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Beware the bagworm

Posted: August 31, 2013 - 11:04pm

Bagworms are caterpillars that make distinctive “spindle-shaped” bags on a variety of ornamental trees and shrubs throughout Georgia. They are not the same insect as the Fall Web Worm. They have been known to attack both deciduous and evergreen trees; most often they are found on cedar, cypress, arborvitae, juniper, spruce and pine. They can also be seen on deciduous plants such as rose, maple, elm, and sycamore.

Bagworms injure host plants by consuming large amounts of foliage. Severe infestations can cause plant or tree death. Bagworms often go unnoticed because their bags can easily be mistaken for developing evergreen cones such as the pine cone.

Bagworms are the larval stage of the adult moth. Only the males of the species develop into moths capable of flight. The bagworm larvae are dark brown with yellow heads and yellow and black spots over the body. Adult females stay in caterpillar form. They are wingless and lack eyes and antennae. The females lay eggs inside the bags formed the previous season. The eggs hatch in mid to late May. The new larvae construct new bags by using silk and small bits of plant material. The new bags resemble tiny upside-down ice cream cones extending from the remnants of plant foliage. As the larvae grow, they continue to add more plant material and enlarge the bag. They use the bags as shelter when disturbed. Bags reach their ultimate size of 1.5 to 2 inches in early fall. The larvae permanently attach their bags to twigs and transform into the pupa stage. Pupation lasts from seven to 10 days, after which the adults emerge. The flying males go out in search of immobile females. After mating, the females lay 500-1,000 eggs and then die.

Because the larval stage of the bagworm is fairly immobile, it is common to see one plant covered with bagworms, and an identical plant nearby is bagworm-free. This time of year, bagworm bags become noticeable.

The easiest method of bagworm control is hand picking. If only a few small trees or shrubs are infested, picking off the bags and disposing of them is often satisfactory. The best time to do this is winter or early spring. This will prevent new eggs from hatching and re-infesting the same plants.

Insecticides are best used when small crawling larvae are observed in spring, usually late May through June. Young bagworm larvae are especially sensitive to the bacterium Bacillus thuringiensis, also known as B.t. It is sold under many common brand names, including “Dipel,” “Thuricide,” and “Caterpillar Killer.”

The bacterium causes a disease that kills the caterpillars while not affecting humans. Conventional chemicals, such as malathion and bifenthrin, can also be used for control of young bagworms.

If you suspect you have bagworm issues in your landscape, scouting is the key to prevention and control. Keep a close eye on susceptible plants, especially evergreens. Evergreens cannot recover from massive defoliation like some deciduous species. Therefore, early identification is crucial.

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