Forest Tent Caterpillar in 2002: What's On The Menu?

The forest tent caterpillar is expected to be widespread again in northeastern Minnesota this summer. About 7.7 million acres were defoliated statewide in 2001. The acreage is expected to be lower this year but there will still be lots of caterpillars and defoliation, according to the Minnesota Department of Natural Resources (DNR).

map of potential defoliated area

The forest tent caterpillar (FTC), Malacosoma disstria, is a native defoliator of hardwoods, especially, aspen and birch trees in northern counties; and basswood and oaks in central and southern counties. The caterpillars are often mistakenly called "armyworms".

The cool spring weather has delayed the caterpillar hatch but is not likely to have any other impact on the population size. Caterpillars will probably hatch by mid-May. The caterpillars don't eat much at first because they are so small. However, by late May or early June they will 3/4 to 1 inch long, moving around a lot and people will start to see the effects of their feeding on trees.

Usually, the caterpillars emerge from their eggs about the time the first aspen begin to get leaves. Defoliation will be noticeable by the end of May, but the heaviest defoliation and the biggest nuisance will occur in June.

The table gives a projected schedule of FTC life stages. The timing varies from year to year depending on weather and location. (Events occur near the earlier date when the spring is early; when May and June are hotter than average; and for locations south of Mille Lacs Lake.)

Projected schedule for FTC life stages and host tree foliage
Host tree buds break Apr 18 - May 10
FTC hatch Apr 18 - May 15
FTC major feeding period June 5 - June 25
FTC pupate End of June
Host trees refoliate Early to mid-July
FTC moths fly and lay eggs July 1 - July 15

Widespread outbreaks of FTC occur every 8 to 15 years and are usually 2 to 4 years in duration. Outbreaks peaked in 1922, 1937, 1952. 1967, 1978, and 1990. Populations collapse due to starvation, predation and parasitism. Populations of the "friendly flies", native parasites, build up as the FTC populations peak and can become a nuisance themselves.

In the forest, FTC defoliation usually does not affect forest health because caterpillar populations usually collapse before tree damage occurs.

When the FTC eat all the leaves off a tree, it will usually "reflush" new leaves by mid-July. Producing new leaves puts some stress on the tree because it must use more of its energy reserves. This can weaken the tree and make it more prone to other problems. But in most cases, the tree will survive the defoliation with no other problems.

For your trees, the best defense is to keep them in a healthy vigorous condition between and during outbreaks by keeping them well watered and avoiding damaging their roots or trunks.

Dealing with forest tent caterpillars can be a real nuisance

During the first three weeks of June, forest tent caterpillars can be a nuisance. They don't cause a health risk to humans, but the presence of hundreds or thousands of caterpillars is not pleasant. Young caterpillars spin threads and fall from trees onto picnic tables, patios and people. Large, mature caterpillars wander widely in search of food and often appear to migrate across roads and open areas. Resting caterpillars commonly form large clusters of thousands of caterpillars on buildings, tree stems, campers, and other stationary objects.

Homeowners may want to adopt two basic strategies

  1. First, identify the small trees, gardens, lawn furniture, and buildings you want to safeguard and ignore the rest (or at least try to). It takes a lot of time and energy to safeguard everything on your property.

  2. Second, be persistent. Some treatments may require daily monitoring and action.


  1. Before they hatch (any time from July to early-May) hand pick all the egg masses off of valuable plants. Destroy egg masses or dispose of them.

  2. When trees are dormant in the fall or early spring, spray the trees and egg masses with a dormant oil to kill the larvae in the egg masses.
  3. Some people think FTC are for the birds. Lure in birds with bird feeders, especially the pine and evening grosbeaks, and they'll do some of the work by eating the caterpillars.
  4. In May and June, hand pick caterpillars off plants and dispose of them. Later, gather and destroy cocoons.
  5. Caterpillars can be brushed off the house, lawn furniture, etc. with a stiff broom or knocked down by a stream of water. Bag, bury or compost the dead caterpillars.
  6. Rig up a barrier around your garden, patio, house or fruit trees (presuming you've already removed the egg masses from the twigs). Here are two popular methods:
    1. Use a wide band of masking tape, tree wrap tape or aluminum foil with a thick layer of Tanglefoot, petroleum jelly, or a coating of vegetable oil spray. This stops the caterpillars from crawling up your tree. This method needs at least daily attention and replacement of the goo. Remember to take the bands down by July 1st so the tree isn't injured by the tape or wrap. And don't put the goo directly on your tree trunk.
    2. Stake up a 2 foot high plastic sheet and weigh down the edge with sand or dirt. Then either apply a 2 inch wide band of Tanglefoot, petroleum jelly or vegetable spray near the middle or let the top 6-8 inches flop down to create a moving and unstable flap the caterpillars can't cling to. You may want to weigh down the flaps so caterpillars don't get flipped into the garden when the wind blows.
    3. Tape a 8 inch wide strip of plastic sheet to the foundation, all the way around your house. Apply a band of petroleum jelly, tanglefoot or vegetable oil to this strip to keep caterpillars from climbing up your house.
  7. forest tent caterpillars
  8. Under some circumstances, you may want to spray an insecticide to protect gardens and trees at risk. The production from fruit trees, raspberries, strawberries and other fruit and vegetable crops will be greatly reduced or lost if the plants suffer even moderate defoliation. Trees at risk include: newly planted woody ornamentals and tree saplings; trees recently damaged by construction, trenching, soil compaction, blacktopping, etc.; birches or oaks that have suffered 2 years of heavy defoliation or have active branch dieback; and, drought stressed trees.
      Insecticides have restrictions as to which plants and sites where they can legally be applied. If applying to shade and ornamental trees, the label should say it is for use on shade and ornamental trees. If applying insecticides to edible fruits and crops be sure it is labeled for that use.
     Microbial insecticides containing Bt, (bacterial products made of Bacillus thuringiensis ) are recommended to use for FTC control in the backyard because of their safety and the low toxicity to non-target organisms. Bt products are only toxic to caterpillars; they do not kill bees, flies, mosquitos, etc. However, Bt products are slightly slower to act since they must be eaten by caterpillars before they take effect. Apply Bt to the leaves of host plants; not to the bark or other non-edible materials. It is most effective on FTC when the caterpillars are small. Please read and follow label directions.
  9. FTC moths are attracted to lights during the nights in early July. Turning out your yard and exterior lights may reduce egg-laying on your trees and thus reduce next year's defoliation.
  10. The most important thing you can do for your defoliated trees is to keep them well watered. Supply 1 inch per week if you do not receive that much in rainfall from May 1 through September 1.
  11. Do not fertilize defoliated trees or use a weed and feed product on your lawn during an outbreak year. Fertilization encourages the tree to produce more leaves which puts an additional stress on the tree.

Additional information and brochures on the forest tent caterpillar can be obtained from your local County Extension Office or DNR Division of Forestry Office.

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