Dr. Grow: Bagworms deserve bad reputation

Say the word bagworm at a dinner party. The response will be "Yuk, oooh, ug, they give me the heebie jeebies, they give me the woolies, they make me want to throw up." These insects have a bad reputation. I have only one friend, Larry, who speaks fondly of bagworms. I suppose I should mention that he is an entomologist by trade.

After researching the subject of bagworms, I have found nothing good to say about them, except that they may be dinner for parasitic insects. I can find no positive role that they play in God's ecosystem. Yet, I'm sure He thinks otherwise.

Several years ago, I heard of one entrepreneur who used bagworms in an innovative marketing ploy. He advertised Christmas trees with natural ornaments on them. After looking at the advertisement, I went to investigate. This enterprising soul had a tree farm infested with bagworms. Normally people would not buy trees infested with insects, but the public was buying trees as fast as he could cut them. In fact customers tried to buy these unique Christmas trees the following year. Sorry, all sold out. His spray program worked the next year.

Feeding bagworms can cause extensive damage on evergreen trees. So how do you make sure they don't eat up your landscape? Know their life cycle. Determine the most vulnerable stage of the cycle and control them at that time.

The bagworm (Thyridopteryx ephemeralformis) overwinters in the egg stage in old bags. They are normally found on evergreens such as juniper, arbor vitae, white pine, bald cypress and spruce. There are 128 different host plants, many of which are not evergreens, recorded for bagworms, along with telephone poles, house gutters, etc.

Egg hatch occurs about mid-May in Southeast Missouri. Upon hatching, the tiny larvae (caterpillars) exit the old bag, lower themselves with silken strands to the foliage of the host plant, and begin feeding.

As the larvae feed, they immediately begin constructing a bag around their body, thus the common name, bagworm. Initially the bag is only 1/8-inch long. This camouflaged bag, made of coarse silk interwoven with bits of leaves and twigs of the host plant, will change appearance depending upon the host plant that it feeds on.

The larvae feed until early August and pupation occurs. At this time the bags are 2 inches long. Pupation is the developmental process that occurs when the larvae change into adults. The female is eyeless, wingless, and without legs, antennae, or functional mouthparts. She never leaves the bag she lived in as a feeding larva. The adult male is a true moth. He emerges from his bag, flies to females, mates and dies in a few days.

Soon after mating, the female produces 500 to 1,000 eggs, which remain in the bag. Following egg laying, the female emerges from the old bag and drops to the ground where she dies. The eggs overwinter in the bag and next May the cycle begins again.

As you can see, the sheer number of eggs in a bag suggest that only 1 or 2 bags on a tree can result in 500 to 2,000 hungry larvae devouring the host plant the following year. Because of this fact, I recommend that if you have plants susceptible to bagworms in your landscape, begin a preventive control program and continue it yearly.

How do you control bagworms? The first effective way is to manually remove any old bags that you see on a shrub or tree. All bags may not have live eggs in them. These bags may stay on a tree 1 to 3 years, before they disintegrate.

The second control method is a consistent spray program each year. Timing is critical. You must spray when the larva are feeding. Remember, feeding occurs after hatch in mid-May and continues until pupation in August. Spraying at any other time will result in no control.

For most effective control, spray when larvae are small. Use B.T. (a bacterial insecticide), sevin, diazanon, dursban, cygon, methoxychlor, or malathion. If you spray in July, B.T. or Sevin will provide no control.

Many nurserymen suggest not planting trees and shrubs that are susceptible to bagworm infestation. Extend this reasoning and you won't have any kids because they may get sick. Don't omit some very interesting and beautiful plants from your landscape because of potential bagworm problems. Just spend a few minutes each Memorial Day and make that preventative bagworm application.