Worm Cocoons in Cedar Trees

by Carolyn J. Randall Google
    Inspect your junipers for brown spots caused by bagworm feeding.

    Inspect your junipers for brown spots caused by bagworm feeding.

    Those brownish cocoons hanging in your cedar trees or shrubs are most likely bagworms, a pest that infests both deciduous and evergreen trees. Each bag contains a worm or caterpillar inside that feeds on the upper surface of the leaves. Bagworms are commonly associated with cedars from the genus Thuja (arborvitae) or Juniperus (junipers.). The bags resemble cones, so they are often mistaken for a natural part of the tree. If left undetected for too long, bagworms can cause extensive damage to landscape plants.

    Bagworm Life Cycle

    Bagworm larvae (caterpillars) construct their cone-like bags from silk and bits of interwoven twigs as they feed. When the tiny, blackish larvae first begin to feed in late May and early June, they carry the bags with them on their back. Later they attach the bag firmly to the plant as they get ready to pupate. By this time, the caterpillar's bag may be as long as 2 inches. Larvae pupate inside the bag, but only the males emerge as moths. The males fly to the wingless females that remain inside the bags and mate. Female bagworms lay 500 to 1,000 eggs inside the bags. The eggs overwinter and hatch in the spring. Since dispersal of the insect relies on the caterpillar stage, a single plant often becomes heavily infested. Caterpillars move to new plants by being windblown on silken threads or by crawling.

    Non-chemical Controls

    Bagworms can be controlled by hand-picking the bags. To prevent the insects from completing their life cycle, do not pull the bags off and drop them on the ground. Bags can by destroyed by crushing or you can kill the bagworms by dropping them into a can of soapy water as you pull them off. Some birds and insects are natural predators of bagworms, but their populations are usually not high enough to control the bagworms effectively. Planting daises and asters near bagworm-infested trees and shrubs supports populations of insects that are enemies of bagworms. Two species of ichneumon parastic wasps, Itoplectis conquisitor and Chirotica thyridopteryx, help control bagworm. You can support bird predation on bagworms by planting fruit- and seed-bearing trees and shrubs. Also, place a birdfeeder near your cedars to attract chickadees, nuthatches and titmice, which are known to eat bagworms.

    Chemical Controls

    Hand-picking can be tedious, especially if you have a lot of cedar trees and shrubs to take care of. It may be more practical to use a pesticide. When choosing a product, read the label carefully and make sure that bagworm is listed as one of the insects it can control. Products that use a bacterium, such as Bt, are considered less toxic to the environment than the traditional stomach poisons. Neem oil insecticides are also considered less harmful to the environment and to beneficial insects. The effectiveness of these pesticides depends heavily on the timing of application. Apply the product during the appropriate life stage of the bagworm. Most products are best applied in the larval stage in June when the caterpillars are still small and actively feeding. Applying insecticides in late August when the larvae have stopped feeding and are pupating inside their bags is ineffective.


    Be vigilant and inspect your cedars in the May and June when the larvae hatch. Early detection and control is the key to preventing older bagworm caterpillars from causing more severe feeding damage. In heavy infestations, bagworms may completely defoliate a cedar tree or shrub and the plants may not survive. Rake up and remove debris around cedar trees and shrubs in the fall to get rid of any bags that may have dropped to the ground. When removing bags by hand, also cut the silk attachment band to prevent the twig from being girdled.

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