Questions about spraying forest tent caterpillars

What are the reasons to control forest tent caterpillars?

To protect valuable ornamental foliage and perennial plants from defoliation. Examples would be fruit trees, small fruit crops and gardens. These plants or areas are often small in size and are suitable for treatment from the ground.

To reduce caterpillar populations in order to reduce the nuisance to people. This could include resorts and private campgrounds which might suffer economically during the June activity of the FTC. Also groups of homeowners or lakeshore associations may wish to preserve the aesthetics of their land. Due to the size of the trees and the acreage involved, this spraying must be done by helicopter or airplane.

To prevent defoliation on high value hardwoods that have suffered drought stress, other defoliators or stem borer (two-lined chestnut borer on oaks) attack in the past two years. These already weakened trees may die from the additional stress of defoliation.

What are some of the alternate methods for controlling FTC?

Before they hatch (any time from July 1 of one year to early May of the next) hand pick the egg masses off valuable perennial plants.

Hand pick caterpillars off plants and put them in a soapy water solution to kill them.

Build a 24" high polyethylene wall enclosing the area. Spray the plastic with vegetable oil to prevent the caterpillars from climbing on the wall. Repeat oil application as needed. This method has not been proven, but has been recommended by many homeowners.

Use a product like Tanglefoot on the trunks of shrubs and trees. This prevents caterpillars from adjacent areas climbing up treated trees. However, caterpillars already in the tree are totally unaffected and small caterpillars can be wind transported on silken threads they secrete from glands in their heads.

Spray an insecticide to kill young caterpillars. Products include chemical insecticides, insecticidal soaps and biological insecticides.

Will spraying this year reduce the FTC problem next year?

Spraying will only reduce this year's population of caterpillars for sure. Moths from untreated areas will likely fly in and lay eggs on treated trees later this year. So next year's population of caterpillars won't really be affected by control work this year.

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 beehives in the area.

If you have large shade trees, it will be difficult to obtain good spray coverage of the foliage by spraying from the ground. Spraying trees of this size should be done by a commercial applicator with equipment capable of reaching the tops of tall trees (either from the ground or with a helicopter).

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

The safest insecticide to use is Bt. Bt is a biological insecticide that contains a bacterial preparation of Bacillus thuringiensis. Bt kills caterpillars of moths and butterflies when they eat leaves that have been sprayed. See the BT fact sheet. Bt is not toxic to other organisms or adult moths and butterflies. Due to its limited toxicities, Bt is the favored insecticide for use near water (lakes, ponds, streams) and homes.

Chemical insecticides, containing malathion, acephate, carbaryl or diazinon, are sprayed directly onto the caterpillars and will kill them on contact. These insecticides can also kill bees and other beneficial insects. These products generally require addition precautions, such as, unsprayed buffers around lakes, ponds and streams.

Is Bt completely non-toxic? If it is why would anyone be against its use?

Insecticide products containing Bt are toxic only to butterfly and moth caterpillars. It is not toxic to other groups of insects, fish, mammals or humans. In fact, Bt must be taken internally by caterpillars where it causes a natural disease which kills them.

Some people are against any type of insecticide application regardless of the product used. Bt is toxic to all caterpillars and in some instances killing them would be of concern, especially if any rare moth or butterfly species are involved. If your neighbor objects to spraying, take precautions to avoid the problem and liability of spraying their land by mistake.

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

FTC is a native defoliator of hardwoods trees, especially aspen and basswood. The first recorded outbreak in Minnesota occurred in the late 1870's and there have been five major outbreaks since 1933. Outbreaks usually last three to six years and cause little or no permanent damage to the trees. The survival of the forest is not threatened and therefore the forest does not need the protection afforded by pesticides. Trees generally refoliate by early July. It is also extremely difficult to obtain a level of control which will actually reduce populations in future years. Control programs can prolong local outbreaks. Controlling FTC on federal, state and county lands can't be justified biologically or economically.

If people want to control the nuisance caused by FTC on private lands, it is both their option and responsibility. For nuisance control, the DNR does not recommend spraying public lands except in areas of concentrated recreational use (i.e., State Parks).