What is Forest Tent Caterpillar?

Introduction
Forest tent caterpillar (FTC), Malacosoma disstria HUBNER, is native to North America and is a defoliator of hardwood trees in Pennsylvania. Like other species of Lepidoptera, it undergoes a complete life cycle and has four life stages- egg, larva, pupa, and adult. This is a cyclic pest in Pennsylvania with populations occasionally reaching outbreak proportions. When populations are high FTC can cause complete defoliation of affected host species. Unlike the similar eastern tent caterpillar, FTC DOES NOT make a silk tent as its name suggests.

Description and Lifecycle
FTC completes one life cycle each year in Pennsylvania. Eggs are laid in a cylindrical mass surrounding a twig in early or mid-summer. Egg masses contain an average of 150-200 eggs and are coated with a foam-like substance called spumaline. Egg masses are preferentially deposited on upper canopy twigs. The egg stage is the over wintering stage for FTC, and egg masses are present until larvae hatch out in early spring.

FTC larvae first start to hatch out in April in Pennsylvania, often at the same time the leaves of host trees first start to appear. Local climate variation may hasten or delay the hatch in certain areas. The larval stage is the main growth stage for FTC and is also the life stage that causes damage. When a caterpillar larva grows, it must periodically molt to accommodate its new size. The periods between molts are referred to as instars. FTC larvae complete five larval instars over a five to six week period. The larvae are generally dark in color, have long hairs, blue stripes, and white markings. The white marks can be variable in shape, but are often described as "footprint-shaped" or "keyhole-shaped", and are dorsally located. These markings are the main diagnostic feature of FTC larvae. There are many similar looking caterpillar species including the eastern tent caterpillar, Malacosoma americanum, and the gypsy moth, Lymantria dispar. The eastern tent caterpillar can be identified by the presence of a "white stripe" in place of the "footprint-shaped" marks. Gypsy moth can be recognized by its' paired red and blue spots.

The last larval instar seeks a protected area, such as a bark crevice, and spins a whitish, silk cocoon, about an inch long, and pupates within. The pupal stage lasts about three weeks and is usually complete in mid-June in Pennsylvania. It is during this stage that metamorphosis into the adult form occurs.

Adult FTC moths are small, with a wingspan ranging between 1 to 1.5 inches. FTC moths are pale yellow-brown, with two thin, dark-brown, vertical stripes on the forewing. Sometimes the area between the two lines on the forewing is shaded brown. Adults live for about five days, and in that time mate and deposit new generation of eggs.

Hosts
FTC will feed on a number of tree and plant species throughout its range. In Pennsylvania the preferred host species differs by region. Historically, outbreak populations have occurred on sugar maple and aspen in northeastern and north central Pennsylvania, and on red and scarlet oak in southern Pennsylvania. FTC will feed on species such as ash, birch, cherry, and basswood, but they are less preferred. Red maple and coniferous species are avoided FTC.

Damage
FTC causes damage during its larval stage by defoliating host species. Most trees recover from one or two seasons of FTC defoliation. Resulting damage includes severely reduced growth, some branch dieback, and reduced sap flow. Multiple seasons of defoliation can lead to mortality. When combined with other stressors like anthracnose, drought, or other defoliators, mortality can occur in as little as one season. Even though historically, more oak has been defoliated in Pennsylvania, sugar maple is the species most severely affected.

Damage to host trees is most severe during outbreaks. Major outbreaks have been recorded periodically in Pennsylvania since the mid-1930's. The most notable include: a 204,800 acre event in Southwestern Pennsylvania on oak from 1969 to 1972, a 29,000 acre event on sugar maple in Northeastern Pennsylvania in the early 1980's which caused 50 percent mortality of over story sugar maple, and 180,000 acres in 1993-1994 on sugar maple in the north and oak in the south. Again, in the 1993-1994 outbreak, sugar maple mortality was high in Northeastern Pennsylvania, with some landowners reporting over 90 percent mortality. This was attributed to impact of additional stressors in the form of anthracnose and an early season frost.

Outbreak populations usually collapse after a few seasons due to the buildup of populations of natural enemies like parasitic flies and wasps. One fly in particular, Sarcophaga aldrichi, or friendly fly, is extremely important in hastening an FTC population collapse. Friendly flies parasitize the pupae of FTC, disrupting completion of the life cycle. In FTC outbreak years, numbers of friendly flies can reach nuisance levels, and they are readily noticed when they swarm on people, animals, and property.

Some naturally occurring pathogens like nuclear polyhedrosis virus (NPV) and the fungus Furia crustosa have been shown to negatively affect FTC populations. However, these pathogens are not usually considered be significant in the collapse of large FTC populations.

Sometimes, conditions for all factors that affect FTC populations allow for an outbreak population to last as many as six years. Because many factors influence the longevity of an outbreak, it is difficult to accurately predict how many years a given outbreak will last.

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