I can see them hanging all over our eastern red cedars right now greedily munching away on the tender young needles from the protection of their “bag”. Curious little devils but very destructive.
Bagworms are not worms at all but the larvae of insects that attack many evergreen (and some deciduous) trees and shrubs. The caterpillars of the evergreen bagworm (Thyridopteryx ephemeraeformis) damage many landscape trees and shrubs by feeding on needles and leaves. A large infestation can lead to almost complete defoliation of trees.
Some of the more common evergreen host plants include their favorites, arborvitae and red cedar, and also fir, hemlock, pine, and spruce. Deciduous host plants include black locust, honeylocust, sweetgum, and sycamore.
The name bagworm comes from the characteristic case or “bag” that these caterpillars construct around them as they feed. The bag is composed of silk and bits of plant material collected from whatever they happen to be feeding on. As the caterpillars grow in size over the summer, they enlarge the bag by adding more plant material to the front of the bag. It is like a “mobile home” during their larval stage and later becomes their pupa case.
The life cycle of these insects is rather interesting. In late May and early June, the eggs hatch from bags that were constructed the previous season. As soon as they hatch, the young caterpillars crawl out and begin to construct a bag around their hind parts. They “wear” this bag as they move slowly through the branches feeding on needles and leaves.
In August, the mature larvae attach their bags to a branch and enter the pupa stage looking very much like small pine cones. The pupa stage lasts about 4 weeks.
The adult males, which are small moths with clear wings, emerge from the cases in September or October and begin to search for females. The adult females remain in their bag, releasing a pheromone to attract the males.
Female bagworms mate, produce eggs, and then die without ever leaving the bag. In fact, the eggs develop within the dead body of the female.
A female bagworm can produce up to 1,000 eggs which overwinter in the bags and hatch out the following May. Yikes, that’s a lot of little caterpillars! No wonder they can be so devastating to our evergreens.
Birds and some insects will prey on bagworms but normally this predation is not enough to stave off an infestation. The following are some tips for getting rid of bagworms if you find them hanging from your trees or shrubs.
As much as is feasible, try to physically remove and destroy as many of the bags as you can. The silk that holds the bags on the branches is quite strong and scissors or shears are usually needed to cut them off. Depending on the number of bags and the size of your trees or shrubs, removing them all may not be possible. We do not recommend using a ladder!
The second most important step is to control the YOUNG caterpillars when they hatch out in the later part of May or June. Young caterpillars can be controlled (without harming beneficial insects) by spraying Bonide BT Thuricide (Bacillus thuringiensis). Bt is a type of bacteria infects and kills the young caterpillars. It is also helpful for controlling young tent caterpillars.
If you miss the younger ones, larger caterpillars (and the young ones too) can be controlled using Bonide Captain Jack’s Deadbug Brew. This product contains spinosad, an insecticide derived from a naturally occurring bacterium. Like Bt, spinosad targets many different plant pests but spares most beneficial insects when used according to the label directions.
Always read and follow the label directions when using any pesticide and NEVER spray any insecticide when trees or shrubs are in bloom!