Forest tent caterpillar homeowner tip sheet

Dealing with forest tent caterpillars (FTC) can be very frustrating!During the first three weeks of June, they can be a downright nuisance. They don't cause a health risk to humans, but the presence of hundreds or thousands of caterpillars can be a real headache. Fortunately, the nuisance associated with FTC outbreaks can be reduced by individual homeowners. The effect of FTC defoliation on shade trees, ornamental plantings and gardens is also important consideration for the homeowner.

Homeowners may want to adopt two basic strategies. First, identify the trees or buildings that you want to protect. Then work to protect the things you selected and ignore the rest (or at least try to). It takes a lot of time and energy to try to protect everything on your property. Second, be persistent. Some treatments may require daily monitoring or retreatment.

How can I keep them off my house, patio, lawn furniture, etc.?
1. Caterpillars can be brushed off the house with a stiff broom or knocked down by a stream of water. If possible, do this daily. Avoid squashing caterpillars on the house. You can also treat lawn furniture, patios, decks, screens, etc. with either of these two methods. The longer caterpillars sit on painted surfaces, the more difficult it is to wash away any staining that may occur.

2. Spray a labeled insecticide on the concrete foundation of your house. Some products containing malathion are labeled for home structural application. Don't spray onto paint or stain as the insecticide may damage the finish. Repeat applications as indicated on the product label. Commercial pesticide applicators may be able to use more effective insecticides.

3. Dispose of dead caterpillars by burying them or mixing them into the compost pile.

4. Cocoons may be difficult to remove by water pressure. They can be brushed off the house with a stiff broom. Bag, burn, bury or compost the cocoons.

Reduce Defoliation
While aspen and hardwoods are the preferred hosts, FTC will feed on any broad-leaved plant with the exception of red maples. In the forest situation, outbreaks usually collapse before tree damage occurs. FTC can consume 60% of a tree's foliage for 2 to 3 years and the tree will show no ill effect. In the suburban landscape, the situation is a little different. First of all, the trees are much more valuable and secondly, they are apt to be subjected to many other stressing agents (soil compaction, construction damage, other insect and disease pests, lawn herbicides, etc.). So urban trees may be at a greater risk of damage than forest trees.

How can I tell if my tree(s) are at risk?
1. Birches and oaks are often at risk because FTC defoliation makes them vulnerable to stem boring insects. Bronze birch borer attack birches and two-lined chestnut borers attack oaks and cause dieback in branches of the upper crown. FTC and stem borers act together to reduce tree vigor causing branch dieback and possibly killing the tree. If your birches or oaks have suffered 2 years of heavy defoliation or have branch dieback, then treatment to prevent FTC defoliation could be warranted.

2. Newly planted woody ornamentals and tree saplings are very vulnerable to any type of stress and with the loss of leaves, some may be killed.

3. The production from fruit trees, raspberries, strawberries and other fruit and vegetable crops will be greatly reduced or lost if the plants suffer moderate defoliation.

4. Shade trees and ornamental shrubs are vulnerable if they have been recently damaged by construction, trenching, soil compaction, blacktopping, etc.

5. Drought stressed trees are also at risk when they become defoliated by FTC or attacked by stem borers.

What can I do to protect my trees, shrubs and garden from heavy defoliation?
1. Before they hatch (any time from July of this year to early-May of next year) hand pick all the egg masses off of valuable plants. Destroy or dispose of them.

2. Hand pick caterpillars off plants and dispose of them.

3. If you can determine that there are no egg masses in a tree or if you have sprayed the tree, you may be able to prevent migrating caterpillars from climbing up the trunk by the use of barriers. Basically, you construct a barrier band around the trunk made of duct tape, tin foil or tar paper and coat it generously with grease (Tanglefoot or vaseline). Never apply grease directly to the tree bark. The barrier band should be in the shade or you run the risk of killing the bark and cambium under the band. Check the barrier band daily to see if more grease or Tanglefoot is necessary. Remove the band in early July after the caterpillars have formed cocoons.

4. Although recommended by homeowners, this next method has not been scientifically proven. To protect an area (garden), build a 24 inch tall enclosure of plastic sheeting and secure its lower edge to make sure that caterpillars can not crawl underneath it. Spray the sheeting with vegetable oil to prevent the caterpillars from climbing the wall. Repeat oil application daily, or more often as needed.

5. Spray an insecticide to kill caterpillars. Each product has restrictions as to which plants and sites where it can legally be applied. If applying to shade and ornamental trees, the label should say it is for use on shade and ornamental trees. Please read and follow label directions.

  1. Biological insecticides containing Bt, (a bacterial product made of Bacillus thuringiensis ) are the recommended products 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 young (small) caterpillars.
  2. Insecticidal soaps can be sprayed directly onto caterpillars or onto plants they infest. Insecticidal soaps are insecticides made from naturally-derived fatty acids. Repeat applications may be necessary.
  3. Chemical insecticides can also be used but would normally be a second choice after Bt, due to safety considerations. Commonly used chemical insecticides contain malathion (Malathion), acephate (Orthene), carbaryl (Sevin) or methoxychlor (Methoxychlor). These products can also kill bees and other organisms, so exercise caution when using them. ( Brand names are not meant to be an endorsement of a particular commercial product.)
  4. Commercially available systemic insecticides (such as acephate in Ace-caps)can be inserted into tree stems to kill FTC. They must be implanted in the tree early, before caterpillars hatch. Such implants require wounding the tree and they should be avoided unless other control measures are unavailable. It is recommended that they be installed by commercial pesticide applicators.
6. FTC moths are be attracted to lights during early July. Turning out your yard and exterior lights may reduce egg-laying on your trees and thus reduce next year's population.

Can I do anything to help defoliated trees and shrubs?
1. The most important thing you can do for your 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.

2. Do not fertilize trees or use a weed and feed product on your lawn during an outbreak. Heavy nitrogen fertilization encourages the tree to produce more leaves which may deplete energy reserves and put additional stress on the tree.

3. Stressed trees are easily attacked by other serious insect or disease pests (two-lined chestnut borer, bronze birch borer, Armillaria root rot). You may need to protect trees from these additional pests in order to maintain their vigor.

Are there any long-term solutions?
FTC have cyclic outbreaks with about 10 years between population peaks. One long-term solution to defoliation and nuisance is to maintain tree vigor. Maintain tree vigor between outbreaks by watering, fertilizing, properly pruning trees and by avoiding root and trunk damage. Another method is to plant trees that are not preferred food hosts. If most of the trees in an area are not hosts for FTC, then fewer caterpillars will be found there. Foliage of red maples and most conifer species is not eaten by FTC.