The tent caterpillars have arrived

StoryImage( ‘/Images/Story//Auto-img-11895391579179.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘A heavy infestation of tent caterpillars can severely damage a tree.‘);

Animals that are large and have an adequate method of defense can afford to lead a solitary life, but those not so endowed frequently band together to live a social life. Not because they particularly enjoy each other’s company, but for mutual protection. The tent caterpillars are in this latter category.

Two species of tent caterpillars are now making their appearance in the Rock River Valley: the much more common eastern tent caterpillar and the forest caterpillar. The latter is more common in the south and southwest and has a row of spots down its back, whereas the eastern variety has a solid stripe there. They love to feed on the leaves of fruit trees including cherries, pears, plums, apple, and a host of others.

We become aware of this insect’s presence when we see an amorphous tent-like structure made of a silky material in the crotch of a tree limb. A closer inspection will reveal the tent is teeming with small, dark, and hairy caterpillars. From the tent, the caterpillars venture forth to feed on the leaves and buds of the tree on which they have set up housekeeping. The tent serves as a protective structure for the “worms,” and they retreat to it at night or during the heat of the day or rainy weather.

If the infestation is severe, a tree may be almost defoliated, and if it is small or under stress for some other reason, it may be killed. A healthy tree is usually able to survive an attack by the caterpillars by growing new leaves.

The caterpillars are actually the larvae of a small, reddish-brown moth with two white stripes running diagonally across each forewing. The adult moths emerge at the end of summer, mate, and the female lays her 300 or so eggs in a ring on a branch of a selected tree. The eggs do not hatch until the following summer.

During mid-summer, the caterpillars will have completed their growth and are ready to go into the pupal or transformation stage. Now they begin to wander from the tent to search for a protected place in which to spin a cocoon to pass the quiescent pupal stage and metamorphose into adult flying moths.

During this wandering stage, the caterpillars may create a sizeable nuisance as they crawl along driveways, other plants, the side of a house, or parked car. It is well to keep in mind, however, that at this stage they have stopped eating, and damage to infested trees is finished.

A note to fishermen is in order. Tent caterpillars are excellent bait for bluegills, other sunfish, and an occasional bass. I affix a caterpillar to a very small # 10 or #12 size hook with a slipknot of thread. The hook is tied onto a small diameter leader on a floating fly line. By making a gentle cast with the fly rod, I am able to present the morsel that seems to be irresistible to members of the sunfish family.

Tent caterpillars are difficult to control. Commonly-used garden insecticides do not reach them inside the tent. Mechanical control is best. Remove the web by winding it on a stick or prune off the limb harboring the tent. I once had a neighbor who became so infuriated with the “worms,” as he called them, that had attacked his pear tree that he assailed them with a propane torch. He got rid of the “worms” all right, but severely damaged his pear tree.

An interesting phenomenon concerning the presence of tent caterpillars and pregnant mares has been reported. In 2001, an abnormal and unexplained thing happened to pregnant racehorse mares in Kentucky when an abnormal number of them spontaneously aborted their fetuses. In 2001, there was an unusually large population of eastern tent caterpillars in the state, and researchers finally made an association between what was termed “Mare Reproductive Loss Syndrome” (MRLS) and the caterpillars. In short, it boiled down to the mares ingesting hairs (setae) from the caterpillars that contained infectious agents. It is estimated that MRLS damaged the horse racing industry to the tune of some $500 million in 2001.

Nature helps to control these pests by providing natural predators and a few diseases. Several types of parasitic small wasps attack the caterpillars and lay eggs inside the victim’s body. The wasp larvae devour the internal organs of the caterpillar and kill it. Some birds are particularly fond of tent caterpillars.

I find it hard to determine what great benefit the tent caterpillars offer the natural world, but this does not mean that they do not fit somewhere into Nature’s grand plan.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

from the Sept. 12 – 18, 2007, issue

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