Curt Swift, Ph.D.
Grand Junction Free Press Gardening Columnist

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February 21, 2013
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SWIFT: Proper treatment for caterpillar nests in trees

Charles Kerr has been having problems with nests of caterpillars in neighboring cottonwood trees. The tents and their associated caterpillars appear in late summer, so does the insect called the fall webworm. These are sometimes confused with tent caterpillars which create nests in aspen, crabapples, and other deciduous trees in early spring just as leaves start to appear. Sometimes, again incorrectly, the insect responsible for the large nests of the fall webworm, are called bagworms. The main reason for getting the names correct is the control and timing of treatment is different for each of these insects.

The eggs of the tent caterpillar are laid around twigs of the host tree in the fall and this band of eggs surrounding the twig hatch in the spring and form tents in the crotches of branches. The eggs of the fall webworm are not laid until summer when the adults emerge from under loose bark and out of the soil. The nests from this caterpillar form in late summer into fall at the tips of branches. The group of insects known as bagworms includes the evergreen bagworm which feeds on evergreens such as Arborvitae and red cedar. The evergreen bagworm caterpillar creates a small nest of twigs, needles, and other plant debris, not the large webby tents we are familiar with. You may have seen the evergreen bagworm hanging from the twigs of Arborvitae if you have ever driven through Oklahoma but you won't find this insect in this area. On occasion we do find infestations of the snailcase bagworm attached to the trunks of trees and the side of home but these cannot be confused with tent caterpillars or the fall webworm as they look like snails. The tents of the fall webworm tend to hang in the trees for several years while the light and airy tents of tent caterpillars disappear within a few months of the caterpillars leaving the nest.

The application of dormant oil prior to leaf emergence in the spring is one of the best ways to control the tent caterpillar. The oil will cover and plug the breathing hole in eggs causing the caterpillars' death. To be effective this needs to be done by the middle of March. This spray will also suffocate overwintering aphids, other insect eggs, and mites that cause damage to your trees. If you have fruit trees this is one treatment that should not be neglected. Another spray that works on the tent caterpillar is the bacterium B.t. There are several strains of the bacterium and the one designed for Lepidoptera (moths and butterflies) is what you need to use.-

An application of dormant oil is not effective on the fall webworm as the timing is incorrect since the eggs of this insect are laid in late summer. The application of the systemic insecticide dinotefuran is however, very effective on this insect. This neonicitinoid insecticide, sold under the trade names Safari, Zylam, and others, is a recent addition to the tree care professional's bag of tricks. When sprayed on the bark at the base of a tree it is quickly absorbed and translocated to the leaves this caterpillar is feeding on 100 feet or more in the air within a few weeks.

If your tree had fall webworm last year, a treatment in early June should prevent the reappearance of the insect this year. A treatment of dinotefuran last fall or even now should control this year's infestation of tent caterpillars. Dinotefuran has been reported to be effective for up to two years so it is a great product to use. Dinotefuran is also very effective on the black pineleaf scale infesting many of the Austrian pine trees in this area.

The neonicitinoid insecticide Imidacloprid is not effective on Lepidopterans or hard-scale insects such as the black pineleaf scale. Imidacloprid is, however, the insecticide I recommended to Connie at All Metals Welding and Fabrication Company for the borers tearing up her black locust. This locust borer is the larvae of a beetle for which imidacloprid is very effective. This insecticide is best applied around the base of a tree in the fall or spring. Unlike dinotefuran, imidacloprid is not effective as a bark treatment.-

Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at or check out his blog at He owns Swift Horticultural Consulting and High Altitude Lavender.

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The Post Independent Updated Feb 21, 2013 02:17PM Published Feb 21, 2013 02:12PM Copyright 2013 The Post Independent. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.