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Tent Caterpillars

Forest Tent Caterpillar and Eastern Tent Caterpillar

forest tent caterpillars on trunk
Forest tent caterpillars on trunk
James Solomon
USDA Forest Service,
http://www.forestryimages.org/
eastern tent caterpillars on tent
Eastern tent caterpillars on "tent"

Forest tent caterpillars (FTC) (Malacosoma disstria) and eastern tent caterpillars (ETC) (Malacosoma americanum) are native to New York State. Population numbers vary over the years from very few and not noticeable, to many and very noticeable defoliation of trees. The ETC is not a major forest threat, as it prefers fruit trees including ornamental crabapples and pears; it is more of a pest in urban and suburban areas and orchards.

How Caterpillars Damage Trees

Tent caterpillars and gypsy moths eat leaves in the spring. Deciduous trees (trees that lose their leaves each fall) can regrow a new set of leaves by July and can usually withstand 2-3 successive years of defoliation (removal of leaves) without being killed. However, defoliation does reduce the vigor and resistance of the tree; it becomes more susceptible to pests and diseases. Mortality can occur when other stresses such as disease or other insect outbreaks attack trees in the same year. Evergreens are eaten when populations of gypsy moths are very high. Evergreens do not regrow leaves as easily as deciduous trees and can die as a result of complete defoliation.

Defoliation is not necessarily a death sentence. Healthy deciduous trees can survive 2-3 successive years of defoliation without dying. Trees defoliated early in the season often grow a new, smaller set of leaves in July once tent caterpillars and gypsy moths stop feeding.

Insect Identification and Look-Alikes

If you notice dark hairy caterpillars eating the leaves on your trees, you may have forest tent caterpillars, eastern tent caterpillars or gypsy moths. See the Comparison Chart for distinguishing characteristics and photos of these three species.

The easiest way to tell these caterpillars apart is by the patterns on their backs. The gypsy moth caterpillar has five pairs of blue spots followed by six pairs of red spots along its back. The eastern tent caterpillar has a white line down its back with light blue and black spots on its sides. The forest tent caterpillar has white footprint-shaped marks down its back and light blue stripes on its sides.

Another caterpillar that is sometimes confused with the eastern tent caterpillar is the fall webworm, because it also makes tents in trees and eats leaves. The fall webworm differs from the eastern tent caterpillar in several ways:

fall webworm tent on end of tree branch
Fall webworm tent on end of tree branch
  • A fall webworm tent always begins at the tips of branches and gradually extends down the branch toward the trunk
  • Fall webworms feed on foliage inside the tent. Eastern tent caterpillars make their tents in the forks of branches and feeds on leaves outside the tent.
  • The fall webworm is hairy, pale green or yellow, and has black or reddish spots along its back and there is usually more than one generation each year.
  • Fall webworms make their tents in July and August, while eastern tent caterpillars make their tents in spring.

Fall webworms are usually not considered harmful to trees, except for the aesthetic quality of the tents. Fall webworms are harmful when they feed on the regrowth of trees that were defoliated earlier in the season by other caterpillars.

Harmless to Humans

Contact with these insects is generally not harmful with a few exceptions. Some individuals may develop skin rashes or irritations from contact with gypsy moth hairs, including those on the outside of egg masses. Researchers have found that ingestion of eastern tent caterpillars only by pregnant horses can cause them to miscarry (mare reproductive loss syndrome). There is no evidence that ingestion of eastern tent caterpillars is harmful to humans or other animals.

forest tent caterpillar old egg mass
Forest tent caterpillar old egg mass

Control Options

For homeowners, the first option to consider is to do nothing. Healthy deciduous trees usually survive defoliation and grow back a second set of leaves in July. Natural control of these species is happening all the time and include birds, rodents, parasites, and diseases. Extremes in temperature can also reduce population numbers.

Manual control on individual trees includes hand removal of egg masses, inhabited tents and pupa, and installation of sticky tree wraps on trunks to capture caterpillars as they move up and down trees. Do not leave egg masses on the ground; drop them in a container of detergent. Do not attempt to burn tents while they are on trees. This is dangerous and hazardous to the health of the tree.

Herbicide Treatment Options

Tent caterpillars are native and a natural part of our ecosystem, and gypsy moths have "naturalized" in our forest communities. These caterpillars will always be around, sometimes in small, unnoticeable numbers. If dense concentrations of tent or gypsy moth caterpillars cause a decline in the trees' health or threaten an economic resource such as a sugar bush, spraying may be an option.

gypsy moth feeding on branch
Gypsy moth feeding on branch
Tim Tigner, Virginia Department of Forestry,
http://www.forestryimages.org/

Various insecticides for tent caterpillars and gypsy moths are available at garden centers. Insecticides are divided into two general groups: microbial/biological and chemical. Microbial and biological pesticides contain living organisms that must be consumed (eaten) by the pest. They are most effective on small, young caterpillars. As they mature, caterpillars become more resistant to microbial pesticides. The most common microbial and biological insecticide is Bacillus thuringiensis (Bt). Bt occurs naturally in soil and on plants. It is harmless to people, animals, and plants, but does affect young moth and butterfly larvae. When Bt is eaten, the caterpillar becomes paralyzed, stops feeding, and dies of starvation or disease.

Chemical insecticides are contact poisons. These chemicals can have a potential impact on a variety of beneficial insects (such as honeybees), so they should be used wisely.

Professional pesticide applicators can be found in the yellow pages under Tree Service. In order to use restricted insecticides, applicators must be certified. For a list, see the link to NYS certified pesticide applicators.

In certain cases it may be economically feasible to spray large areas. Maple syrup producers may be interested in aerial spraying since severe defoliations can reduce maple syrup production.

Spraying is not effective against pupae or egg masses, and is less effective once caterpillars reach 1 inch long. Nesting birds, beneficial insects, and other animals could be endangered by use of chemical insecticides.

There is currently no State program in New York to spray for tent caterpillars of gypsy moths on State or private property. There are no State funds available to property owners for managing tent caterpillars or gypsy moths.

Woodlot Recommendations

We recommend delaying timber harvests in defoliated areas for two or three years after an outbreak to minimize additional stress on trees. Time will also allow you to see which trees remain healthy and alter harvesting plans if necessary. (See link to NYS DEC Forest Tent Caterpillar Defoliator Report 2005.)

Predicting Future Defoliations

tree defoliated by forest tent caterpillars
Tree defoliated by forest tent caterpillars
James Solomon, USDA Forest Service,
http://www.forestryimages.org/

FTC and ETC are natural parts of our native forest ecosystem and are here to stay. Their populations fluctuate - after a few years of high numbers, their populations usually drop. Particularly large populations of tent caterpillars typically occur every 10 years and usually last 2-3 years.

If you are interested in sampling your woodlot or forest to determine the likelihood of defoliation by FTC for the following growing season, sampling protocol is provided (see important links). If you do sample your woodlot or forest, your data is valuable to us for tracking annual populations. We would appreciate if you mail or email you sampling data to:

NYSDEC
Division of Lands and Forests, Bureau of Invasive Species and Ecosystem Health
625 Broadway
Albany, NY 12233-4253
Telephone: 518-402-9425 Fax: 518-402-9425
Send an email to the Forest Health team.


More about Tent Caterpillars:

  • Caterpillar Comparison Chart - A chart comparing the characteristics of Forest tent caterpillars, Eastern tent caterpillars and Gypsy moths