Garden Clippings: Bagworms

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Bagworm on a tree limb.
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Have you ever thought about the wonders that surround us? For instance, technology has brought conveniences that were dreams only a few decades ago. The Internet, which can provide information on almost any subject, is available at the tips of our fingers, cell phones keep us in touch with others, no matter where we are, and debit cards allow us to buy things without having money in our pockets.

However, there are wonders that are even more remarkable than technology, and some have been around for thousands of years.

It’s remarkable that certain birds migrate each year to winter in warmer climates. What a feat it is for my backyard hummingbird, with its tiny wings, to fly thousands of miles each fall and spring.

It’s amazing that the mole, a mammal that evades a gardener’s trap so well, does not have the use of sight.

But some of the most astonishing characteristics of any group in nature belong to the insect group. A grasshopper can leap a distance over 20 times its length. If I could do that, it would be about a 120-foot jump. The Monarch butterfly migrates even farther than my hummingbird. But one of the most unique insects in the garden is the bagworm.

Bagworms are the scourge of gardens in the eastern half of the United States. If their voracious appetites don’t destroy a host plant’s foliage and ruin its aesthetic appeal, then the bag will remain on the plant for several years giving it an ugly Christmas tree ornament look. But I do admire the insect for the distinctive way it lives.

For those of us who live in areas the bagworm calls home, now is the time the insects become active. The caterpillars feed on many plants, but are most detrimental to evergreens. Deciduous plants attacked by the insect will lose foliage from the feeding and re-leaf, but evergreen plants can be ruined if feeding populations are high enough.

The life cycle of this insect is fascinating and worth knowing about for the purpose of eradication and control. Eggs hatch in late spring, and small caterpillars emerge from the bags created the previous year. The small larvae spin a thread of silk and drop down on foliage, or are caught by the wind and carried to neighboring areas. Once the caterpillars begin feeding, they form a bag around themselves with the silk and pieces of foliage from the plant they’re attached to.

The bagworm will feed until around mid-August. After that, the male will emerge from the bag as a moth and fly about seeking females in the bags. The female will lay 300 to 1,000 eggs, which will winter in the bag until the process starts over again.

Bagworms can be controlled culturally, biologically or chemically. Cultural control is achieved by simply picking off the bags in the fall and destroying them. This will improve the appearance of the landscape, as well as rid the garden of the hundreds of eggs in each bag.

The use of a bacterium (Bacillus thuringiensis, or what is commonly called Bt) in a nonchemical spray is effective in controlling the insects while they are young. However, it has little effect once they begin to mature.

There are many chemicals labeled for control of this insect, and all are effective if applied while the worms are small. Sevin, Malathion and Orthene are a few. Sevin is very effective before the worms mature, and it isn’t that unpleasant to apply. Orthene will do a good job if the caterpillars are older, but it has an offensive odor.

Because bagworms live inside the bag and retreat to the shelter at any disturbance, a spray application of any chemical must be applied before feeding stops in late summer if it is to be effective. Sprays will not penetrate the bag, which means the pesticide needs to be taken in by feeding.

It’s no wonder this insect is so successful in populating our landscapes. It has few natural enemies, and it has unique survival instincts.