Bagworms are the larva of a small, hairy, black male moth and a wingless gray female. Bagworms will hatch from eggs in late April or early May to begin several months of ravenous feeding. Their favorite hosts are junipers and other conifers, but they will sometimes be found feeding on maples, oaks, Indian hawthorn, hollies, and other plant species.
Bagworms derive their name from their habit of attaching pieces of foliage from the plant on which they are feeding to a silken bag that envelops them. The bag serves both as protection and camouflage. Generally, the caterpillar’s head and front legs will protrude from the bag while it is feeding, but it will retract within the bag when disturbed or resting.
By late August, the bagworm has completed feeding, and it will attach its bag anywhere which it may hang freely, such as a branch. The bagworm then pupates for about one month and becomes a moth in early October.
For several weeks the winged male flies about and mates with the wingless female that remains within the bag. The female dies shortly after laying 500 to 1000 eggs within the bag. The eggs overwinter in the bag and hatch the following spring.
Bagworms are especially serious pests of junipers or other needle evergreens because these plants do not have the ability to generate new growth from branches that have been defoliated. If the entire plant is defoliated, then the plant will die; therefore, it is essential that these pests are controlled if you wish to save the plant.
Control is not difficult if the caterpillars are detected early. When the young caterpillars first emerge in the spring, they are very small – less than one‑quarter inch long. The easiest way to see them is to stare blankly from a distance and look for movement among the needles. Many insecticides are available for use on junipers that will be effective against young caterpillars to include organic products. For more information on specific control products, see HGIC 2017, Integrated Pest Management (I.P.M.) For Evergreen Bagworms.
As the caterpillars become larger, they are more difficult to control with insecticides. You may have to resort to handpicking and destroying the caterpillars if insecticides are having limited success.
The bags can even be picked through the winter months. The bags which contain the dead female moths and the eggs can be removed and destroyed to prevent the pest from appearing the following spring. Male bags often have the shell of pupae sticking out of the bottom. The bags are held on a stem with very strong silk and may have to be cut from the stem with a boxcutter or similar tool. The silk can often girdle and eventually kill the stem.