Young spruce trees in Calgary may be subject to infestation by an insect pest known as the yellow-headed spruce sawfly (YHSS). The sawfly infests and feeds on spruce trees that are growing singly or on the edge of a group of trees. Infestations can begin when a tree is only 3-5 years old. The infestation can continue until trees reach a height of 8 meters.
The larval form of the insect resembles a caterpillar and is a voracious feeder on spruce needles. The damage is a complete defoliation of branches and twigs with only a few chewed brown needle stubs remaining. 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 current year's growth on lower branches close to the ground. On repeatedly infested trees these lower branches will appear "skeletonized". Repeated severe infestation over a three year period can kill trees.
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 structure) has serrated teeth resembling those of a wood saw. The adults are reddish brown in colour and 8-10 mm long. When first hatched the sawfly larvae are 3-4 mm in length with yellowish bodies and yellow-brown heads. The mature larvae are 16-20 mm in length and dark glossy green with lighter stripes down the sides, and the heads are reddish-brown in colour.
Lifecycle
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 30-40 days until they mature. At maturity the larva drop to the ground and spin cocoons in which they overwinter. In the following spring the adult sawflies emerge, mate, and repeat the yearly cycle. There is only one generation per year.
Control
YHSS 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 for sawflies, and larvae on stripped trees may succumb to starvation, thus providing a small level of control. However, active control measures are still recommended.
Active control measures for YHSS 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 is an effective non-chemical method of control.
The City of Calgary inspects individual spruce trees in parks and green spaces and applies low environmental impact controls as necessary. However, we do not spray spruce trees along residential streets or on private land. For trees along residential streets we recommended spraying the trees with high pressure water soon after the larvae hatch or removing the sawflies by hand.
![](http://www.calgary.ca/imageserver/plumtree/portal/public/img/sp.gif")
