Forest Insect and Disease Newsletter

Frequently asked questions: Controlling forest tent caterpillars

What are the reasons to control forest tent caterpillars (FTC)?

  1. To protect valuable ornamental foliage and perennial plants from defoliation. Examples would be fruit trees, small fruit crops and gardens. Because these plants occur in small areas, they are suitable for treatment from the ground.
  2. To reduce nuisance to people caused by caterpillar populations. FTC nuisance occurs from Memorial Day to mid-June during the third and fourth years of an outbreak. Homeowner's or lakeshore associations, resorts or private campgrounds may wish to reduce nuisance. Due to the size of the trees, spraying must be done by aircraft.
  3. To prevent defoliation of high value oaks or basswoods that suffered drought stress or FTC defoliation or root injury from construction in recent years. These already weakened trees may die from the additional stress of defoliation and continued drought. Due to the size of the trees, spraying must be done by aircraft.

What are some of the proven methods for controlling FTC?

  1. Before the egg masses hatch (any time from mid-July until April of the next year) hand-pick the egg masses off valuable perennial plants and shrubs. Egg masses should be burned, buried or sealed in the trash. Do not compost them, as they may still hatch.
  2. Hand pick caterpillars off plants and put them in a soapy water solution to kill them.
  3. Build a 24" high wall of plastic sheeting to enclose the area you've chosen to protect. Spray the plastic with vegetable oil to prevent the caterpillars from climbing up the wall. Repeat oil application daily or as needed. Caterpillars already on plants within the enclosure are unaffected.
  4. Use a product like Tanglefoot on the trunks of shrubs and trees. This prevents caterpillars from adjacent areas climbing up treated trees. Caterpillars already up in the tree are unaffected.
  5. Spray an insecticide to kill young caterpillars. Products include chemical insecticides, insecticidal soaps and biological insecticides. Products containing Btk are preferred because they are bio-rational and organic.

Will spraying this year reduce the FTC problem next year?

No, spraying will only affect this year's population of caterpillars. Since the moths are strong fliers, forested areas will be reinvaded during moth flight this summer and eggs will be laid on trees and they will be not be affected next year when they hatch. Spray programs can reduce defoliation and nuisance but do not affect the next year's FTC population. Spray programs can actually prolong local outbreaks, especially if the areas are sprayed annually.


Can I spray to protect the trees in my backyard?

If you have fruit trees or small, ornamental trees, you can probably spray them from the ground with hand held equipment and be fairly effective. Do not spray fruit trees with chemical insecticides while they are flowering. This could seriously affect bees and beehives in the area and prevent your fruit trees from being pollinated. If you have large shade trees, you physically will not be able to reach the foliage spraying from the ground and invasion of caterpillars from adjacent, unsprayed trees will occur. Spraying trees of this size likely will require a licensed commercial applicator or aerial application.


Which insecticides can be sprayed? Are some better than others?

The DNR strongly recommends that private and public landowners use biological insecticides. Biological insecticides containing Btk, which is a bacterial preparation, only kill caterpillars that eat leaves treated with the product. The Btk stands for Bacillus thuringiensis var. kuristaki, the active biological ingredient in the insecticide.

Because of the limited toxicity, Btk products are the favored insecticides to use for nuisance control or for use near lakes and homes. Btk insecticides are slower to act since they must be eaten by the caterpillars before they are effective. If eaten by bees, ladybugs or adult moths and butterflies, those insects will not be harmed.

Chemical insecticides can also be used but would normally be a second choice after Btk, due to safety considerations. Commonly recommended chemical insecticides can be found on the MN Dept. of Agriculture website. Some of these products can also kill bees and other non-target organisms, so exercise caution when using them. Always read and follow label directions.


Is Btk completely non-toxic? If so, why would anyone be against its use?

Some people are against any type of insecticide application regardless of the product used. If your neighbor objects to spraying, take precautions to avoid the problem and liability of spraying their land by mistake.

Insecticide products containing Btk are toxic only to caterpillars of butterflies and moths that actually consume the Btk-treated leaves. In some instances killing them would be of concern, especially if any rare or endangered species are involved. Btk is not toxic to other groups of insects, birds, fish, aquatic invertebrates, mammals or humans. For more information about Btk-containing products.


Why don't governmental agencies aerially spray FTC to control defoliation?

FTC is a native defoliator of hardwood trees, especially aspen, oak, birch and basswood. The first outbreak was recorded in Minnesota in the late 1870's and there have been six major outbreaks since 1933. Outbreaks usually last three – seven years and cause no permanent damage to vigorous trees. Trees generally refoliate by early July. The survival of the forest is not threatened so the forest does not need the protection afforded by pesticides. Controlling FTC in forests on federal, state and county lands can't be justified biologically or economically.


Private land spraying is the responsibility of the landowners.

If people want to control the nuisance and defoliation caused by FTC on private lands, it is both their option and their responsibility.