Larch sawfly (Pristiphora erichsonii)

From Bugwoodwiki

U.S. Department of Agriculture, Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State and Private Forestry, Forest Insect and Disease Management., Broomall, PA. p. 123, illus.(USDA Forest Service, Northeast Area State and Private Forestry Publication. NA-FR-4)

The larch sawfly is the primary defoliating insect of native and most exotic species of larch. It attacks old foliage first, but at high population densitites will completely defoliate trees. On good sites, larch trees can withstand 15 or more successive defoliations before mortality occurs. On less favorable sites, mortality may follow three or more complete defoliations.

Winter is spent in the prepupal stage. Pupation is completed in the spring, and most adults appear from mid-May to August (although some pupae may pass a second winter before emerging as adults). Sawfly eggs are deposited in the late spring in slits cut into the bark of new terminal growth. This causes the twig to curl or "pigtail," with the egg slits on the inner curve. The long adult-emergence and egg-laying period results in larvae of various sizes being present throughout most of the summer. The larvae, which are pale grayish-green with black heads, tend to feed in colonies. The larval feeding period lasts about 20 days.

When mature, the larvae drop to the ground and spin the overwintering cocoon in the duff layer. New cocoons are light brown and about ½ inch long. There is usually one generation per year.