Summary: Commonly observed as a cigar-shaped bag camouflaged with pieces of bark, foliage, the bagworm is notoriously shy. However, there are effective means of keeping these insects from infesting your precious shrubs.
The bagworm gets its name from the characteristic bag it builds around itself using silk and bits of plant material, dirt, and other components.
There are three distinct types of bagworm in the U.S. characterized by the type of plants it prefers to eat. One, the snail shell bagworm, is so named because its bag closely resembles a snail. Bet you didn't see that one coming. The evergreen bagworm is the variety most considered to be a pest of ornamentals. These creatures tend to prefer cedar, pine, juniper, spruce, and other evergreens as its food supply, but it will attach to and eat from certain deciduous trees as well. The grass bagworm is a larger problem in gardens for vegetable, legume, and ornamental plants.
Bagworm eggs winter in the bag of the female that lays them. Her body is generally mummified around the eggs as an added layer of protection. In early spring the eggs hatch and the tiny larva exit the bag and begin to spin its own bag around its lower body. They often drop down on a single silk thread and spin a sort of balloon that allows them to be carried on the wind to the next plant they will infest. Others may simply crawl to another part of the same tree or plant and start its bag.
As bagworms grow, it increases the size of its bag and incorporates bits of plant material inside it to make an excellent camouflage. This keeps the majority of them safe from birds and other animals that would prey on them. Thus predation is not a major form of control for these pests.
During spring and summer it is difficult to find these pesky creatures. Typically the only way to know they are present is when damage to the plant becomes extensive. In late fall, it tends to shape its bags into something resembling a pine cone and attachs it to a limb with a strand of silk that wraps around the limb of the tree. It then hang upside down like a bat and pupates.
The adult males emerge from its bags in the form of a moth with clear wings and a furry body. Females do not change their form and do not come out of its bags. Instead, it emits a pheromone signal that calls the males to them for mating.
Control of bagworms is difficult under the best of conditions. Certain pesticides such as Sevin, diazinon, permethrins and Talstar One have been shown to be effective against the very young bagworm larva if applied at the right time. It is necessary to spray the entire plant and hope the pesticide leaves a slight residue because the bag protects the larva from direct contact.
In summer and fall the bagworms have a lot in common with nails. Both are tough. Pesticides are largely ineffective at this point. The most effective means of control is also the most difficult and time consuming. The individual bags must be plucked from the plants and smashed. This is not easy because the bags often appear to be a part of the plant or the cones produced by evergreens.
When removing the bagworm bags it is essential to remove the wrapping bands of silk that hold the bags in place. Failure to do so can result in the limb being choked off by the silk as it grows.
It is very rare for a bagworm infestation to do enough damage to a plant to cause its demise. It can damage plants in ways that create asymmetrical growth cause damages to the plant's appearance. Left unchecked the bagworm could multiply resulting in defoliation severe enough to kill fully mature trees and choke off the growth of new limbs.