Yellowheaded Spruce Sawfly larva feeding on new needles. It is this stage of the insect’s lifecycle that causes the damage.
Typical Yellowheaded Spruce Sawfly damage to spruce trees which is often mistaken for natural causes.
Yellow Headed Sawfly
Although many different species of sawfly larvae cause damage to trees, the Yellow-headed Spruce Sawfly is the most common and serious pest in our region.
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 and can continue until the tree reaches a height of 8 meters.
The larval form of the insect resembles a caterpillar and voraciously feeds 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.
The adult sawfly is not a fly at all, but a stingless wasp. 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 their heads are reddish-brown in colour.
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 which hatches 5-10 days later. The larvae feed ravenously for 30-40 days until they mature. At maturity the larva drops to the ground and spin cocoons in which they overwinter. In the following spring the adult sawflies emerge, mate, and repeat the cycle. There is only one generation per year.
Active control measures for 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 repeated spray of Trounce is an effective control.
Source: City of Calgary
