Problem pests

Over the last several summer seasons, Minnesota's anglers who camp or own cottages may have noticed an increase in feeding activity by a couple of insects that most years do minimal defoliation. However, with the right environmental conditions, such as a mild winter, early spring and low predation, these insects can survive and emerge in larger numbers, causing significant defoliation. They are the Forest Tent Caterpillar and the Spruce Sawfly.

What they do
The Forest Tent Caterpillar can reach outbreak proportions in some years, causing severe defoliation in localized areas of central and northern Minnesota. Areas of defoliation can cover many square miles, resulting in serious weakening of trees, especially during drought years. With several years of heavy defoliation, tree mortality can occur. Common host trees include aspen, basswood, birch, elm, maple (sugar maple, not red maple) and poplar.

Caterpillars emerge from overwintered eggs on branches in the spring about the time new leaves have formed. These larvae are gregarious and may be seen in clusters on branches and tree trunks. Tents of webbing may or may not be produced. The larvae are blackish, hairy and less than a half inch long at first. As they develop, the hairs disappear and a bluish color becomes evident along the sides of the body. When full grown they will be about two inches long and will have a row of white keyhole spots along the center of the back. Development of the caterpillars requires several weeks, and as the larvae increase in size, so does their appetite. Indeed, about 80 percent of their defoliation occurs as the larvae are reaching full size.
The other insect noted around the lakes in recent years is the Spruce Sawfly. This insect feeds on many types of spruce, but favors white spruce. Full-grown larvae are olive green above and lighter green below, and are about three-fourths inch long. The head is brown or reddish-yellow. Each side of the body bears a gray-green longitudinal stripe near the midline of the back, a broad one beneath this, and a darker one further down. There is also a dark spot above the base of each leg.

Adults, which are wasp-like in appearance, emerge from late May to mid-June. Eggs are deposited in slits cut in the current season's (new growth) needles, usually at the base and usually only one per needle. Larvae prefer to feed on new needles, but older larvae will eat old needles if the new needles have been consumed. The larvae become full grown in early to late July and drop to the ground. Here they spin cocoons in the duff or top soil area.

Be alert for early signs of defoliation by these or other defoliators this coming season. It might be a good idea to make a regular habit of checking your trees once a week from early to mid season, and if it appears as though defoliation is starting to occur, treat when the insects are still small and damage is still light. Don't wait until these insects are large and defoliation is severe.
If you need to control

You can still control the insects when they have reached their full growth potential, but by this time the damage is done and treatment will not make holes in leaves disappear, or needles on spruce re-grow. In other words, "revenge spraying" might make you feel better but will mostly be to no avail except, perhaps, to reduce infestation potential for the next season.

Since many of us have bird feeders, do not spray any insecticide directly toward the feeders, or allow spray to drift onto feeders. If insect control is necessary, it would be best to spray toward evening when bird activity is subsiding and wind is calm.

do not allow insecticide spray to draft toward the lake area. Some insecticides can be toxic to fish and other aquatic organisms.
Several insecticides with EPA registration for tree spring include: Diazinon, Dursban, Malathion, Pyrethrin and Sevin. Dipel, a bacterial-type insecticide, is very safe for the environment and will control Tent Caterpillars, but not sawflies.