Bagworm

The Pest
The aptly named Bagworm is often seen during the winter hanging en mass from defoliated evergreens and dormant trees and shrubs in neglected landscapes. The one to two inch long, spindle-shaped, brown bags, constructed of bits and pieces of the host plant, appear lifeless in the gray cold of winter. In fact the bags that housed males last summer are empty, but the female bags may be jammed with from 500-1000 eggs.

The Feeding Begins
Those over wintering eggs begin hatching in late spring, about the first week of June in our area, when Mock Orange are in full bloom and you see the first red fruits on Honeysuckle. The larvae immediately begin feeding and constructing their protective 'houses' out of silk and bits and pieces of their host plant. They prefer arborvitae and juniper but aren't picky or nimble enough to move much and will devour just about any tree or shrub, evergreen or deciduous, that Mama bagworm last munched on. That's why the bags on maples look different from those on arborvitae: different building material.

As the pest grows its bag expands to comfortably accommodate the worm as it awkwardly, but surprisingly quickly, searches about the plant for choice feeding areas. Everywhere the worm goes, the bag goes.
Bagworms feed on the foliage of evergreens as well as on a wide variety of deciduous trees and shrubs. They partially or completely defoliate whatever plant they're eating but deciduous trees and shrubs usually don't die. They look nasty with no leaves and all those bags hanging on them but they usually can survive. A defoliated evergreen is another story. It is ugly and dead. Next year's bagworms will have to find another food source.

Life Cycle
In late summer the worm has eaten its fill and begins the 7-10 day transition to adult, pupation, in its bag. The males emerge as a furry black moth with almost clear wings. The adult female is not so lucky. She is wingless, legless, eyeless and has no functioning mouthparts. Her only reason to exist is to mate and lay eggs. She lays her brood at the top of the bag, packing them in securely with her own body parts. Then she dies. Fortunately for the homeowner there is only one generation a year.

Eliminating Them
Efficient control is a matter of timing. Spray is best applied while the worms are small, soon after all eggs have hatched, around June 15. Don't wait too long because later sprays are much less effective. The larger the worms, the harder they are to kill with chemicals.

We recommend a natural occurring, target specific caterpillar killer bacteria called Bacillus thuringiensis. The caterpillar ingests it, the good bug paralyzes its digestive system, the bagworm, or any other caterpillar, cannot feed and slowly starves to death. A fitting demise for these eating machines. The bacteria is found as Thuricide, a concentrated liquid; Dipel, a dust or powder; and under many other names, even, simply enough, Bt.

Wet or dust all the foliage thoroughly and repeat if rain falls within 48 hours. Forget about spraying late in the summer. Once feeding is complete you can't kill them. But you can substantially reduce the population by picking off and destroying the overwintering bags before egg hatch in the spring.