than a few kilometers per generation. It seems more probable that the spatial synchrony is driven by forces that act on a regional scale, such as climatic variation. Even infrequent climatic events may be adequate to synchronize population cycles across broad geographic areas (Royama 1984).

Our research supports the hypothesis that D. frontalis population dynamics are influenced by both density-dependent interactions with T. dubius, and environmental effects on host trees. T. dubius adults at natural densities can kill up to 52% of the D. frontalis adults attempting to colonize a tree (Fig. 5). By comparison, moderate drought stress can increase resin flow in trees and reduce D. frontalis reproductive success by 63-85% relative to irrigated trees (Fig. 3 and 4). In both cases, these are probably underestimates of the potential effects on D. frontalis population dynamics. T. dubius larvae feed on D. frontalis larvae beneath the bark, but we presently can only estimate effects of adult predation. Similarly, tree water balance and physiological status probably influence the growth and survival of D. frontalis larvae, but we presently can only estimate effects on attacking adults. The expected effects of these two forces on D. frontalis population dynamics is quite different. T. dubius has a long development time, which makes it a good candidate for producing outbreak cycles, while climatic effects on host-tree suitability may influence the amplitude of outbreaks and synchronize population cycles on a regional scale.