By Lisa Stewart
Extension County Director
What has covered and killed my knock-out roses?
Bagworms are the most common insect pests of juniper, arborvitae, cedars and other needle-bearing evergreens. They also occasionally attack broadleaf trees and shrubs like river birch and Indian hawthorn, or knock-out roses.
Infested trees are made unsightly by defoliation and the presence of the bags. Heavy infestations can cause total defoliation, making it necessary to replace the affected plant.
The caterpillars live inside silken bags that they cover with pieces of foliage from their host plant. The caterpillars also pupate inside these bags and adult females never leave the bags. They simply mate with the winged males, deposit their eggs inside the bags and die. Bagworms overwinter inside these bags as eggs, which hatch the following spring. Because there is only one generation per year, proper spray timing is the key to good control.
Control Bagworms: Removing the bags by hand is a good control option for small trees with light infestations, but you may need to use scissors or hand pruners to cut the strong silk fibers that attach the bags to the tree. For heavy infestations or large plants, spray with an insecticide containing spinosad (Fertilome, Bonide, Monterey and Greenlight companies all sell spinosad products).
For best control, apply sprays shortly after eggs have hatched in the spring and before caterpillars have grown large enough to cause significant damage. This is about the time crape myrtles have finished leafing out. Bagworms do not normally occur on crape myrtles, but bagworm egg-hatch coincides with spring leaf development of crape myrtles.
Avoid spraying with products like carbaryl, malathion, acephate or pyrethroid insecticides (permethrin, cyfluthrin, bifenthrin). These products will kill bagworms, but they are also prone to trigger spider mite outbreaks, and home gardeners have no good treatment options to use against spider mites.
Because of our crazy summer, bagworms are appearing later this year.
This column is made up of questions presented to the Webster County Extension Service. Please get in touch in person or by phone (258.3971) or e-mail (firstname.lastname@example.org) if you have questions you would like to see discussed here.