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.
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.
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.
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.