Bagworms Of Leyland Cypress

Disease Information


Bagworms (Thyridopteryxephemeraeformis) infest many shrubs and trees, but conifers (evergreens) are the preferred hosts. Damage to plants results from feeding by the caterpillars, which causes loss of needles. Mild infestations of this pest slow the growth of Leyland cypress. Heavy infestations can kill a plant.


The adult male bagworm is a dark-colored, hairy moth with a 1-inch wingspan and clear wings. The adult female does not have wings or legs, is yellow, and appears almost maggot-like. The larvae (immature forms, aka caterpillars) range in size from about 1/8 inch to 2 inches depending on maturity.


Each larva produces a carrot- or cone-shaped bag that it carries as it feeds. The bag is formed from silk that the larva produces. As it feeds, the larva adds bits of plant material to the bag for camouflage. The bag is about 2 inches long when complete. Home gardeners sometimes mistakenly identify it as a pine cone.

COMMENTS on disease

bagworms survive the winter as eggs in a bag. The larvae hatch during May. Each one produces a strand of silk that allows it to be blown by the wind to a new location on the same plant or to a new plant. They soon begin to spin their cases. When mature, each larva pupates (transforms to an adult) within its bag. An adult male moth emerges from its bag in late summer (August/September). It locates an adult female in her bag. After mating, the female lays 500-1,000 eggs in her bag and dies.


 Several parasites and predators feed on bagworms, generally keeping their numbers under control so that damage is not noticed. Removal of the egg-containing bags during winter and early spring is a very effective method for preventing problems before the next growing season. Once removed, the bags should be destroyed or placed in a deep container (5-gallon bucket), which allows beneficial parasites that may also be present in the bags to escape while retaining the bagworm larvae.

If an infestation is severe or the bags are out of reach, spray with the bacterial insecticide, B.t. This insecticide contains spores of the bacterium,Bacillusthuringiensis, which when eaten, kill the caterpillar. Young larvae are much more susceptible to B.t. than are older larvae. As such, apply this pesticide in the spring as soon as bagworms are seen (usually in May) and repeat two weeks later. Control is most effective when spraying is done in late afternoon or early evening. This insecticide is very safe to use. Once the bags have reached ¾ inch long, the efficacy of B.t. sprays decreases rapidly.

Sprays applied later in the season (May and June), when bagworms are larger must be with a contact insecticide, such as permethrin, cyfluthrin, lambda cyhalothrin, carbaryl, malathion or acephate. Note that these insecticides will also reduce populations of beneficial insects (predators and parasitoids) that help control spruce spider mites, which can result in an outbreak of this occasional Leyland cypress pest. Soil application of neonicotinoid insecticides, such as imidacloprid or dinotefuran only give minimal (less than 10%) control from bagworm damage, and should not be substituted for spray control.


Share this article