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Growing Green: Forest Tent Caterpillars

It's that time of year again, the notorious forest tent caterpillar (FTC), AKA the army worm, will soon be raining from the trees! What are these little buggers, and how can we control them? Here are the facts:

These caterpillars live throughout the US, and nosh on hardwood trees like balsam poplar, basswood, oaks, ashes, birches, alder, fruit trees, and their favorite: quaking aspen. When they are done munching on trees, they often move to other vegetation, including vegetables, small fruits and nursery crops. They rarely eat red maples or conifers, such as pine and spruce. FTCs appear in mid-May when new leaves are sprouting. They feast until late June, when they begin to spin their cocoons. Moths emerge two weeks later and lay from 100-350 eggs in their short, 5 day, lifespan. These eggs over-winter in the trees to hatch the following spring.

The good news: outbreaks of FTC are cyclical, lasting 3-6 years, and Minnesota has only had 5 major outbreaks since 1933. Although they are a nuisance, FTCs are rarely responsible for the death of trees. However, repeat infestations (3 years or more) can slow down a tree's growth. Trees suffering from drought related stress or disease are much less tolerant of the defoliation caused by these worms, and will succumb much more rapidly to an infestation. Widespread spraying of forests and public lands may kill this year's FTC, but is an ineffective way to control future populations. (Moths from un-treated areas fly into treated areas and lay their eggs for next spring's generation.)

What can you do if you just can't live with these pests in your trees? Remove and destroy over-wintering egg masses from the branches of small trees (they look like brown bubble wrap). Use a stiff broom to brush caterpillars off infested areas, or use a strong spray of water to knock nests off structures and trees. Insecticides are effective early, when FTCs are small (no more than an inch). BT is an excellent insecticidal option that doesn't kill beneficial insects. Other beneficial safe products include insecticidal soap, spinosad (Conserve), and azadirachtin (Azatin). Additional insecticides for home use include carbaryl (Sevin), Malathion, acephate (Orthene), and permethrin. Additional early measures include using a product like Tanglefoot on the trunks of shrubs and trees, wrapping the trunk of the tree with oil coated plastic wrap (oil side out) or hand picking caterpillars off plants and putting them in soapy water to kill them. Remember, these measures will kill some of this year's worms, and will protect trees already under stress, but will do little to decrease next year's population.

Hope this answers your most pressing "worm" questions. As always, please contact me by phone at 320-762-3890, or email at if you have any questions you would like me to answer.

Until next time, happy gardening!