Forest Tent Caterpillars

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The forest tent caterpillar (Malacosoma disstria Hübner), sometimes wrongly called armyworms due to the caterpillar’s habit of crawling together over the ground while searching for food,  is an important defoliator of deciduous hardwood trees. In addition to eating leaves of desirable trees, these bugs leave behind unsightly mess of fecal maters, which may render one’s backyard useless.

HOW DO I KNOW IF I HAVE FOREST TENT CATERPILLARS?

  • One of the identifying features of forest tent caterpillars is their damage and symptoms: the crowns of heavily infested trees may appear thinner and stripped of their leaves.

  • Even though their name suggests it, forest tent caterpillars do not build tents. Rather they leave silken mats on tree trunks and branches where they travel and rest.

  • When food is scarce, caterpillars congregate on side branches, move toward the main trunk, and then migrate downward and disperse in yards, on buildings, and on sidewalks in search of more food.

  • Forest tent caterpillars will build cocoons on houses, lawn furniture, and other structures.

  • The forest tent caterpillar adult stage is a moth, which is attracted to light. The moths can become a nuisance factor at night, especially when they are found indoors hovering around light sources.

WHAT DO FOREST TENT CATERPILLARS LOOK LIKE?

  • Forest tent caterpillars have pale bluish lines along the sides of a brownish body and reach about 2” in length at maturity.

  • Forest tent caterpillars have a row of 10-12 white footprint-shaped markings down the middle of their backs.

  • The larvae are lightly covered with whitish hairs.

  • Adult moths are buff-brown, with darker oblique bands on the wings.

  • Egg masses of 100 to 350 eggs encircle the twigs and are covered with frothy, dark brown cement (USDA Forest Service 1996).

There are many types of caterpillars that can be mistaken for forest tent caterpillars:

Eastern Tent Caterpillar (Malacosoma americanum)

  • Central yellow stripe

  • Small blue spots on the side

  • Black head

Gypsy Moth Caterpillar (Lymantria dispar)

  • Prominent red or blue dots

  • Beige head with dark marks

ARE FOREST TENT CATERPILLARS DANGEROUS?

No. Tent caterpillars do not bite or sting, although some people may have an allergic reaction to handling them.

WILL FOREST TENT CATERPILLARS HARM MY TREES?

  • Forest tent caterpillars will probably not kill your trees or even cause permanent damage.

  • For most trees, forest tent caterpillar infestations over several years result in only a slower growth rate.

  • As forest tent caterpillars continue to grow, they eat more and more leaves, causing considerable damage to the tree’s foliage.

  • Any defoliation from forest tent caterpillars usually causes little damage to healthy trees and most hardwoods develop a second set of leaves by mid-July.

  • However, if the trees are already stressed from drought, root damage, or age; defoliation can make the trees more vulnerable to disease or other insects that may eventually kill the trees.

WHY ARE FOREST TENT CATERPILLARS IN MY TREES THIS YEAR?

  • In Minnesota, forest tent caterpillars outbreaks typically occur every 6-16 years (MN DNR). 

  • Outbreaks of forest tent caterpillar can last from three to seven years.

  • During the peak of an outbreak, caterpillars can number from 1 to 4 million per acre. (MN DNR)

  • The last outbreak of forest tent caterpillars was in 2001. (MN DNR)

  • Extreme weather conditions, such as deep freezes during mid-winter, low temperatures in early spring, as well as high temperatures later in the spring, can cause many forest tent caterpillars to die, reducing the impact of an outbreak.

WHAT DO FOREST TENT CATERPILLARS EAT?

  • Forest tent caterpillars prefer the leaves of hardwoods, such as aspen, birch, basswood, ash, and oaks.

  • Forest tent caterpillars are known not to feed on balsam fir, spruces, pines, or red maples.

  • Adults have siphoning mouthparts and do not feed.

WHAT IS THE LIFECYCLE OF THE FOREST TENT CATERPILLAR?

  • There is one generation of forest tent caterpillars each year.

  • In spring, eggs usually hatch when enough foliage of the host tree is available for the first larval instar feeding activity and when the temperature is at 50°F or more.

  • The time varies with weather and locality.

  • Caterpillars go through four molts during the next five to six weeks.

  • By the end of June, fully-grown tent caterpillars seek protected places around buildings, bushes, and lawns to spin their yellowish cocoons and pupate.

  • The presence of these hard-to-remove cocoons may frustrate homeowners.

  • Adult moths emerge about 10 to 45 days later (usually by mid-July).

  • When first emerged, females will secrete a sex pheromone to attract males for mating.

  • Mated females usually deposit their eggs in masses around small twigs (eggs look like a wide gray ring).

  • Each female, may lay an average of 150 eggs per egg mass.

  • Adult moths will not live for more than two weeks.

  • Forest tent caterpillars over-winter in the egg stage.

TWO INTERESTING FACTS ABOUT FOREST TENT CATERPILLARS:

  1. In 2001, forest tent caterpillars defoliated more than 7.5 million acres of hardwoods, the most ever recorded in Minnesota.

  2. Only the caterpillar stage has mouthparts capable for biting and chewing leaves and it is this stage that is responsible for all the damage.