Three different types of historical population data on the fall webworm, Hyphantria cunea Drury, are examined graphically. Oscillations in population density have peaks at intervals varying from 8 to 16 years, and are generally synchronous over very large areas in eastern and central Canada. Temperature in late summer is a key factor affecting the rate of change in density, particularly during the development of gradations, and this explains the synchronism of the oscillations. Climate and vegetation appear to be the main factors determining differences in mean density from one area to another. However, oscillations about the mean density have discrete limits, which are usually below the limits imposed by the food supply even where climate is generally favorable, and population decline often occurs despite favorable weather. These limits are imposed by other factors, such as the parasite Campoplex validus Cress. which has a delayed response to webworm density in eastern Canada.

It is concluded that similar analyses for other insects, perhaps based on the sort of data being accumulated each year by the Forest Insect Survey, would contribute to the development of population principles and would suggest many useful short-cuts when more intensive work is undertaken on additional species.