Time to Cut Those Bagworms!

Last night, during my evening walk I noticed an unusual number of bagworms on a few evergreens. Bagworms are common pests in the Washington DC metro area. What I saw weren’t the bagworms themselves (Thyridoptery x ephemeraeformis) but their “homes,” 2-inch long “bags” they have created from their spun silk and plant debris. These bags were hung like small, brown ornaments on relatively new plantings in someone’s front yard. Some of the needles were clearly brown and dead.  Interestingly, one bag was hanging from the neighbor’s chain link fence, creating a very visible view of the threads wrapped round the metal.

Bagworms are moths, native to North America. They can attack more than 120 different types of trees but we tend to see them on evergreens such as juniper, arborvitae, cedar, spruce, pine, and Leyland cypress.

In the beginning of the summer, the eggs hatch and the larvae move out of the bags. The tiny caterpillars, 2 millimeters long, eat foliage and/or move to other trees via their silk threads. When they settle on their host tree, they spin a small bag of silk and plant debris. As they grow, their bags become bigger with more material collected from the host plant. By August, they have matured and the bags are very visible. During August and September the male adult bagworms, i.e., moths, emerge and fly to find a female to mate. The females cannot fly, they are grub-like and never leave the bag. Mating occurs through the bag and after mating, the female lays 500 to 1,000 eggs within her pupal cast skin and dies. The eggs overwinter and hatch next year in May or June.

Bagworms can defoliate and kill trees, especially evergreens. Although they can attack deciduous trees, most are not defoliated enough to be killed. Bagworms can also kill twigs by winding their silk around the twigs too tightly.

note white silk thread wrapped tightly around chain link fence

Now is the time to look for the bags and remove them by cutting them off–not pulling–bagging and disposing. Do not put them in your compost bin. If they are on the perimeter they will be easy to find but don’t forget to move branches aside and look within the tree. It goes without saying that a tree that has a bag will be damaged repeatedly each year, weakening the tree and possibly killing it.

Another option is to spray the tree with Bacillus thuringiensis (Bt) in May or June when the caterpillars have hatched and are on the foliage. This is a type of bacteria that kills the worms but does not harm the tree. If the tree is too tall, call an arborist. Spraying them later in the season after they have created their bags will not work since the bags will protect them. Or replace the evergreens with more resistant plants. But go out there now and inspect your plants, you need to remove them before the males emerge!