AIR-MASS CLIMATOLOGY OF ONTARIO NORTH OF LAKE HURON AND LAKE SUPERIOR BEFORE OUTBREAKS OF THE SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (CLEM.), AND THE FOREST TENT CATERPILLAR, MALACOSOMA DISSTRIA HBN. (LEPIDOPTERA: TORTRICIDAE; LASIOCAMPIDAE)

Previous work showed that ideal physical conditions for the development of the spruce budworm, Choristoneura fumiferana (Clem.), occur when the weather is relatively dry and clear. These conditions have tended to occur in summers when the annual number of cyclonic centers passing over the area was declining, and have preceded past outbreaks. On the other hand, the physical requirements of the forest tent caterpillar, Malacosoma disstria Hbn., include warm, humid, cloudy weather during much of the larval stage, and outbreaks of this species in Ontario have begun after an increase in the annual number of cyclonic passages. While the annual number of cyclonic passages is declining in periods before spruce budworm outbreaks in northern Ontario, the number of these passages in the summer months falls below average. Furthermore, the majority of the centers that do pass in these months contain air masses of polar continental or maritime origin. The more humid southwestern air masses are usually barred from the area by a southward shift of the whole circulation pattern. This situation is reversed in periods before M. disstria outbreaks. While the annual number of passages is increasing, the number occurring in the summer months is above average, as is the proportion of southwestern air masses occurring in these months. Northern and western air masses are usually active farther north, owing to a northward shift of the whole circulation pattern.