Eastern tent caterpillars are highly visible in Wisconsin this spring

State forestry health specialists report that people may see caterpillar tents in the following Wisconsin counties this month: Adams, Ashland, Columbia, Dane, Green Lake, western Lincoln, Marquette, Sauk, Waupaca and Waushara.

“Eastern tent caterpillars are often confused with webworms and gypsy moth caterpillars,” says Andrea Diss-Torrance, Suppression Program Coordinator at the Department of Natural Resources. “Tent caterpillars make tents, gypsy moth caterpillars don’t.”

Infestations are commonly seen on open grown trees and trees on roadsides or along fence rows. The eastern tent caterpillar favors crabapples, apple, wild cherry and wild plum trees though it also feeds on some other deciduous trees, such as oak.

The caterpillars hatch when the buds begin to open, and construct a silken tent on a fork of branches. In high numbers, this caterpillar can strip the leaves off of infested trees, but they rarely do lasting damage. Native trees such as wild plum and cherry have co-evolved with this native caterpillar and will put out new leaves in a week or two. Yard or fruit trees should be protected as the loss of leaves will prevent flowering and fruit production.

Controlling eastern tent caterpillars is fairly easy in many cases, though it is most effective in May when the caterpillars and their tents are small. Each evening, the caterpillars return to their tent. Near dark, go to the tent and wearing rubber gloves, pull the tent and its inhabitants out of the tree and plunge the bundle into soapy water. Next morning the caterpillars will be dead and you can discard the mess. If the tent is out of reach, tear up the tent with a stick and spray in soapy water or a contact insecticide.

“Don’t cut branches off of your tree to remove the tents. This causes much more damage to your tree than the caterpillars ever would,” says Diss-Torrance. “Also, do not burn the nests or webs out of the tree. This also kills the branches.”

Insecticides that contain the bacterial insecticide Bacillus thuringiensis (Bt) work well on tent caterpillars but they are best applied in May when the caterpillars are small and the damage is not already done. Bt must be eaten by the caterpillars to work, so this insecticide is usually sprayed over the entire tree.

Soapy water or other general use insecticides that are labeled to control eastern tent caterpillars are best in June. Be sure to follow manufacture’s label directions. For more information about pest management and pesticides, please read a UW Extension booklet, titled “Woody ornamentals: pest management in Wisconsin (A3597)” online at [learningstore.uwex.edu].

Tent caterpillars are nearing the end of their larval stage and will soon be entering the pupal stage to change into little brown moths. There is only one generation per year and typically high populations will only persist in a particular area for a year or two. The tents will remain until they weather out of the tree. Nests seen in roadside trees in August and September are likely another native pest, the webworm.

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