Forest Insect and Disease Newsletter
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 an important consideration for the homeowner.
Remember, FTC have only one generation per year. The most noticeable defoliation and annoyance occurs over a 4 to 5 week period from late May to mid- June. Defoliated trees will refoliate by mid-July.
Homeowners may want to adopt two basic strategies. First, identify the gardens, 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 effort.
How can I keep them off my house, patio, lawn furniture, etc.?
- Caterpillars can be brushed off the house with a stiff broom or knocked down by a stream of water. If possible, do this daily. Be careful not to squash caterpillars on the house or buildings since this may cause staining. 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.
- Spray a labeled insecticide on the concrete foundation of your house. Some pesticides are labeled for home structural or foundation application. Don't spray on 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.
- A 12 to 24 inch strip of plastic sheeting can also be fastened to the foundation of a building to discourage caterpillars from climbing up. Leave the top 4 inches loose so it flops over and makes it difficult for caterpillars to climb over the top.
- Dispose of dead caterpillars by burying them or mixing them into the compost pile.
- Cocoons may be difficult to remove from buildings. They can be brushed off the house with a stiff broom or with a pressure washer. Bag, burn, bury or compost the cocoons.
While aspen and hardwoods are the preferred hosts, FTC will feed on any broad-leafed plant with the exception of red maples. In forested situations, outbreaks usually collapse before significant tree damage occurs. FTC can consume 60 percent 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?
- Birches and oaks are often at risk because FTC defoliation makes them vulnerable to additional damaging insects. Bronze birch borer attack birches and two-lined chestnut borers attack oaks. Both insects 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.
- 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.
- 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.
- Shade trees and ornamental shrubs are vulnerable if they have been recently damaged by construction, trenching, soil compaction, blacktopping, etc.
- Drought-stressed trees are also at risk when they become defoliated by FTC or attacked by stem and wood boring insects.
What can I do to protect my trees, shrubs and garden from heavy defoliation?
- After eggs masses are laid by the moths and before they hatch (any time from July to mid-April of the next year), hand pick all the egg masses off of valuable plants. Destroy or dispose of them.
- In May and June, handpick caterpillars off plants and dispose of them.
- 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 tree wrap tape, tin foil or tar paper and coat it generously with 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 vaseline or Tanglefoot is necessary and to scrape off the caterpillars. Remove the band in late June after the caterpillars have formed cocoons.
- 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 cannot crawl underneath it. Leave the top 4 inches or so hang loose and flop to the outside of the fence to make it difficult for the caterpillars to climb over the top.
- Spray an insecticide to kill caterpillars. Each product has restrictions as to which plants and sites 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.
- Biological insecticides containing Btk, (a bacterial product made of Bacillus thuringiensis var kurstaki) are the recommended products to use for FTC control in the backyards, public and private forests and along lakeshores because of their safety and the low toxicity to non-target organisms. Btk products are only toxic to caterpillars; they do not kill bees, flies, mosquitos, etc. However, Btk products are slightly slower to act since they must be eaten by caterpillars before they take effect. Apply Btk to the leaves of host plants not to the bark or other non-edible plant materials.
- 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. A similar home remedy uses a little dish soap in water and is applied in a similar manner. Remember that dish soap builds up in the soil and can harm tree roots.
- Chemical insecticides can also be used but would normally be a second choice after Bt, 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.
- FTC moths emerge from cocoons late June or early July and lay eggs for next spring. During the night, FTC moths are attracted to lights. Turning out your yard and exterior lights may reduce egg-laying on your trees and thus reduce next year's FTC population.
Can I do anything to help defoliated trees and shrubs?
- 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 freeze-up in the fall.
- 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 the tree's energy reserves and put additional stresses on the tree.
- Stressed trees are easily attacked by other serious insect or disease pests (two-lined chestnut borer, bronze birch borer, Armillaria root disease). 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 ten or more 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 ones that FTC do not feed on, then fewer caterpillars will be found there. Foliage of red maples and most conifer species is not eaten by FTC.