Those Webs in the Trees

By Carl Hoffman, University of Minnesota Extension

No, they are not “army worms”.  Those conspicuous white masses of webby material that are seen in the forks of branches of trees are created by the eastern tent caterpillar.  Populations of this insect fluctuate from year to year and judging from the number of webs present, 2007 definitely appears to be a year of high populations. 

The silken webs or tents are constructed by the caterpillars to protect themselves from birds and other predators and from temperature extremes that occur at night or on cool or rainy days.  The caterpillars come out of the web to feed on the foliage of the trees when the weather is warm and sunny.  Although they will feed on many trees, they are most often found on apple, cherry, flowering crabapple, plum and chokecherry trees.

Eastern tent caterpillars start feeding in late April or early May in central Minnesota and are usually through feeding by late May or early June.

After they finish feeding, the caterpillars leave the web nest and spin a silk cocoon in protected places on the trunks of trees, fences or buildings.  About three weeks later, they emerge as reddish-brown moths which mate and the female lays eggs in band-like masses on small branches of trees.  They overwinter as eggs and hatch again about the time the buds break in the spring.

Eastern tent caterpillars are more of a nuisance than a serious pest of trees.  They rarely kill healthy, mature landscape plants.  The damage is primarily cosmetic because of the unsightly tents and the trees often appear ragged until they replace the lost leaves.

These insects can be controlled without the use of insecticides.  In early spring, the small webs can be removed by hand.  Larger webs can be removed with a stick or broom.  Put the caterpillars in a pail of soapy water or crush, bury or burn them if it is permitted.  Do not try to burn the web in the tree because you will cause more damage to the tree than the caterpillars would.

It is not effective to spray the caterpillars inside the webbing with an insecticide because they are protected by the web. Wait until a warm sunny day when the caterpillars leave the tent to feed on the leaves and apply the product to the foliage.  Bacillus thuringiensis (BT) is a bacterial insecticide that is very low toxicity and is effective if applied when the caterpillars are less than one inch in length.  Insecticide products that are effective include acephate (Orthene), carbaryl (Sevin) and malathion.   If the tree is flowering, use Bacillus thuringiensis which will not kill the pollinating bees.  Use insecticides with care.  Read the label before you purchase the product and again before you use it.  Follow all directions and safety precautions.