Master Gardener: Here come the Eastern tent caterpillars

By Jerry Vitalis, Chisago County Mster Gardener

I had started an article on another topic but since I am receiving so many calls on small worms and webs on trees, I decided to write on the Eastern tent caterpillar.

After doing some research I realized that I was giving some wrong information when I said that the Eastern tent caterpillars are the same as armyworms.  The true armyworm is a distant species of insect that belongs to the cutworm family.  It is primarily a pest of grasses, small grain, and corn but also will feed on a wide variety of plants.  Armyworms most severe and costly infestations result from moth migrations from the south when they attack crops in mass.

When people think about caterpillars they usually are thinking about the forest tent caterpillar that some years ago, defoliated millions of acres in northern Minnesota.  The forest tent caterpillar is native to 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.  Outbreaks occur every 10 to 15 years and populations usually collapse because of starvation, predation and parasites.

The caterpillars that we are seeing are the Eastern tent caterpillars.

The host plants are apples, crabapples, pear, plum, and wild cherry trees.  They also can be found on forest and other shade trees.

The eggs overwinter on tree twigs and hatch as tree buds begin to unfold in the spring.  The larvae construct a web or tent, which grows as they develop and from which they emerge to feed.  They produce one generation a year.

The weblike "tent" of the Eastern tent caterpillar can be found around the county right now. Photo by MaryHelen Swanson

People are noticing the white webs in tree forks on host trees.  If they have yet to leave the web, cut the branch or twig and destroy.  By now, most are outside the web during the day.

Wait until evening when they go back to the web to keep warm in.  You can either take the branch off and dispose of it, or spray an insecticide on the web.

Although they look ugly and may eat some foliage, the plant mortality risk is low.

If you want to control them with a chemical use Bacillus thuringiensis (Bt) to protect pollinating bees.  If you use a chemical pesticide containing acephate, carbaryl, malathion, permethrin, etc. Be sure to read the label carefully and be sure it is used to control Eastern tent caterpillar.

Our plant sale and distribution has been completed but we may have some, blueberries, rhubarb, and asparagus still available.  If you are interested please call me at 651-257-4496 and leave a message and I will return the call.


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