Effects can be direct on the physiology of the organisms, and indirect through interactions with physiological changes in host plants or natural enemies. Considering the direct effects of warming, most insect species would survive more successfully over winter. Also, many insect species would shift their ranges toward higher latitudes and elevations, but this response is partly dependent on concomitant host and enemy changes. Species with flexible life histories could produce more generations per unit of time, as has already happened with the spruce bark beetle in Alaska. In summary, direct effects of warming are likely to increase the frequency, level, and geographical extent of disturbance by insect pests.
Indirect effects are more subtle and therefore even more difficult to predict.
Population changes of defoliators such as gypsy moth and spruce budworm are governed by complex interactions with hosts and natural enemies. For example, the effects of increased CO 2 on insects is mediated by changes in foliar nutrition and presence of defensive chemicals in foliage. Monitoring may be the only viable activity given that prediction may be impossible. If drought frequency changes as a function of altered precipitation, temperature, or both, trees and forests may become chronically stressed, leading to progressive deterioration in tree health and increasing susceptibility to secondary organisms and decline diseases. Pathogens could spread into new areas where they are presently limited by low temperature.