Treating bagworms |

Treating bagworms

Richard Snell
Barton County Extension Agent

Well, it is that time of year, which you have been nervously anticipating dealing with that insect pest called the bagworm. Yes, bagworms are here!! Typically the time between Memorial Day and July 4th is when we want to treat for bagworms.

Newly hatched caterpillars (larval stage) are very difficult to detect because they tend to blend in with plant foliage. In addition, although we won’t admit it, as we get older (or more mature) our eyesight tends to diminish, which also makes it difficult to detect small bagworms. In the spring, caterpillars climb to the tops of trees and hang-out on one to three foot strands of silk. These strands are eventually caught on wind currents and detach, becoming streamers that allow the caterpillars to remain aloft for hundreds of feet to several miles, depending on wind speed (or velocity) and the occurrence of up-drafts. This process is referred to as ballooning. The caterpillars float through the air until the silk catches onto a plant or other object. It is important to note that caterpillars can balloon from nearby or even distant plants (e.g. trees).

In general, young caterpillars are small and cause only minimal damage to foliage; feeding on the epidermal and mesophyll layers, which create light areas on leaves. It is typically recommended to avoid applying any insecticide for at least two weeks after egg hatch to allow adequate time for the caterpillars to complete the ballooning process, settle down, and initiate feeding. An insecticide application during this time maximizes regulation of bagworms resulting in higher mortality levels. A second application is usually required a week or two later.

A female bagworm still hanging on a tree from last year may contain between 500 to 1,000 eggs. In general, newly hatched caterpillars emerge from the bottom of the bags from late May through early June. Each caterpillar creates a small silk bag, or case, covered with material from the host plant to fed upon. Caterpillars remain in the bag for the remainder of their life. Young (or early instar) caterpillars are 1/8 to 1/4 inches in length and initially feed on the epidermal tissue on one side and the mesophyll layer, which may cause leaves to appear white before turning brown. Young caterpillars typically initiate feeding at the top of trees and shrubs.

Deciduous trees and shrubs will usually produce new growth and are thus able to survive an infestation of bagworms. Bagworm caterpillars, in general, feed for approximately three months. On certain host plant species, female bags are located at the top, whereas male bags are distributed near the bottom of the plant canopy. This arrangement allows the females to effectively disperse pheromones that attract the winged males and increases the possibility for mating and fertilization of eggs.

In late summer (around mid- to late-August and sometimes into early September), caterpillars develop into a pupal stage inside the bags. Bagworms take about seven to 10 days to change from a pupa to adult; however, this depends on temperature. The males, which are black moths with clear wings, emerge through the bottom of the bag and disperse to mate with females. Females never develop into adult moths because they lack eyes, legs, antennae, and wings. As such, the females remain inside the bag, producing eggs before dying. The eggs are the overwintering stage and there is one generation per year in Kansas.

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If feasible, hand-picking and destroying bags from fall through mid-spring is very effective in removing the overwintering eggs before they hatch. If you only have one small tree or a few shrubs this can be very effective, but if you have many plants or larger trees, it isn’t practical. The bags can be placed into a plastic container with soapy water or into a sealed Ziploc bag and then disposed of. We don’t recommend placing the bags into a container filled with kerosene and then lighting it!

A number of insecticides labeled for regulation of bagworms include Bacillus thuringiensis. kurstaki (Dipel), cyfluthrin (Tempo), trichlorfon (Dylox), indoxacarb (Provaunt), chlorantraniliprole (Acelepryn), carbaryl (Sevin,) malathion, permethrin, spinosad (Conserve). Some of these active ingredients are often available and sold under different trade names. Furthermore, several of these materials may not be available to homeowners. Insecticide applications are most effective on the young caterpillars.