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Time to battle bagworms

Bagworms on a tree near Hardyville.

by David Moore
and Dan Nortman
Middlesex Extension

The bagworm is a general pest of trees and shrubs. The caterpillar of a moth, the bagworm constructs a carrot-shaped, or ice cream cone shaped, case or bag formed from plant material. They attack both deciduous trees and evergreens, but are especially damaging to Leyland cypress, juniper, arborvitae, spruce pine and cedar (most conifers). 

Large populations of bagworms can strip plants of their foliage and cause them to die. A bagworm infestation often goes unnoticed at first because bagworms are inconspicuous when young. By the time they are fully grown, the branches are bare as a result of their feeding and the 1-2 inch bags are more visible.

Bagworms pass the winter as eggs inside the bag that contained the previous year’s female. In mid-to-late May, the eggs hatch and small larvae crawl out from the end of the bag in search of food. By using silk and bits of plant material, they soon construct small bags that we begin to see by mid-summer.

As larvae continue to feed and grow, they enlarge the bag. Older larvae strip evergreens of their needles and consume whole leaves of susceptible deciduous species. By early fall, the bags reach their maximum size of 1.5-2 inches. At this time, the larvae permanently suspend their bags from twigs and transform into the pupa or resting stage before becoming an adult.

We have planted a huge number of Leyland cypress evergreens here in the Middlesex area. We have done this because Leylands grow fast and give us privacy along property lines. We did this not realizing the pests that would follow this attractive, fast-growing tree. 

Leylands also are prone to damage from high winds and have problematic root diseases that may cause gradual death by the time the tree reaches 15-20 years of age. Consult your nursery/plant supplier for suggestions for replacements for Leyland cypress. 

For more information about Leylands and their diseases visit UGA’s article Diseases of Leyland Cypress in the Landscape.

Hand-picking the bags from infested plants and destroying the cases are simple ways of reducing bagworm numbers. This procedure is especially advantageous during the spring, late summer, and during the fall and winter. For effective control, you must get every bag, collect them and discard or burn. When bagworms are too numerous to hand-pick, an insecticide may be applied.

Remember, however, that the older the bagworms become, the more difficult it is to kill them with pesticides. The best time to treat with pesticides is mid-May to early-June. Treatment may need to be repeated again by mid-June to control the staggering egg hatches. 

A big mistake that homeowners make is that they wait too late to treat.  They call for control recommendations in August and September after the bags have become large and difficult to penetrate with pesticides. Please remember to do your chemical controls in May and June.

Bacillus thuringiensis (Thuricide, Dipel), carbaryl, cyfluthrin, neem, acephate, esfenvalerate, pyrethrin, malathion, dimethoate and permethrin (these are active ingredients) are all labeled for homeowner use. Insecticidal soap may give limited control, but must be applied early before and during bag formation. Pesticides mentioned are generally listed as the active ingredient or common chemical name. Read the pesticide label to determine if the correct active ingredient is present. Regardless of the product you choose, be sure the plant and/or pest you want to control are on the label. For more information about bagworms in the landscape visit UKY’s article Bagworms on Landscape Plants.

Trade and brand names are used only for educational purposes, and Virginia Cooperative Extension does not guarantee or warrant the standards of the product, nor does Virginia Cooperative Extension imply approval of the product to the exclusion of others which may also be suitable.

posted 04.22.2009

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