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Forest Pest Leaflet #75: Gypsy Moth

Introduction

The gypsy moth, Lymantria dispar (L.), is a Eurasian defoliator of mainly deciduous trees. Accidental introduction of the European race of the moth to eastern North America in 1868 has caused severe infestations in parts of New England, New York and Pennsylvania. In these states, and at other eastern U.S. locations, it is considered a major forest pest and a serious threat to urban shade trees and ornamentals.

Female (left) and male gypsy moth

In Canada, the gypsy moth has spread from the U.S. to become well established in Ontario and Quebec, and has been threatening parts of southern New Brunswick and Nova Scotia for many years. Though not established in British Columbia, repeated introductions over the past 20 years emphasize the need for continued monitoring, eradication programs, and public awareness.

The gypsy moth threat in the Pacific Northwest was increased in 1991 by the discovery in Vancouver of an Asian race of the moth; the Asian gypsy moth has broader host preferences and spreads faster than the European gypsy moth. Since 1991, the insect has become a major concern for the forest industry, government agencies, and several British Columbian municipalities. This concern is shared by the United States which has also had introductions of the Asian race since 1991.

Hosts

There are over 300 known host plants for the gypsy moth. In North America, the long list of preferred hosts includes oak, cherry, white birch, maple, alder, willow, elm and trembling aspen. The Asian race also does well on coniferous trees such as larch.

Gypsy moth hosts

Many valuable plants are at risk in British Columbia. In some cases, plants less acceptable to young or well-fed caterpillars can be defoliated later when the larvae mature or when the preferred host plants are depleted. The long list of host plants in British Columbia includes the following trees, in addition to most other broad-leaved plants:
	Mature larva

Distribution

The gypsy moth is a native insect of Europe and North Africa, and it occurs across Asia to Japan. In North America, the European race is established in the northeastern United States and the provinces of Quebec and Ontario, having expanded from the original introduction at Medford, Massachusetts in 1868. During the 1970s and 1980s, it was found in numbers sufficient to cause concern in the western states of California, Utah, Oregon and Washington as well as in British Columbia. These occurrences were caused by the transportation of egg-laden materials from eastern sources. In 1991, federal inspectors found egg masses of the Asian race on Russian ships at Vancouver waiting to load with grain; male moths were subsequently trapped on shore. British Columbia therefore now faces the threat of gypsy moth introductions from sources to the east, south and west.

Description

Egg:

In masses ranging from about 15 to over 40 mm in length, containing up to 1000 eggs. Shape is usually roughly oval and slightly raised, distinct from the spherical shape of spider egg masses. Color is buff to beige due to covering of hairs from the female, fading with age to dirty white. Laid indiscriminately on objects such as tree trunks and branches, rocks, buildings, vehicles, outdoor furniture and, with the Asian race, on stored containers and ships near shore. Egg masses of the Asian race are more commonly found on tree leaves.

Larva:

Newly hatched larvae are typically hairy and about 2-3 mm long; mature larvae are up to 60 mm long. Very distinctive, with two rows of large spots along the back usually arranged in five pairs of blue and six pairs of red from head to rear; many long hairs cover the body. Usually found feeding on tree foliage or, in early instars, hiding in shady spots.

Pupa:

Dark reddish brown, usually with some yellowish hairs attached; length is variable with females (15 to 35 mm) often larger than males (15 to 20 mm). Found in protected places such as bark fissures or crevices, underneath loose moss, and in litter (European race), and also on foliage or hanging from branches (Asian race).

Adult:

Male moth is tan to brown with irregular black wing markings; plumose (feather-like) antennae; wingspan 37 to 50 mm; strong flier. Females are often larger than males with a wingspan ranging from 37 to 62 mm; they are whitish with faint darker and wavy bands across the forewings. Females of the European race are flightless; those of the Asian race are strong fliers. Adults can be found just about anywhere when abundant.

Note: On average, all stages of the Asian race are significantly larger than the corresponding measurements of the European race given above.

Life history and habits

One generation of the gypsy moth occurs annually. The insect overwinters in the egg stage, usually surviving winter in northern locations under a protective layer of snow. Hatching depends on warming weather and commonly occurs from mid to late April, occasionally extending to late May even at the same location.

The newly hatched, small and hairy larvae move up host plants to newly emerged foliage where they begin feeding. Some may be blown by the wind to new locations, assisted by their light, hairy bodies and long silk threads produced by special glands in their heads. This spring "ballooning" is a major means of natural dispersal for both races, and is the primary natural means of dispersal for the European race. Although ballooning has been recorded over 50 km, it usually adds about 5 km per year to new infestations.

Feeding usually continues for six to eight weeks, varying with weather, host conditions and location. There are normally five male and six female larval instars. Larvae in the first three instars feed mainly at night; those in later instars feed day and night and consume the largest quantity of foliage. When larvae are numerous and food becomes insufficient they may disperse along the ground in search of more foliage, becoming a greater nuisance in urban areas. Contact with the histamine-containing hairs of the caterpillars induces allergic reactions, such as a skin rash or respiratory problems, in many people.

Feeding is usually completed by late June or early July and most larvae move to protected locations, such as bark crevices, to pupate. The pupal stage lasts an average of 10 days for individual females and 13 days for males, and occurs over a month (July) or more for a local population.

Moths start emerging in July, peaking about mid-month and extending into August in eastern North America. The female moths emit a pheromone to attract males. Adults mate, do not feed, and live for up to several weeks. After mating, indiscriminate laying of egg masses occurs from late July to September.

Only the males of the European race of gypsy moth can fly, while both sexes of the Asian race are strong fliers. The attraction of Asian gypsy moths to light, at times over considerable distances, greatly increases the potential for egg deposition on vehicles, ships and aircraft under the brightly lit conditions commonly found at urban parking lots, seaports and airports. Preventing introduction, establishment, and spread into British Columbia is therefore difficult, and more effective methods preventing introductions are required.

Female gypsy moths laying egg masses

This information is a condensed version of the Forest Pest Leaflet #75: Gypsy moth. The full version is available, free to download, from the Canadian Forest Service Bookstore.

Selected references

There are thousands of published reports, brochures and articles on the gypsy moth, many of them located at universities and government research establishments concerned with forestry and agriculture. Please consult your local librarian for assistance in locating literature about the gypsy moth.

Nealis, V.G.; Erb, S. 1993. A source book for management of the gypsy moth. For. Can., Great Lakes For. Cent., Sault Ste. Marie, Ont., Min. Suppl. and Serv. Cat. No. Fo42-193/1993E, ISBN 0-662-20907-9, 48 p. * Now available from The Pacific Forestry Centre, 506 W. Burnside Road, Victoria, BC V8Z 1M5*

Agriculture Canada. 1985. Understanding the gypsy moth threat. Proc. Inf. Symp. B.C. Plant Prot. Adv. Council, 5 Nov. 1985, Vancouver, B.C. A collection of 10 papers available from Agriculture Canada, 202-620 Royal Ave., New Westminster, B.C. V3L 5B3

Agriculture Canada. 1986. Gypsy moth. A destructive pest of forests and shade trees. Publ. 1811/B, 8-panel brochure in English and French Available from Communications Branch, Agriculture Canada, Ottawa, Ont. K1A 0C7

Benoit, P.; Lachance, D. 1990. Gypsy moth in Canada: behavior and control. For. Can. Inf. Rept. DPC-X-32. 22 p. Available from Canadian Forest Service, 351 St. Joseph Blvd., Hull, Quebec. K1A 1G5

British Columbia Ministry of Forests. 1984. B.t. and biological control of insect pests. PesTopic No. 16. 2 p. Available from B.C. Forest Service, 31 Bastion Square, Victoria, B.C. V8W 3E7

British Columbia Ministry of Forests. 1985 (Revised). Gypsy moth: a forest threat from the east ... and the south. PesTopic No. 9. 2 p. Available from B.C. Forest Service, 31 Bastion Square, Victoria, B.C. V8W 3E7

Cram, W.A. 1989. Gaining support for British Columbia's gypsy moth wars 1978-1988. B.C. Min. For., Victoria, B.C. Pest Manage. Rept. No. 12. 35 p. Available from B.C. Forest Service, 31 Bastion Square, Victoria, B.C. V8W 3E7

Doane, C.C.; McManus, M.L. 1981. The gypsy moth: research toward integrated pest management. U.S. Dept. Agric. For. Serv., Tech. Bull. 1584. 757 p.

Gansner, D.A.; Herrick, O.W.; Mason, G.N.; Gottschalk, K.W. 1987. Coping with the gypsy moth on new frontiers of infestation. Southern Journal of Appl. For. Res. 11:201-209.

McManus, M.L.; Houston, D.R.; Wallner, W.E. 1979. The homeowner and the gypsy moth: guidelines for control. U.S. Dept. Agric. Home and Garden Bull. No. 227. 34 p.

McManus, M.L; Schneeberger, N.; Reardon, R.; Mason, G. 1992. Gypsy moth. U.S. Dept. Agric. For. Serv., For. Ins. and Dis. Leaflet 162. 13 p.

Miller, J.D.; Hanson, P.E. 1989. Laboratory studies on development of gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), larvae on foliage of gymnosperms. Can. Ent. 121:425-429.