Native to North America, the forest tent caterpillar (Malacosoma disstria) has captured attention for hundreds of years. From mid-spring to early summer this Lepidopteran insect strips the foliage from trees, often extensive areas. Healthy trees can withstand several consecutive years of defoliation without suffering extensive damage or death.

If trees become stressed, or if more than three years of heavy defoliation occurs, trees may suffer branch dieback, and eventual mortality. These defoliating caterpillars build cocoons on the sides of buildings, which are hard to remove, and the larvae become messy as they crawl on sidewalks, patios, and driveways where they become squashed.

Susceptible Hosts

A number of deciduous trees including aspen (Populus), oak (Quercus), birch (Betula), ash (Fraxinus), maple (Acer), elm (Ulmus), basswood (Tilia), water tupelo (Nyssa), sweetgum (Liquidambar), red alder (Alnus), willow (Salix), and cherry (Prunus).

Pest

Malacosoma disstria

Symptoms

• Shotholes or stripping of the foliage from mid-spring to early summer.

Signs

• Shiny, black egg masses encircling small twigs.

• Young larvae are black, hairy, and about 1/8 inch (3 mm) long.

• Mature caterpillars are about two inches (50 mm) long with pale blue lines along the sides of the dark brown body. A row of white footprint-shaped spots run down the backs of the caterpillars, and numerous hairs line the edge of the body.

• Yellow, silk colored cocoons in a folded leaf, bark crevice, or other protected area. • Silken mats on the trunk or branches.

Life Cycle

• Larvae emerge in mid-spring and the caterpillars feed for about 5-6 weeks.

• In late spring and early summer mature caterpillars move from their feeding sites in search of a protected area to pupate in their silky cocoons.

• Approximately two weeks later adult moths emerge and lay shiny, black egg masses around small twigs.

• The eggs over winter on the twigs.

• One generation per year.

Distinguish from Look-alikes

• Many caterpillar species infest certain hosts making knowledge of susceptible species and proper identification of the host plant helpful in diagnosis. In addition timing of symptoms and particular marking characteristics are key distinctions between caterpillars.

• The forest tent caterpillar can be properly identified by the pale blue lines along the sides of the dark brown body, and the row of white footprint-shaped spots running down their backs.

• Eastern tent caterpillar (Malacosoma americanum) form tent-like nests in the crotch of branches rather than silken mats on the trunk or branches.

• Fall webworms form tent-like nests that enclose leaves at the branch terminals rather than silken mats on the trunk or branches.

Feeding from forest tent caterpillar larvae in mid spring and can completely defoliate foliage from trees. Healthy trees can tolerate a single defoliation event, however, multiple defoliation events can cause dieback and when combined with abiotic stress events.

The forest tent caterpillar is easy to control if treatment applications are timed correctly in the spring of the year. Soil applications are a feasible option to replace foliar sprays as it can be challenging to spray large urban trees. Lepitect soil applications provide greater window of application timing than foliar applications.

Egg masses may also be pruned out and destroyed in the spring or fall to reduce populations, but is very labor intensive.

U.S. Forest Service

University of Minnesota

Ohio State University

University of Massachusetts

Johnson, W.T., Lyon, H.H. 1991. Insects That Feed On Trees and Shrubs.