Caterpillars are no cause for alarm

By Jim Moll, Extension Cords
Published March 6, 2008

Each spring, the cry rings out: "The caterpillars are falling out of the trees!"

Those who call the Cooperative Extension Service about this menace learn that the creatures falling on their heads are tussock moth caterpillars. They drop down from oak trees like rain. Besides the annoying presence of numerous hairy caterpillars, these insects may spin their difficult-to-remove cocoons on houses, boats, picnic tables, slow-moving pedestrians and other outdoor items.

Hatching occurs during late February and early March, at the same time oak trees are beginning to leaf out. The newly hatched caterpillars move from the egg mass in search of newly expanding spring leaves so they can feed on them.

Once the caterpillars start falling out of the trees by the hundreds, they are essentially done with the oak trees. They have been feasting for several weeks on the tender new leaves. By the time they fall on your head, they have finished feeding and are completing their life cycle.

Should the gardener worry about the oak trees while these caterpillars are feeding on the foliage?

Caterpillars that feed on mature oak trees have little effect on the overall health of the tree. Large trees retain enough energy to put out a second flush of new foliage with no permanent damage to the tree. It is difficult and nearly impossible to spray large trees with insecticide. Therefore, spraying big trees is really a waste of money and unnecessary.

Small trees, on the other hand, may need your help. Your smaller trees and shrubs could be defoliated if a wind-blown caterpillar happens to land on them.

If the tree is small enough, handpick the caterpillars and drown them in a bucket of soapy water for quick, easy and inexpensive control.

Nature provides plenty of natural caterpillar predators like birds and predatory wasps. When the predators cannot keep up with the caterpillar population, control can be obtained with the natural insecticide B.T., sold under the trade names Dipel and Thuricide. This product offers excellent control and is harmless to the caterpillar's natural enemies. The caterpillars must ingest this product, so they must still be feeding on the foliage for effective control. By the time they are falling out of the trees, it is too late to spray them.

Another annoyance created by the caterpillars is the cocoons they spin. Walk around the house with a broom and sweep the climbing caterpillars into a pail of soapy water before they have a chance to spin their cocoons.

Remember, when they reach this stage, they are not eating anymore, so spraying them with insecticide is useless. Spraying them off with a hose will just waste a lot of water. Should a few make cocoons in unwelcome places, you can remove them, but it is not easy.

Try to remove them from structures by slipping tweezers or a similar small tool between the cocoon and the wall. Wear a long-sleeved shirt while doing this job. The hairy caterpillars can shed irritating hairs. A good pressure washing after caterpillar season goes a long way toward freeing your house of the cocoons.

In the long run, these caterpillars are unpleasant but not a huge problem. With patience, this plague will be over soon and you can enjoy your piece of paradise once more.

Jim Moll is the urban horticulture agent for the Hernando County Cooperative Extension Service. He can be contacted at jimm@hernandocounty.us.

[Last modified March 5, 2008, 22:30:32]