Have You Noticed Your Arborvitaes Turning Brown?

Have you noticed your arborvitaes turning brown? Do you see pine cone-like objects hanging on the branches and they appear to be moving? After closer observation, do you notice a dark brown to black caterpillar sticking its head out? What is this caterpillar?

This is no ordinary caterpillar. It is a native bagworm moth larvae. Adult male moths are about 3/4 inches long and black. They are made with females in their bags, which look like cones hanging from the host plants. The mature female larvae in their bags are one to two inches in length. The silken bags that the larvae create are covered with plant parts from the host plant. They construct and add to them throughout their life. Silken bags of live larvae have green material on the outside of them. There is one generation per year.

The bagworm has been known to attack many types of plants when food sources are limited. Some of the more common plants damaged by bagworms are cedar, arborvitae, juniper, leyland cypress, white pine, sycamore, honeylocust, willow, oak, and maple. In high populations, they have even been found on roses and perennial flowers. Only the growing larvae feed on host plants.

The damage is most serious and obvious on foundation conifers, such as arborvitae and juniper. The bagworms can completely defoliate branches of the host plant. On large trees and shrubs, exfoliation is less evident. Most people don’t notice the damage until a large branch area has browned out. By that time, it may not be too late to control the bagworms, but maybe too late for the branch area to recover. How can you monitor the bagworms?

In early June, begin looking for new bags on the host plants, especially where there is the presence of old large brown bags. First, look on the outer foliage in the full sun for bags. In the fall and winter, you can manually pull these old bags off and throw them away. Each old bag may contain up to 1,000 overwintering eggs. If there are too many bags or they are too high up in the plant to reach, call us at Virginia Green Lawn Care to put these plants on our Tree and Shrub Program. We can help you protect your valuable trees and shrubs from these devastating pests.