Bagworms on Leyland Cypress

Leyland cypresses (x Cupressocyparis leylandii) are sterile hybrids of Alaskan cedars (Chamaecyparis nootkatensis) and Monterey cypresses (Cupressus macrocarpa). They have a natural pyramidal form and produce feathery, deep green or blue-green foliage. Young trees are fast growing, and mature trees can reach heights of 100 feet. They are often planted as hedges and screens in the southeastern United States, and grow in U.S. Department of Agriculture plant hardiness zones 6 through 10. Leyland cypress trees are susceptible to infestations by bagworms, which can cause significant damage or kill the trees.

Bagworm Identification

Bagworms (Thyridopteryx ephemeraeformis) are the larvae of moths. Newly hatched caterpillars are about 1/25 inch long, while mature caterpillars reach 2 inches. These pests have drab grayish bodies and dark head markings. The adult female remains larval in form and has a yellowish in color, with no eyes, mouthparts, antennae, legs or wings. Adult males are the only ones are able to fly. They have a 1-inch wingspan, with clear wings and dark bodies. Larval bagworms are most easily identified by the cone-shaped bags they carry with them as they feed. These sacs are made of insect silk and plant parts, so you may mistake them as part of the tree.

Life Cycle

Adult female bagworms never leave their bags. In fall, they release a mating pheromone that attracts male moths. Females lay between 500 and 1,000 eggs inside their bags, and then fall to the ground and die. The eggs spend the winter inside the bags and hatch in early summer. The emerging larvae spin threads and allow the wind to blow them to different parts of the tree, or to other nearby plants. They create bags of their own and carry them with them as they feed, adding to their bags as they grow. They continue feeding and growing for approximately six weeks during the summer, and pupate for around four weeks at the end of summer before emerging as adults.

Feeding and Damage

Newly hatched and small bagworms feed on the top surface of Leyland cypress's scalelike leaves, leaving brownish spots on the damaged areas. Older bagworms devour the entire leaf. The damage does not usually become apparent until late summer when the caterpillars mature, but by then Leyland cypress trees may suffer from extensive leaf loss. Trees with mild infestations may grow more slowly, while severely infested trees may die.

Biological Control

Several predators feed on bagworm larvae. Two types of parasitic wasps attack the caterpillars and help keep their populations under control. Steinernema carpocapse, a type of nematode, is used as a form of biological control. Spray the nematodes into Leyland cypress trees before the females lay their eggs. The nematodes infest the females and prevent them from reproducing.

Mechanical Controls

For a small tree or two, you can remove bagworms by hand. Pull the bags from the tree in late winter and spring before the eggs hatch. Place the bags in a bucket and leave them for a while to allow helpful parasites an opportunity to escape. Then, throw the bags away or destroy them.

Chemical Control

Insecticides may help when infestations are too severe to be controlled through biological or mechanical means. Bacillus thuringiensis and spinosad work well on young bagworms, while trichlorfon and cyfluthrin are more effective on mature larvae. Apply the insecticide about two weeks before the eggs hatch, and make sure to cover all parts of the tree.

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