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Insecticide for Bagworms

By Bonnie Grant ; Updated September 21, 2017
Bagworm cocoons in pine trees mimic the male pine cones and are difficult to find.

Bagworms are sneaky pests that can cause serious harm to trees and ornamental plants. The insects seem to prefer conifers, but they can be found on other species occasionally. Bagworms build a nest out of whatever materials are on the host tree, which means it blends into the plant. The larvae grow and feed inside the bag, which they carry with them as they forage. The bags are waterproof, which makes it difficult for insecticides to penetrate them.

Identification

Bagworms are a species of moth whose females are eyeless and wingless. The larvae begin life inside the bag and cart it around with them, enlarging it as they grow. The bags eventually are about 2 1/2 inches long and the larvae are full grown. They stop eating and tie their bag off to a twig. Then they close the bag and pupate. The males emerge in four weeks, while the female lives the rest of her life in the bag.

Damage

The bagworm feeds on the foliage of the host plant. The defoliation that ensues can be severe enough to kill an evergreen. The tree can't produce carbohydrates or plant sugars for energy without foliage. Deciduous plants are not usually killed because they will grow new foliage the following year. Female moths overwinter in the bag and can lay 300 to 1,000 eggs each. This can mean overwhelming infestation. Hatching begins in mid-June, so control measures need to be taken in early June.

Mechanical Control

In lightly infested trees, it is appropriate to simply pluck off the bagworm cocoons. Look in fall or winter for the nests and pull them off and destroy them. This ensures that they do not have time to hatch and effectively reduces the chances of an infestation in June. The hand picking method is not practical on taller trees and you need to be completely thorough since even one cocoon can cause a huge infestation. There is only one generation of bagworms per year, so this method works quite well on individual infestations.

Insecticides

You can use some very effective bio-insecticide measures against bagworms. Bacillus thuringiensis is a naturally occurring bacteria that attacks the nervous system and kills the larvae. Spinosad is also a safe insecticide that has shown good control. Cyfluthrin, trichlorfon, acephate and carbaryl are all prescribed for bagworm control. Every leaf and needle needs to be saturated to ensure that none of the larvae escapes. Insecticides used in June will be ingested on the leaves of the plant by the larvae and kill the insect.

 

About the Author

 

Bonnie Grant began writing professionally in 1990. She has been published on various websites, specializing in garden-related instructional articles. Grant recently earned a Bachelor of Arts in business management with a hospitality focus from South Seattle Community College.