Larch and Tamarack
Appearance and Life Cycle
Adult larch sawflies emerge from May to August. They resemble small wasps, being approximately 8 mm long and black in colour with a broad, orange band around the abdomen. Females deposit eggs in slits cut under the bark of developing green terminal shoots. Full grown larvae are greyish-green with shiny black heads and are approximately 18 mm long. During the fall, the larvae drop to the ground, form cocoons and overwinter. The following spring or summer most of the larvae will pupate and emerge as adults. However, a portion of the larvae will overwinter for a second or third year before emerging as adults.
The larch sawfly may cause damage two ways: one is by defoliation during the larval stage and the other is from egg laying. When the egg slits are cut into the terminal shoots, especially early in the growing season, the shoots usually curl, become distorted, turn brown and die. Larval feeding may defoliate the entire tree. Complete defoliation does not kill the tree because of its ability to refoliate the same season. However, the damage does cause a thinning of the foliage, reduced annual growth and branch mortality. Several consecutive years of defoliation may cause tree mortality.
Larch sawfly populations are regulated by many natural factors including a fungus disease, parasites, predators and flooding of cocoon sites. Larch sawfly populations on a few ornamentals can be reduced by raking up and burning cocoons and debris from beneath trees in the spring. Larvae on small trees can be picked off and destroyed when they are first noticed. Young larvae are easily controlled with insecticides such as: carbaryl; diazinon; dimethoate; malathion and permethrin. Due to prolonged adult emergence the trees must be sprayed several times throughout the summer to achieve control.