Bagworms Spell Disaster for Trees

Roger Hart, writing for the News-Messenger in Fremont, Ohio, received the following e-mail and responded.

Mr. Hart, We were told by a local lawn care service that the evergreen bushes (arborvitae) around our pool have bag worms. What are they and how do we get rid of them? The man from the lawn care service wasn't sure what to do about them until he did some research. J.S., Fremont.
  
Dear J.S.: Bagworms are the larvae of a kind of moth. The males develop into flying moths but the females remain grubs and live in cocoon-like bags until they lay their eggs in the fall, after which they die.

Bag worms feed on all deciduous trees but, for some reason, are especially attracted to evergreens varieties such as arborvitae, juniper, spruce cedar and pine. If left unchecked, they can be devastating, defoliating entire trees or shrubs.
The cycle of the bagworm starts in the spring (usually sometime in May) when eggs, laid the previous fall, hatch and the larva emerge to feed on the host. As they grow, the larvae start to weave bags out of silk and bits of the leaves they are actively eating. As they grow, so does the bag, until it is large enough for them to retreat into, if anything attacks or disturbs them.

The larger, older larvae are extremely ravenous and can eat the leaves off entire branches. In early fall, the male bagworms emerge from the pupal stage as small black furry moths. The females don't develop wings or legs; instead, they attach their cocoons to a branch of the host plant and remain inside secreting powerful pheromones, which attract the searching male moths. Once she has mated, the female dies and falls through the bottom of the bag, leaving behind several hundred eggs.

On evergreen trees and shrubs, the bags look like tiny pinecones- approximately 1-2 inches long- but, on closer inspection, you will be able to tell that they are made up of bits of leaves.

At this time of year, the best thing to do is to handpick the "bags" off. Spraying with pesticide now will not do much good because the eggs won't be affected by the spray and they are done feeding for the year anyway. Check again in early spring for any bags you may have missed.

Another option is to wait until the middle of May or the beginning of June- or both- and spray them with sevin, malathion or diazinon while the larvae are still small. Larger grubs are more resistant to these types of pesticides, so it is important to spray early in the season. When spraying, be sure to wet the entire surface of the leaf thoroughly.

For severe infestations, or in cases where the bags are in large trees, it is probably best to call on a professional to take care of the problem. Hope this helps, Roger.

After receiving this e-mail, I went out, checked my own arborvitaes, and found a significant number of cocoons dangling from the branches. Therefore, I advise you, dear reader, to get out there and inspect your evergreens for "bags." If you find that your plants have been infested start picking- be sure put to them in a securely tied plastic bag and dispose of properly- the more you pick the fewer you have to worry about hatching in the spring.

Roger Hart is owner of Mole Hill Farm, a market garden and greenhouse business in Clyde, Ohio.