In the course of studies on Neodiprion swainei Midd. carried out in 1958 and 1959 in the region of Lake St. John, Que., mass migrations of Neodiprion swainei Midd. were observed. This paper gives a brief description of this phenomenon, which has never been reported heretofore, and discusses some of its implications.

On hatching from the eggs which are laid on the needles of the current growth of jack pine, Pinus banksiana Lamb., N. swainei larvae crawl to the old foliage where they feed in colonies of 40 to 70 individuals. At times, when larval populations become so great that all the foliage is destroyed, mass migration begins. The larvae gather in large clusters at the top of the tree where they remain for two or three days. At this time some of the larvae moult. The larvae then abandon the tree by crawling down the tnink or by dropping to the ground. They then creep on the ground, up and down stumps, in search of jack-pine trees with foliage. The migrating larvae are pale yellow in colonr, probably as a result of their starved condition, and large nutnbers perish before they can become established on proper food. Those that manage to reach new food supplies quickly recover their original dark-green pigmentation.