In the spring, spruce budworm (Choristoneura fumiferana (Clem)), emerge as second-instar larvae from hibernacula. This emergence may be sudden and is readily detectable in the field. In northwestern Ontario, the sudden appearance of larvae in large numbers was especially spectacular in 1948; that year, very few insects were observed on May 11, but on the following two days, countless larvae could be seen on strands of silk streaming out from trees. Many of the insects were carried away by air currents. The surface of Big Canon Lake, a body of water close to four square miles in area, was covered with larvae, at an estimated density of 100 larvae per square foot of surface. At other times, however, emergence may be gradual or there may be two distinct periods of abundant emergence. Furthermore, the time of abundant larval emergence through six years has varied by as much as four weeks. Because of such differences, prevailing spring temperatures were examined to determine what relationship might exist between temperature and the time and character of emergence. This paper points out a relationship between emergence and April and May temperatures, and an apparent temperature threshold for emergence.