Garden Column for the Salisbury Post

Darrell Blackwelder

July 24, 2002

I've received a large number of calls over the past few days concerning bagworms on leyland cypress and other shrubs. These immature insects are now out and force and could kill leyland cypress and other evergreen trees and shrubs if not controlled. They are a major problem on junipers and arborvitae. It is very important to thoroughly check susceptible plants for these insects.

.           Bagworms are 1/8 to almost 2 inches long depending on age. Each bagworm is covered by a bag de of white silk with bits of the host plant spun onto the outer surface. The pupae are dark brown. The papal stage occurs inside the bag. Female bagworms are wingless, legless and grub-like (they never leave the bag). Males are small, brown hairy moths with dark wings that clear with age. Eggs are extremely small and found in the mother's bag inside her pupal cast skins.

They  hatch in May and June. The newly hatched larvae spin down on silken threads and are blown about by the early spring breezes. Most of the larvae land on the original host plant but some small worms are carried for some distance on the silk thread.  The worm begins to spin its bag and as it grows, it incorporates some of the host plant foliage into the bag for camouflage making it difficult to recognize. As the bagworms grow, it enlarges the bag and adds fresh plant material to the outside. In August the worms mature and molt into the pupal stage. The bag is firmly attached by a sturdy silk band, which the bagworms usually wrap around a twig. During August and September, male moths emerge from their bags to mate. After mating, females lay their eggs inside the pupal cast skins and die.

Apparently when the newly hatched larvae reach a plant, which are different from its parent host plant, these insects often have difficulty in adapting to it and may die or may produce only a few off-spring. After several years of struggling to keep from going extinct, the population may hit on the right combination of genes for the "new" plant and "suddenly" the new plant is covered with bagworms.

Small numbers of bagworms can be removed with scissors or a sharp knife. Bagworms can be parasitized by several kinds of parasitic wasps, however, if there are great numbers on the plant, insecticides are the only logical way to control the pest. Insecticides are effective if the bagworms are small and relatively sensitive so it is very important to control these insects now while they are small. These insects are almost impossible to kill if allowed to pupate.

Insecticides such as Orthene, Dipel Sevin, Malathion or Conserve SC are labeled to control these insects. Complete coverage is necessary to control these pests. Always read and follow the label before applying any pesticide.

07/25/2002