Weather and Climate in Forest Entomology

Abstract

Some basic references to the general field of insects and climate are provided, and some of the more recent research trends are indicated. However, to avoid compressed, confusing descriptions of the diversity in habits and requirements found among insects, the reactions of only two dissimilar species are described in detail, to show how field and laboratory observations lead to climatological investigations. The two species, the spruce budworm, Choristoneura fumiferana (Clem.), and the forest tent caterpillar, Malacosoma disstria Hbn., are both native to North America. Although active during the same part of the year, they have different physical requirements. For example, although both are adversely affected by prolonged rain, the tent caterpillar develops better during moderately warm, humid, partly cloudy weather, whereas the spruce budworm develops best in dry, sunny weather. Hence, in areas where both occur, their outbreaks begin during different years that, in each case, are preceded by a few years during which the specific favorable weather recurs.

Before spruce budworm outbreaks, the area is influenced most by northern and western air, whereas, before tent caterpillar outbreaks, it is influenced more by southern and southwestern air. In general, tent caterpillar outbreaks tend to begin in suitable parts of the Boreal Forest two to four years after the continental complex of storm tracks has shifted northward, or after a marked increase in cyclonic activity. On the other hand, spruce budworm outbreaks tend to occur after southward shifts of the tracks, or after decreased cyclonic activity.

Several fields for further research are suggested, including a study designed to show effects of short-term climatic fluctuations on population trends of groups of insects occupying similar and dissimilar types of microhabitats on a single tree species.

Contribution No. 67, Forest Biology Division, Science Service, Department of Agriculture, Canada.