Spruce budworm (Choristoneura spp.) biotype reactions to forest and climate characteristics

The spruce budworm (Choristoneura fumiferana) is the most destructive insect defoliator of forests in North America. Climatic influences on this species' life history are considered a major factor in restricting the extent and intensity of outbreaks. We examine the life history traits of the spruce budworm and related Choristoneura populations with respect to forecasting the conifer-feeding responses of these insects in changing environments. Analysis of the evolutionary relationships between Choristoneura entities, including their hybridization, genetic distances, and their degree of sympatry leads us to distinguish 15 possible Choristoneura 'biotypes'. Population trend has been associated with recruitment to the feeding stage, and two indicators of recruitment, egg weights and phenological development, are both 'biotype' and climate dependent. Among Abietoid feeding 'biotypes' and among spruce budworm populations, those from locations with extreme winters tend to have heavier eggs than those from the more benign environments. In spruce budworm, this genetically based adaptation allows populations to increase their potential recruitment substantially where winters are mild. All biotypes feed on the newly developed shoots of their host trees in spring, and are thus vulnerable to the uncertain timing of budbreak. Genetic control of spring emergence is weak so larvae from a single family typically exit from hibernacula over a prolonged period. This guarantees some synchronization with budburst. However, hybrid populations have high heritabilities. This allows rapid adaptation to new conditions (e.g. mixed host-species stands). Geographic variation in phenological development after establishing feeding sites is largely genetically controlled. The importance of variation in these traits is examined with respect to competing population dynamics theories to evaluate their utility in forecasting future trends in defoliation. We finish with a plea for jointly using alternative approaches in forecasting spatiotemporal patterns of defoliation.