Some shrubs easy targets for hungry bagworms

Published 12:00 am Friday, July 18, 2008

By Darrell Blackwelder
For the Salisbury Post
Leyland cypress and arborvitae have the potential to become seriously damaged or completely killed by bagworms. A homeowner contacted me earlier his week and the immature insect is now feeding at a rapid rate. Homeowners need to check their shrubs carefully to determine if bagworms are present. Bagworms first appeared about seven years ago on Leyland cypress and other evergreen shrubs. Over-planting of Leyland cypress and arborvitae make these shrubs easy prey for insects.
Moth larvae hatch in the early summer and spin down on silken threads. The immature ones are blown about by the early spring breezes, landing on adjacent trees and shrubs. The larva forms a protective cocoon or bag made of spent twigs or leaves. The bag is firmly attached by a sturdy silk band which the bagworms usually wrap around a twig.
Scores of larvae feeding during June and July have the potential of defoliating the shrub. Apparently, when the newly hatched larvae reach a plant, which is different from its parent’s host plant, these insects often have difficulty in adapting to it and may die or may produce only a few offspring. Unfortunately, after several years of struggling to keep from going extinct, the bagworm population develops the right combination of genes for a new plant and the new plant is covered with bagworms.
These insects can completely defoliate a shrub in a matter of days, easily killing the plant.
Many people have informed me that they have returned from a weeks’ vacation and their shrubs are completely defoliated. Bagworms particularly like Leyland cypress. Sprays of Sevin or Thuricide (a biological insecticide) in the early larval stage easily kill the insect. Shrubs with a few larvae can hand pick them off the shrub. As the insect matures, they increase in size and become resistant to insecticides. As the summer progresses, the larvae become very difficult to control. After these pupate, they are impossible to kill. Large shrubs or tree-like shrubs can be difficult or in some cases impractical to spray. Insecticidal sprays to immature insects provide the best control.
Darrell Blackwelder is an agricultural agent in charge of horticulture with the North Carolina Cooperative Extension Service in Rowan County. Call 704-216-8970 or fax 704-216-8995.
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