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Twitching cocoons on a Leyland cypress are probably bagworms

By Al Cooke
Posted Saturday, July 16, 2011

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Pittsboro, NC - “Twitching cocoons” on a Leyland cypress are probably bagworms. You may want to confirm that by comparing with pictures here. The cocoon is generally broadest in the center, narrowest on both ends, and covered with vegetation from the host plant. Bagworms occur throughout North Carolina. Bagworms are usually found on conifers such as arborvitae, spruce, juniper, cedar, and Leyland cypress. However, bagworms have a very wide host range and can feed on many plant species including deciduous trees and shrubs.

Bagworms are common landscape pests because they feed on many of the most common ornamental plant species. They can be readily identified by the cone-shaped bag they spin from silk and embed with bits of host plant and other debris. The bags range in size from 1/4 inch to over 2 inches to accommodate the growing caterpillar inside. Though they are rarely seen outside of bags, the caterpillars are 1/8 to almost 2 inches long depending on age. The head and forward parts are dark with a hardened head capsule. The rest of the body is paler and soft. In late summer dark brown pupae can be found in the bags. Male pupae are slender and female pupae are fatter. Adult 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. In fall the female lays hundreds of spherical or oblong eggs within her pupal cast skin that overwinter and hatch in spring

Bagworms have one generation per year making it pretty easy to estimate their current stage of development and to predict how and when development will progress. Generally by early June all eggs have hatched and it’s easy to kill the young caterpillars with organic sprays such as B.t. (Biobit, Dipel, Foray) or spinosad (Conserve). Later in the summer as they mature and feed less it will likely require a synthetic product such as Orthene, Sevin, or Mavrik. Since you can observe them twitching, it’s probably not too late for the organic products. But you should deal with them soon or plan on losing another plant eventually.

It’s always appropriate, if not always practical, to remove the cocoons by hand. By late summer, mature males will leave the cocoons, find a female, and mate. Females never leave the cocoon, lay eggs after mating, and die there. Cocoons left by the end of summer are either empty (vacated by a male) or contain hundreds of eggs that will start hatching about next May. From June to early August is the only window for effective treatment with insecticides. Hand removal of the cocoons is always appropriate.

Female bagworms lay 500-1000 eggs in their bag before they die in the fall. The eggs overwinter and 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 may be "ballooned" for some distance on the silk thread. Upon reaching a suitable host, the worm spins a tiny bag of silk and plant debris that looks like an upside down ice cream cone (figure). As the caterpillar grows the bag grows also and more host plant material is added to the outside for camouflage. In addition, the larger bags hang down from branches like pine cones rather than sticking straight up. In August the caterpillars 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, adult bagworms emerge as moths from the pupal case. Females are flightless and never leave the bag so male moths emerge from their bags in search of females to mate with. Mating occurs through the bag. After mating, females lay their eggs inside the pupal cast skins and die.


Apparently when the newly hatched larvae reach a plant which is different from its parents' host plant, the insects often have difficulty adapting and may die or may produce only a few offspring. 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.

Older larvae consume entire leaves. New bagworm infestations often go unnoticed until late in the summer when caterpillars are large and consuming a lot of plant material and branches begin to appear defoliated. Trees that do not produce new foliage will not recover and may die.

Typically, bagworms do a little damage in the first year on a plant and a lot in the second year; few plants survive a third year of feeding. Monitoring for bagworms is easy when the bags are large and easy to spot on trees and shrubs. Look for bags in fall or early spring before eggs hatch. Anytime you see old bags on a plant there are probably bagworms present because the females lay eggs in their bags. Thus the same trees and shrubs will be damaged year after year as populations build.

Leyland cypress has become one of the favored plants but junipers and arbor vitae are also prime targets.

Al Cooke is the Horticulture Extension Agent st the Chatham County Center, N.C. Cooperative Extension

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Twitching cocoons on a Leyland cypress are probably bagworms

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