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Eastern Tent Caterpillars

How to control these and other web-builders

Those white webs expanding in the crotches of cherry, crabapple and Juneberry trees are made by eastern tent caterpillars. Last summer and early fall, the adults laid their eggs in these favorite trees. As the larvae emerge, they spin a web around the nest, giving it protection from the weather. In the evening, the larvae crawl out from under the web to feed on nearby tender young leaves. Just about the time the sun rises, they return to the web for protection. As the population of larvae increases and the larvae increase in size, so does the webbing of the nest.
    As long as the larvae remain under the protection of the web, they are protected from birds and the elements as well as from insecticidal sprays. You will never see birds feasting on these webs. If you poke your finger into one, you will see why birds do not bother them.
    “The defoliation usually does little damage to trees, and rarely do trees die from an infestation,” says Maryland Agriculture Secretary Joe Bartenfelder.
    The lack of damage is due to timing. Because the caterpillars hatch as soon as the young leaves unfurl in the spring, the tree has put little energy into the leaves and typically re-foliates in June, seemingly no worse for wear.
    Do not try to control the eastern tent caterpillar by torching the nests. Torching with a flaming kerosene-soaked rag tied to the end of a pole is not only dangerous but also causes permanent damage to the tree.
    The best method of control is to spray the foliage nearest the web with an organic pesticide such as Thurcide or Dipel. These pesticides contain the BT bacteria that kill the feeding larvae from the inside out. They are approved for use by organic gardeners. To obtain maximum effectiveness, apply in the evening to the foliage in the feeding area. A single application will provide protection for three to four days; it will take a few days before you see evidence of the treatment. The smaller the larvae, the better the control. As the larvae grow larger, they become more difficult to kill with BT.
    Use a fresh supply of these organic pesticides, not an unused portion from last year. Once the bottle is open, the effectiveness decreases with time. Unless you are going to be using them to control other pests in your garden, such as cabbage loopers, bagworms or corn-ear worms, purchase the smallest container possible.
    In mid to late summer, you’ll see similar webs on a wide variety of woody plants. These are created by the fall webworm. The same treatments can be used to control these pests. In July, you may also see webs on two-needle pines such as Virginia pine and mugo pine. To control the pine sawfly creating them, you’d need the hard pesticide Sevin.


Is My Compost Safe to Use?

Q I shredded some sunflowers in my composter this past fall. I forgot that they are like walnut trees and put out a mild toxin that can negatively affect other plants. Can I use this ­compost? Or should I just throw it out?

–Mike Morgan, Bowie

A The composting process destroys the enzymes that cause the allelopathic effect, so you can use your compost.


Ask Dr. Gouin your questions at DR.FRGouin@gmail.com. Please include your name and address.