Cocoons in Evergreen Trees
The bagworm (Thryridopteryx ephemeraeformis) is one of the most destructive pests on evergreen trees, especially junipers. In severe infestations, they attack deciduous trees and ornamental plants in the landscape, eating all the foliage. The first sign of a bagworm infestation is often the appearance of cocoons they weave in which to pupate and lay eggs. Control of bagworms is most effective if you are aware of their life cycle and feeding habits. Does this Spark an idea?
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Bagworm Identification
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Bagworms are usually noticed in the fall when they form 1/2-inch-wide, 1- to 2-inch-long cocoons that appear along the branches of target plants, especially evergreens. The cocoons are brown, cone-shaped and are made of plant material, such as sections of leaves. The larvae are so small when they begin feeding in spring, they are difficult to see until damage is evident. They are easily seen in summer because they form a protective covering of plant material that moves along with them as they feed.
Life Cycle
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Once the 1/8-inch-long, brown or tan bagworm arrives on a target plant in spring, it builds the protective bag of plant material around its body, and begins feeding on the target plant. It feeds all summer, working from the top down, before attaching itself to a branch with a strand of silk. The silk used to attach the cocoon to the branch is so strong, it alone can girdle and kill a branch.
The larva pupates inside the cocoon and matures in early fall. The female bagworm remains in the cocoon, while the male emerges as a moth, leaves the cocoon and flies to find a female. The male dies after a few weeks, but the fertilized female lays up to 1,000 eggs in her cocoon before dying. The eggs hatch in spring. Each of the tiny larva spin a fine silk thread as they drop from the cocoon. The slightest breeze picks them up and, using the thread as a "balloon," they fly to a new host plant and start the cycle anew.
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Damage
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Damage to evergreens is swift once the worms arrive and start feeding. All the foliage on an entire limb can be removed over a period of a few days. Bagworms often kill or severely weaken the target plant. Evergreens are particularly susceptible to damage because they cannot quickly regenerate foliage lost to the caterpillars.
Organic and Chemical Control
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Removing the bag, or cocoons, by hand in early spring before the eggs hatch helps control large numbers of bagworms. Put cocoons in a plastic bag or other container and dispose of them by smashing the cocoons while they are in the bag and placing the bag in the garbage.
Organic sprays that contain Bt (Bacillus thuringiensis), or spinosad, are effective if sprayed while the worms are actively feeding in late spring and early summer. Spinosad has a longer active life that Bt. Both Bt and spinosad are made from naturally occurring bacteria. Thoroughly cover all parts of the tree when treating with either product.
Chemicals available for bagworm control change frequently. For a list of chemicals currently approved for bagworm control, contact your local county agricultural extension office.
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