By Island County Extension Agent, Donald B. Meehan

In 1985 our office was beseiged by call and all indications were that we were going to have a banner year for Tent Caterpillars that year. A great deal of concern has been expressed by various callers that if something isn't done about the infestation we wouldn't have any trees left. To help you better understand this problem I have written some information which you can apply to your own situation here on Whidbey Island. The trees which tend to be attacked most often are Alder, Ash, Birch, Willow, Cottonwood, fruit trees and roses. Since we have a significant number of Red Alder growing in Island County it comes as no surprise to see large populations of Tent Caterpillars in those stands. The caterpillars generally eat all the leaves on one branch before moving on to the next. The entire tree can be completely defoliated if the infestation is large enough. The important thing to keep in mind is that it is very unlikely that the tree will die from a defoliation. This is especially true if the tree is healthy. Often healthy trees will grow new leaves by mid summer. Unhealthy trees are going to be more damaged by a complete defoliation. Regardless of the health of the tree a defoliation will increase the tree's susceptibility to disease and freezing hazards. I doubt we have much to worry about unless we get a couple of years of serious damage taking place. Of course, if you have Tent Caterpillars eating your landscape trees I would encourage you to try controlling them. There are a variety of methods, all work fairly well. The most common method we have all heard about is the prune out and burn technique. WSU no longer recommends burning caterpillar nests still in the tree. I suspect someone got carried away and burned something they hadn't intended. If you prune out your nests you will want to do so in the early morning or late evening when it is cool. This way you will get more of the caterpillars since they generally return to the nest when it cools down outside. If you should decide spraying is the best method for you, you'll want to do that in the early morning or late evening as well. Two types of sprays are available, biological or chemical. The biological is the safer of the two types and involves the use of a bacteria called "BT" (Bacillus thuringiensis). This bacteria is selective to caterpillars. It interrupts the caterpillar's digestive tract when ingested. The caterpillars stop eating shortly after ingestion but may take several days to die. This spray is quite safe to use since the bacteria pose little to no threat to humans. Chemical sprays include Malathion, Diazinon, Carbaryl (Sevin), and Zectran. All of these sprays pose a health hazard to humans and should be used with caution and proper protective clothing. Carbaryl (Sevin) poses a serious hazard to bees. All chemical sprays should be applied in the early morning or late evening. Since it is difficult to control the caterpillars feeding high up in the Alder trees we may have to wait for Mother Nature to do her part. She controls such pests by a variety of biotic factors. Eggs, larvae, and pupae are killed by various insect parasites: larvae (caterpillars) are consumed by predaceous beetles or bugs; and eggs, larvae, and moths are eaten by birds. The larvae are even attacked by viruses. O course, all of this takes time. All we need to do is have enough patience. What will happen to the caterpillars we are watching now? Will they keep eating all summer or will they eventually go into another stage of their life cycle? The latter is the case. In a few weeks, when they reach maturity, they will fall to the ground forming cocoons. Moths emerge after about two weeks during mid summer. They lay their eggs on twigs and branches. The eggs overwinter and new larvae will appear about the time new leaves appear in the spring. There is only one generation each year. The caterpillars are primarily a nuisance and do not pose a health threat to humans. Our office has a publication called "Biology and Control of Tent Caterpillars, EM-4037", available to the public. Call us if you want a copy.



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