Bagworm Problems

September 9, 2008 at 7:00 am

Many people have noted the many plants being ravaged by the bagworm this year. Bagworms have been thriving on many coniferous plants this summer, especially arborvitae, spruces, and junipers. But bagworms can also feed and thrive on a long list of deciduous trees and shrubs including sycamore, buckeye, crabapple and honeylocust trees. These trees may not be killed by the bagworm; however, the caterpillars can make them look very bad. The infested trees can also act as a reservoir for bagworms to spread throughout the landscape in future seasons. Heavily infested plants are easy to spot from a distance because the tops are thin and turning brown as the bagworms chew off the foliage. An up-close inspection will reveal hundreds of bagworms hanging down from the browning branches.

Unfortunately we are nearly past the time where an insecticide application will have any effect on bagworms, unless the bagworms are still actively feeding. The bagworm caterpillars are getting too big and old for insecticides to be effective. Also, bagworm populations have reached a point in their feeding and growth where they are ready to move on to the next stage in their development, pupation. When bagworms are ready to pupate, they will spin silken bands around stems of the host plant. Once a firm anchor has been spun, the caterpillar will suture the opening in the end of the bag closed. After a short period, the caterpillar pupates.

Some populations may currently have numerous small bags sewed up while bigger bags still have caterpillars poking out of the end ravenously devouring everything it their path. The small bags are most likely males which require less food to finish their development than females, so they pupate sooner. Once the bagworm seals the end of the bag, there isn’t much hope of managing the population with an insecticide this season.

If bagworms are found on your trees they should be picked off the plant and drowned in a bucket of dishwashing liquid and water (make sure you throw the bag in the water too). The dish washing liquid saturates the bag and coats the eggs. Of course burning them is a good option if you live outside the city limits. This can be done easily in the late fall when deciduous foliage has been dropped or the bits of plant material on the bags turn brown and can be easily located on evergreens.

To prevent bagworm infestation next year, plan for a June insecticide application of Sevin, Malathion, Biobit, Dipel, Thuricide, or Caterpillar Attack. This application should be applied to susceptible plants, and may require repeat applications for thorough control of bagworm. For more information on bagworm, read the OSU Extension Factsheet on bagworms at

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