Bagworm Cures for Fir Trees

Identification and Damage

• Bagworms are the larval stages of the Theridopteryx ephemeraeformis moth. Only the winged, adult males leave the cocoons after they pupate. The females are sightless, wingless and legless, and they never leave their protective cocoons. Even during mating, they remain inside while the male moths fertilize them through tiny openings in the cocoons. The female lays her eggs inside the cocoon where she lives, and the hatchling larvae exit and spin their own protective bags. The larvae leave their cocoons to feed on fir needles but retreat into them during resting stages. Their voracious feeding kills fir needles and, in severe infestations, defoliates trees. As the larvae grow, so do their cocoons, which often escape detection because of the unique way bagworms weave fir needles into the silk casing.

Handpicking

• Fir trees grow in U.S. Department of Agriculture plant hardiness zones 3 through 9, depending on species. Understanding how bagworms reproduce underscores the need to eradicate them completely, so that you can break their life cycle and rid your fir tree of these pests. Removing the bags by handpicking them is a viable solution if the height of your tree is not out of reach from the ground or a ladder. But if you miss only one of these cocoons, it may be harboring a sessile female that is capable of laying 1,000 more eggs that could reinfest your tree.

Encouraging Predators

• Birds, particularly finches and sparrows, are primary predators of bagworms. Encourage them to your garden by offering plants that produce seeds and fruits, or by placing seed feeders near your fir trees. Other predators that feed on bagworm larvae and pupae include tachinid flies and parasitoid wasps. Flowering plants that produce nectar and pollen invite wasps and flies to your garden, which helps control bagworm populations. If you use chemicals to control bagworms, choose a minimal-risk product to reduce the toxic threat to beneficial insects.

Chemical Controls

• Biorational insecticides pose minimal risks to beneficial insects, but they are effective in killing bagworm larvae. Purdue University Extension recommends using products that contain spinosad or Bacillus thuringiensis (Bt), which are labeled for caterpillar control. Typically, you mix 4 tablespoons of a product containing spinosad in 1 gallon of water and spray your fir tree’s foliage thoroughly to control bagworms. Mix Bt products at a lower concentration, generally 4 teaspoons in 1 gallon of water. Products vary, so it’s important to observe label directions for application rates and restrictions.