Pest Management Fact Sheet #5024
James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician
Description & Biology
Gypsy moth caterpillars feed on most hardwood trees, except ash. They prefer oak, poplar, gray birch and fruit trees. When half grown or larger, the larvae are also likely to feed on evergreens. As the number of gypsy moth larvae in an area increases, their favorite food sources become depleted. The larvae are then likely to feed on other ornamental trees and shrubs.
Healthy hardwoods can usually withstand two to three years of defoliation; however, secondary attack by insects or diseases can shorten or affect the life of weakened trees. Weakened, diseased, insect-damaged or shaded trees, particularly those already struggling with poor soil or moisture conditions, are especially vulnerable. Severely defoliated evergreens are less likely to survive than other trees.
Gypsy moth infestations are heaviest in central and southern Maine. In addition to defoliating trees, they are also nuisance pests because of wandering caterpillars, droppings, pupal cases and egg masses on homes and the spinning down of young caterpillars. Tiny hairs from the caterpillars can irritate the skin of some people. In some cases, severe reactions result in rashes and/or itching. If consulted, a doctor should be informed of the possibility of contact with gypsy moth caterpillar hair.
Overwintering gypsy moth eggs hatch in May. The young, 1/16 inch, hairy, black caterpillars have a small knob on each side of the head and are first seen near clusters of eggs. It takes up to a month for all the eggs to hatch. For this reason, it is common to find caterpillars of various sizes at the same time. The larvae soon crawl into treetops, where they spin down on lines of silk to be blown to other places.
Feeding by the young caterpillars on underside of leaves usually goes unnoticed. By the time they are half grown, 3/4 to 1 inch long, entire leaves are eaten. At this stage, the caterpillars are mostly black, except for orange markings down their backs. Careful selection and use of insecticides and biological controls at this time can provide effective control of wind-blown caterpillars.
The caterpillars molt five or six times before pupation. When the caterpillars are 1 inch or more in length, their feeding is more noticeable. They move up into the trees to feed at night, or on cool days. During hot, dry, sunny days, they move down to rest on lower branches, trunks or on lawns.
The late stage caterpillars are hairy, dark and easily distinguished by five pairs of blue spots on the front body segments and six pairs of red spots on the back body segments. Fully grown larvae are 1 ½ to 2 ½ inches long. About 70 percent of their food is consumed during this stage of development; they are able to strip a tree overnight.
In July, the mature larvae seek protected areas which could be almost anyplace, where they form reddish brown pupae. Male pupae are about 3/4 inch long; females about 1 inch long. In 10 to 15 days, still in July, the moths emerge.
Male moths are excellent flyers. They are usually dark brown. Front wings display black wavy bands and V-shaped markings. Hind wings are light brown trimmed with dark brown. Their wingspan is about 1 ½ inches. Male moths have conspicuous, feather-like antennae.
Female moths are larger than males and predominantly white with a few faint, wavy brown or black bands and V-shaped markings on the front wings. Marginal dark spots are found on both front and hind wings. Female moths have a wingspan of 2 to 2 ½ inches, but they cannot fly.
Because they cannot fly, females do not move far from the pupation site to lay their 500 or more eggs. The eggs are covered with a tan velvet-like mass made from the female’s body hair. Eggs are the stage in which the gypsy moth overwinters. Moths die soon after mating and egg laying. There is only one generation of gypsy moth each year.
Several factors can affect gypsy moth populations; weather, predators, parasites and diseases. One night of -20°F or colder kills many overwintering eggs not covered with snow.
Eggs can be crushed, but it is difficult to crush all the eggs in an egg mass on bark or other rough surfaces. Eggs can also be scraped or otherwise removed and destroyed. They must not be allowed to fall to the ground, where protective snow cover will increase the chance of hatching.
Burlap can be wrapped around the trunks of trees to collect caterpillars and prevent them from climbing onto branches to feed. Collected caterpillars can then be killed. Crushing any female moth, caterpillar, pupae or eggs helps protect ornamental trees, but only if a high percentage of the population is killed.
Commercially available sticky petroleum materials are available to keep caterpillars from getting into trees. Tin, plastic, etc., can be placed around the tree trunk to accomplish the same purpose. Some people put petroleum jelly, oil or grease on the wrappings for additional protection, but greasy substances must be kept off the trees and removed when the caterpillars are gone. Most petroleum products, sticky products, oil, grease or diesel fuel should not be put directly onto trees, because they will penetrate the bark, especially of younger trees. Trees four inches in diameter have been killed in one season when grease was applied to their bark. If you get grease on the bark, scrape it off as soon as possible, and keep it off the ground to avoid contaminating the tree root zone. Barriers are most effective on isolated trees, if only a few caterpillars are in the tree when the barrier is applied, and if an insecticide was applied earlier.
The insecticides Bt (Bacillus thuringiensis) for young larvae, neem, spinosad, carbaryl (Sevin), cyfluthrin, orthene, and malathion can also be used to control caterpillars. Spray caterpillars anytime, but for best results spray before they are half grown at 3/4 to 1 inch long. Foliage should be sprayed in late afternoon or early evening for best results and greatest protection of bees, predators and parasites.
When Using Pesticides
ALWAYS FOLLOW LABEL DIRECTIONS!
Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
17 Godfrey Drive, Orono, ME 04473
1.800.287.0279 (in Maine)
Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.
© 2016, 2018 | Reviewed: 2020
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