GardenLine | Common Saskatchewan Pests | Yellow-Headed Spruce Sawfly

The yellow-headed spruce sawfly (Pikonema alaskensis) is a serious pest of ornamental and shelterbelt spruce trees. The larval form which resembles a caterpillar is a voracious feeder on spruce needles. A severe infestation over a three year period can kill trees. Trees from the ages of 3-5 years in an open growing situation are the most susceptible to infestation. The infestation can continue until the tree reaches a height of 8 meters.

Damage appears as complete defoliation with a few chewed brown needle stubs. The larvae feed first on new foliage and continue on to consume the older needles. The damage is likely to be noticed first in the top most growth.

Description

The adult sawfly is not a fly at all, but a stingless wasp. The insect is called a sawfly because the female's ovipositor (an egg laying organ) has serrated teeth resembling those of a wood saw. The adults are reddish brown in colour and 8-10 mm long. The larva are caterpillar like. When first hatched the sawfly larva are 3-4 mm with yellowish bodies and yellow brown heads. The mature larva bodies are 16-20 mm and dark glossy green with lighter stripes down the sides. The heads are reddish brown in colour. If disturbed the larva will secrete spittle from its mouth and arch both its front and back end.

Life Cycle

The adult females lay eggs in June. They make a slit in the bark of a spruce branch at the base of a needle and deposit a single egg. Eggs hatch 5-10 days later. The larva feed ravenously for 30 to 40 days until they mature. At maturity the larva drop to the ground and spin cocoons in which they over winter. In the following spring the adult sawflies emerge, mate and repeat the yearly cycle. There is only one generation per year.

Control

The yellow-headed sawfly has some natural predators, such as birds, and small mammals. However natural predators rarely provide satisfactory control. Trees that have been stripped of needles have fewer feeding and nesting sites, and larvae on stripped trees may succumb to starvation, thus providing a small level of control. Active control measures should be considered.

Active control measures for the yellow-headed spruce sawfly, should begin in mid-June. Regularly inspect trees for signs of damage or colonies of feeding larvae. It is possible, to simply remove the larvae by hand, particularly if there are only a few spruce trees, and a low level of infestation. For situations where there are many trees, a high pressure blast of water with a little dish soap mixed in to the spray is an effective non- chemical method used for control. One teaspoon of dish soap per gallon of water should be strong enough to offer control.

Chemical sprays are effective on the yellow- headed sawfly. Permethrin is effective and is sold under tradenames such as Ambush. Permethrin quickly paralyses insects, and has a low toxicity to vertebrates. Other chemical controls are the common insecticides, malathion, diazinon, and Sevin. Malathion should only be used when temperatures are above 20 degrees C. These chemicals have a higher toxicity than permethrin.

When using chemical controls be sure to follow the label directions and take proper safety precautions.

Sources:

Forestry Canada. Yellow-headed spruce sawfly. Pamphlet

Johnson, Warren T., Howard H. Lyon. Insects that Feed on Trees and Shrubs. Ithaca: Cornell University Press, 2nd Ed. 1988.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.