Yellowheaded spruce sawfly is a serious pest in our area and is found in most types of spruce. Early July is when the olive green larvae first appear and can be found on the branches of spruce. Generally speaking the larvae are similar to budworm in the fact they prefer new growth for feeding. Once the finish with this they have been known to move on to the older growth on the tree. One way of identifying this pest is to disturb them while on the branch. They will usually rise up both their head and tail and produce a liquid from their mouths.
Adults emerge in early to mid June and resemble a small wasp in appearance. These adults begin laying eggs in openings they create at the base of new growth needles. Unlike budworm these eggs are laid singularly. As the larvae emerge they immediately begin feeding on the edges of the new growth needles. As they feed and grow they will move on to feed on the entire needle. This process continues for about a month. At this time the larvae will fall to the ground to pupate and over-winter in the ground beneath the tree.
Weather, parasites, and predators generally will keep populations of sawfly under control. This pest tends to prefer small, young trees growing in the open and on its own. A tree growing for 3 to 4 years in your front yard is ideal for this pest to take advantage of. They begin by eating single branches but can move on to spread to the entire tree. If defoliation occurs repeatedly the tree may be killed. In the event you are required to use an insecticide ensure you follow all label directions carefully. These insecticides should be applied once the sawfly larvae begin to feed. Generally this will occur about 10 days after new growth sheds their bud caps. When your tree is young and small enough you can use cultural controls to combat this pest. By taking the time to handpick these larvae you will minimize the amount of insecticides used on your trees.