In forests managed for timber and pulp production approaching typical rotation age, forest managers are encouraged to consider regenerating stands defoliated for three or more years in a row by forest tent caterpillar. By doing this, managers will salvage the value of timber that would otherwise die with additional defoliation or stress such as drought.
Defoliation from forest tent caterpillar usually causes little damage to healthy, vigorous trees, as most hardwood trees develop a second set of leaves by mid-July. Trees suffering drought stress, root damage, or are over-mature might be killed, but this allows other tree species to grow that are better adapted to the site.
The upside of forest tent caterpillar outbreaks is that natural enemies and other natural factors eventually control large populations.
- Starvation is the biggest reason outbreaks come to an end. After a year or two of buildup, the caterpillar population needs more foliage than is available. Starvation typically kills up to 95 percent of the caterpillars during the final year of an outbreak.
- A native fly (called the "friendly" fly because of its persistent habit of landing on people) becomes common in the environment as it parasitizes forest tent caterpillar cocoons. It strongly resembles a large house fly.
- The caterpillars are a food source for birds, rodents, and other animals.
- Bacteria and viruses that attack caterpillars help to knock down populations as they reach their peak.
Weather also plays a role in knocking down young populations of forest tent caterpillar. Late spring frosts that kill the first flush of leaves cause tiny caterpillars to starve. Cool, wet spring weather slows down caterpillar development and makes disease transmission easier.
Because of the natural ebb and flow of forest tent caterpillar populations, the DNR does not use insecticides to control large populations. Changes in forest dynamics and regeneration patterns have been linked to large outbreaks, and it is likely the forests we see in our state today are the result of periodic forest tent caterpillar outbreaks happening for thousands of years.
That said, if you want to reduce the numbers of forest tent caterpillars in your own yard, try these methods:
- Brush caterpillars and cocoons off houses, picnic tables, or decks with a stiff broom or brush or knock them down with a forceful spray of water.
- Turn off outside lights to avoid attracting adult moths. This may help reduce the number of egg masses laid on nearby trees.
Forest tent caterpillars rarely cause severe damage to trees, and the forest does not normally need the protection of pesticides. However, for some landowners such as resort owners, applications of an insecticide to control the caterpillars may be an option to consider. Private landowners may want to protect trees from defoliation, preserve trees' appearance, or provide nuisance control. Insecticide treatments only reduce caterpillar numbers and defoliation during May and June when the caterpillars are active; there is no effect on next year's population because moths from outside the treated area can enter and lay eggs. For detailed information on pesticide choices, see Forest tent caterpillars, UMN Extension.