By Jerry Vitalis, Chisago County Master Gardener
In an earlier article I wrote on the Eastern tent caterpillars because so many of their webs were found in trees and shrubs. The forest tent caterpillar is native to Minnesota and is the one that can defoliate millions of acres in Northern Minnesota. Their favorite food is aspen, but they also consume the leaves of birch, oak, basswood, ash, apple trees and various shrubs and berry bushes.
The larvae of the forest tent caterpillar hatch when leaves are beginning to unfold in mid-to-late May. Colonies of larvae stay together and move single file much like the larvae of armyworms. They don’t make silken tents like the Eastern tent caterpillar, but rather cocoons that are made from within folded leaves or bark cracks. In mid summer, eggs are deposited in black masses that encircle twigs. The black masses overwinter on twigs and there is one generation a year.
Young larvae cause shot hole damage to foliage. Mature larvae can defoliate forest and shade trees, reducing their growth and vigor, but rarely cause tree death.
Look for shot holes in foliage from mid to late May when larvae begin to be active, feeding is complete by the end of June and it is important to detect serious infestation early to prevent defoliation. If possible egg masses can be cut from trees and destroyed. Small populations of larvae can be physically removed.
A large grey parasitic fly often becomes very abundant during forest ten caterpillar outbreaks. Known as the friendly fly, this native parasite lands on anything including people, but is important to naturally reducing caterpillar outbreaks.
If you need to control the forest tent caterpillar but want to use harsh chemicals, you could use chemicals that have spinosad, pyrethrin, insecticidal soap, Bacillus thuringiensis (Bt), etc. in them. For more severe outbreaks you could use chemicals that include acephate, bifenthrin, carbaryl, malathion, etc., in them.
Be sure to read the label carefully and be sure it is used to control forest tent caterpillar.