Hemlock Sawfly

Hemlock sawfly is a small wasp that feeds on the foliage of western hemlock throughout Southeast Alaska. Larvae feed on old foliage. This defoliation can result in growth reduction and top-kill. Defoliated trees appear thin and gray. Sawflies cause the greatest amount of damage when found simultaneously with black-headed budworm. Sawflies are more numerous and outbreaks tend to be of longer duration south of Frederick Sound, especially along Clarence Strait, the most southern and warmest area in Southeast Alaska. The largest and earliest recorded outbreak occurred in 1952-3 on four million acres from Frederick Sound to the south end of Prince of Wales Island. An outbreak in 1983-4 covered approximately 70,000 acres.

Sawflies typically overwinter as eggs on host foliage, but uncommonly spend the winter as pupae in forest litter. Females cut notches in the edge of hemlock needles where they deposit single eggs. Each female lays approximately 70 eggs. Eggs hatch in June and larvae feed gregariously on old foliage. Larvae are unable to complete their development cycle by feeding solely on new foliage. Pupation takes place in cocoons on host foliage or in forest litter, usually during August and September. Adults emerge in September and October.

Natural controls typically reduce epidemic sawfly populations to a few years. Warm and wet summers help reduce populations by favoring conditions for fungi that are known to infect immature forms of the sawfly. Cooler temperatures delay sawfly development and reduce opportunities for successful oviposition. Depletion of host foliage is likewise known to impact sawfly populations resulting in starvation and poor nutrition of actively feeding larvae. Soon after sawfly larvae spin a cocoon parasitic wasps lay their eggs on these larvae. A large number of cocoons, 39%, can be parasitized.

Young larvae usually feed in colonies and are almost completely black. Later instars turn dark green. Full grown larvae develop a dark longitudinal stripe along each side. Mature larvae will attack new foliage if all the older foliage is gone. Larvae in all stages of development have black head capsules. Mature larvae are 15-20 mm long. Sawflies pupate in brown, capsule-like cocoons that are approximately 7 mm long. Adults are small, thick-waisted wasps approximately 5-8 mm long. Females are larger than males, are olive green in color, and have serrated antennae. Males are black and have plumed antennae.

REFERENCES:
Hard, J.S., Torgersen, T.R. and D.C. Schmiege. 1976. Hemlock Sawfly. USDA Forest Service, Forest Insect and Disease Leaflet 31. 7 p.

Holsten, E.H., Hennon, P.E. and R.A. Werner. 1985. Insects and Diseases of Alaskan Forests. Alaska Region Report 181. Juneau, Ak. USDA Forest Service, Region 10, Revised October 1985. 181 p.

Torgersen, Torolf R. 1969. Hymenopterous parasites of the hemlock sawfly, Neodiprion tsugae Middleton, in Southeast Alaska, with a key to larval remains. J. Entomol. Soc. Brit. Columbia, 66. p53-62.