WW-02363     Revised 1994  

Gypsy Moth: a Threat to Minnesota

Copyright ©  2002  Regents of the University of Minnesota. All rights reserved.

Arrival of Gypsy Moth in Minnesota

After its introduction from Europe to Massachusetts in 1869, the gypsy moth spread throughout hardwood stands in the northeastern United States. It first appeared in the lake states in the mid-1950s when moths were collected in Michigan and Wisconsin. In Minnesota, the first verified collection occurred in 1969 when several egg masses and pupal cases were found in Duluth. The first male moth was captured in 1976 in Hennepin County, and, in 1980, 26 male moths were collected in the Twin Cities area. These moths entered the state on infested nursery stock and as hitchhikers on belongings of persons who moved from eastern outbreak areas. Moth captures in Minnesota are shown in Table 1.

Table 1. Gypsy moth pheromone trap captures in Minnesota.

Year	# Moths Caught	# of Traps		# of Counties
		Total	w/Catches
1980	26	3590	17	3
1981	111	4561	65	7
1982	105	2777	50	5
1983	196	8785	88	12
1984	580	7792	91	7
1985	126	11238	91	6
1986	10	7967	9	6
1987	329	7128	112	7
1988	64	7524	19	6
1989	162	7531	71	5
1990	122	7159	96	12
1991	51	7770	39	8
1992	84	8445	60	16
1993	97	7358	64	15
Trap data provided by MDA and USDA-APHIS-PPQ.

The early catches of gypsy moth stimulated trapping in Minnesota and moths have been collected annually since 1980. Statewide totals and locations are shown in Table 1. Since traps only attract the adult male gypsy moth, trap catches do not necessarily indicate that the insect is established in an area (see Detection, Evaluation and Control in Minnesota). Although eradication efforts have been required in specific areas (Table 2), there are no known infestations at this time (1993) and Minnesota is considered to be gypsy moth free according to federal authorities.

Table 2. Gypsy moth eradication sites in Minnesota.

Year	Location(s)	Area Treated	Treatment*
1980	St. Paul	177 Trees	Fumigation
1983	St. Paul	300 Acres	Sevin-4 Oil
	Woodbury	130 Acres
1984	St. Anthony	30 Acres	Bacillus thuringiensis var. kurstaki (Bt) 2X / Site
	Sauk Rapids	20 Acres
	Stillwater	32 Acres
1985	N. Apple Valley	57 Acres	Bt 2X / Site
	S. Apple Valley	51 Acres
	Lakeville	38 Acres
	White Bear Lake	41 Acres
1986	N. Apple Valley	145 Acres	Bt 3X / Site
	S. Apple Valley	135 Acres
1988	Oak Grove Township	85 Acres	Bt 3X / Site
	Oak Grove Township	2 Acres (core)	Dimilin 2X / Site
	Delano	2 Acres
1990	Minnetonka	35 Acres	Bt 3X / Site
	Big Lake Township	25 Acres	Bt 2X / Site
*Insecticide Types: Sevin (carbaryl) - conventional insecticide; Bacillus thuringiensis var. kurstaki - microbial insecticide; Dimilin (diflubenzuron) - insect growth regulator. Treatment data provided by MDA and USDA-APHIS-PPQ.

Potential of Gypsy Moth in Minnesota

The gypsy moth will continue to enter and be captured in Minnesota. The locations of introductions will be monitored by the trapping program and where isolated areas of infestation are identified an eradication program (see Eradication vs. Management) will be implemented. However, as the gypsy moth infests neighboring states, introductions from those infested areas will become more common, eventually resulting in the gypsy moth becoming a permanent resident in some regions of Minnesota. Gypsy moth has been detected in Wisconsin along Lake Michigan from Milwaukee to north of Green Bay; however, no infestations have been detected in western Wisconsin to date (1993). Eradication efforts are being attempted in Wisconsin.

Distribution

Gypsy moth eggs may be killed when temperatures at the site of the overwintering egg mass drop below -25° F for more than 24 hours. Egg mortality will be correlated with temperature, length of exposure and snow cover. Given the gypsy moth's ability to overwinter in other northern areas (for example, Quebec), it is unlikely that Minnesota's winter temperature extremes will limit its spread northward in the state. It appears that gypsy moth will survive in Minnesota wherever suitable hosts are present, regardless of temperature.

Damage

What can we expect if large numbers of caterpillars occur in an urban or forest area? As a defoliator, the gypsy moth can totally strip the foliage from a wide variety of trees (Photo 1). While oak, poplar, birch and willow are a few of the preferred hosts (Table 3), the gypsy moth will feed on most deciduous trees and even on coniferous trees when hardwood foliage is not available and the caterpillars are in their final development stages. There are, however, plants that are usually avoided by the gypsy moth (Table 4). In anticipation of increasing problems with gypsy moth, non-preferred tree and shrub species such as these should be selected for landscape plantings.

Table 3.

Common landscape plants most at risk to defoliation by gypsy moth. Apple and crabapple (Malus) Aspen and poplar (Populus) Basswood (Tilia) Birch (Betula) -grey, paper, river Blue spruce (Picea) Oak (Quercus) White pine (Pinus) Willow (Salix)

Table 4.

Common landscape plants generally avoided by gypsy moth. Arborvitae (Thuja) Ash (Fraxinus) Dogwood (Cornus) Honeylocust (Gleditsia) Juniper (Juniperus) Lilac (Syringa) Northern catalpa (Catalpa) Sycamore (Platanus)

Gypsy moth feeding damage.

In urban areas, the decline and death of unhealthy hardwood trees will be accelerated when they are defoliated for two or more consecutive years. Growth loss in healthy trees will be roughly proportional to the amount of defoliation above 50%. Healthy deciduous trees can enter a period of decline following consecutive years of defoliation. Most coniferous trees will not survive one complete defoliation in a single season, though complete conifer defoliation by gypsy moth caterpillars is rare.

Homeowners and recreationists in infested areas will also have to deal with the considerable annoyance of gypsy moths. In addition to the unappealing appearance of defoliated trees, masses of caterpillars will drop or crawl from heavily infested trees onto sidewalks, trails, picnic areas, cars, homes, cabins and other objects. Fecal droppings also abound in outbreak areas and the larval hairs can cause welts and rashes when susceptible individuals come in contact with the larvae. While not dangerous, these allergic reactions add to the annoyance of gypsy moth for some people.

The forest areas most susceptible to gypsy moth defoliation damage are the considerable oak forests found throughout the central and southeast regions of the state (see Map). Overstocked oak stands on droughty soils are the most susceptible to gypsy moth buildup and damage. Mortality of oak species may occur following two or more consecutive years of defoliation as the stressed and declining trees are attacked and killed by root rots, wood borers and bark beetles. Following severe outbreaks, oak trees will be reduced and replaced by less susceptible species. Ash and maple will replace oak on high quality soil sites, while pine is most likely to grow in poor soils.

The Asian gypsy moth (also classified as Lymantria dispar), a near twin of the North American gypsy moth, is threatening to establish itself in the U.S. To date, detection and eradication efforts have taken place only in the Pacific Northwest; no moths have been detected in Minnesota. The Asian gypsy moth presents new problems due mainly to the ability of female moths to fly, which contributes to faster spread of the population. Recently, a hybrid of the North American and the Asian gypsy moths has been detected. Like the Asian gypsy moth, this female can fly, which will present problems similar to that of the Asian gypsy moth.

The Insect

The gypsy moth, Lymantria dispar, belongs to the insect order Lepidoptera, which contains the moths and butterflies. Gypsy moth eggs hatch into caterpillars which feed for several weeks on tree foliage, then form a stationary, non-feeding pupal stage before becoming adult moths (Table 5). Gypsy moth is like other defoliating caterpillars but has received notoriety because it is larger than many similar caterpillars, has a voracious appetite, can feed on many different tree and shrub species, has hairs which can cause skin irritation to susceptible people, and has been under federal quarantine for many years. When the insect becomes abundant it can cause extensive defoliation, tree or limb mortality and irritation and annoyance for people living in or visiting the infested areas. The annoyance caused by gypsy moth probably accounts for the greatest pressure to control the gypsy moth in urban and recreational areas.

Table 5. Gypsy moth life cycle.

Gypsy moth females and egg masses.

Egg Stage

The female gypsy moth normally deposits her eggs (Photo 2) in a well-formed mass in mid-July to mid-August. An egg mass contains from 300 to 1000 eggs and is covered with a fibrous mat of hairs. At first, it is buff colored but will later bleach with exposure. The gypsy moth overwinters in the egg stage.

Gypsy moth larvae begin emerging from egg masses sometime in early to mid-May. All of the surviving eggs from an individual egg mass will hatch in 3 to 5 days, while individual egg masses in an infested area may hatch over a period of 2 to 3 weeks.

Larval Stage

There are several distinct stages of growth (called instars) for gypsy moth larvae. Depending on sex, the gypsy moth has five or six larval instars each lasting from 4 to 10 days. Newly hatched larvae (Photo 3) are small (3mm) and buff colored, but turn black within a few hours after emergence. If temperatures are below 40° F or if it is raining, newly hatched larvae remain on or around the egg mass for several days.

When conditions are favorable, larvae leave the egg mass and climb trees or other objects. When the larvae reach the outer branches or tops of trees, they drop on silken threads that are easily broken by light gusts of wind. Although larvae may be carried for several miles by wind, most spread only within a small localized area of up to a few hundred yards.

After dispersing, first or second instar larvae seek a highly preferred host (oak, aspen, birch, apple, willow) to begin feeding. Usually they have two or three feeding periods during the day, chewing small holes within the perimeter of the leaf. When not feeding, they rest on a mat of silk they make on the underside of the leaf.

Newly hatched gypsy moth larvae on egg mass.

Nearly full grown gypsy moth larva.

Early instar gypsy moth larvae.

Second and third instar larvae feed at the leaf margins and characteristically stay in the tree tops, migrating to the undersides of branches and twigs when not feeding. They are hairy, and blackish with several light orange dots on the last half of the back (Photo 4).

After larvae molt to the fourth stage, their behavior changes dramatically. Caterpillars are now 1" to 2" long and can be easily recognized by the five pairs of blue spots toward the front and six pairs of red spots toward the rear on their upper sides (Photo 5). Larvae feed during the night, then descend at dawn in search of protective locations where they rest for the remainder of the day. It is during this stage of development that defoliation can be severe enough to be noticed and masses of larvae cause a nuisance as they crawl on lawn furniture, buildings and sidewalks. Last stage larvae stop their daily migration and remain on foliage feeding 24 hours a day. Larvae usually complete their development in late June to mid-July and attach themselves to any surface with strands of silk and transform into pupae.

Pupal Stage

The pupa is the transformation stage between caterpillar and adult moth. Pupae have been found attached to picnic tables, rocks, lawn furniture, firewood, tents, campers, automobiles, trees, ladders, shutters, trash cans, shrubs, barbecue grills; the list is endless. Hanging as mahogany-colored cases, the pupae await their change to adult moths (Photo 6). After about 10 to 14 days, the adult moths emerge leaving the pupal cases behind.

Gypsy moth pupae.

Gypsy moth egg masses on car.

Gypsy moth adult female (left) and male (right).

Adult Stage

The male moths are dark brown with black markings (Photo 7), are strong fliers and are most active from mid-July through August during daylight hours. They fly in rapid zig-zag patterns searching for female moths. A single male is capable of mating with dozens of females. Females are larger than males, whitish in color with black markings (Photo 7), and do not fly, but crawl short distances from the place they emerge. They release a potent sex attractant (pheromone) that attracts male moths. Soon after mating, the female deposits her eggs in a single cluster, then dies. Egg masses can be found anywhere, and it is in the egg stage where most transport of gypsy moth occurs from infested to uninfested areas. Careful inspection of firewood, picnic tables, automobiles (Photo 8), campers and other protected locations will help reduce the spread of gypsy moth to uninfested sites. The gypsy moth has one generation per year.

Detection, Evaluation, and Control in Minnesota

Detection efforts to find gypsy moths in Minnesota began in 1969 and continued with limited trapping into the early 1970s. Since that time, detection trapping has been improved and intensified in locations where gypsy moth introductions are most likely. Eradication efforts began in 1980 when egg masses were discovered on nursery stock shipped to the City of St. Paul from infested east coast nurseries and have continued since (Table 2).

Detection

Pheromone traps (Photo 9) are used to detect the presence of gypsy moths by attracting and catching adult male moths. Once a male moth enters the trap, a sticky substance prevents it from leaving.

Gypsy moth pheromone traps.

Detection surveys using pheromone traps are conducted annually from late June to mid-September under the direction of the United States Department of Agriculture (USDA), Animal and Plant Health Inspection Service (APHIS). In areas where male moths are caught, the trapping intensity increases the following year depending on how many moths are found in the area. It is generally accepted that potential establishment of gypsy moth is indicated only when more than one male moth is found in a trap, particularly if this occurs in the same location in consecutive years. Single trap catches are not retrapped as intensely since they may be new hitchhiking introductions or males flying away from a nearby infestation. Areas with multiple moth catches in single traps are more intensely surveyed to determine the extent of any infestation, since the possibility exists that an infestation may be present. These areas also receive intensive ground surveys to check for egg masses or other life stages (larvae, pupae).

In the fall, the ground checking begins and survey entomologists under the direction of the Minnesota Department of Agriculture (MDA) and USDA-APHIS, carefully examine the immediate area searching for viable egg masses or pupal cases. In urban areas, this often means door-to-door surveys with specific emphasis on locations where people have recently moved from or visited gypsy moth infested areas outside Minnesota. Not until one or more of these alternate life stages is found in the same year is the area normally considered infested.

Evaluation and Control

Recommended control operations can range from limited ground applications to more extensive aerial applications of an appropriate insecticide. Before control recommendations are finalized by the Minnesota Commissioner of Agriculture, residents in the affected communities are encouraged to participate in the decision-making process through public information meetings.

Eradication vs. Management

Two approaches are commonly considered when dealing with recently introduced insect pests: eradication and population management. Eradication attempts to completely eliminate the insect from an area so that no individuals remain. Population management attempts to regulate the insect below a level that causes damage. The insect remains in the managed areas.

Eradication is not advisable once a population becomes established and widespread. However, eradication can be successful when dealing with low numbers of pests found in relatively small, localized areas. Therefore, a means of locating just a few insects is required for eradication to be considered.

To date, eradication efforts in Minnesota have been successful (Table 2) because pheromone traps are capable of detecting low-level populations and pinpointing the insect in an area. The control agents Bacillus thuringiensis var. kurstaki, and diflubenzuron (Dimilin) in combination with mass trapping, are the most effective means of controlling early instar gypsy moth larvae at this time.

Gypsy moths have been introduced into Minnesota by people moving contaminated items from eastern outbreak areas. These introductions have remained localized, allowing for early detection and reaction which make eradication possible. However, as neighboring states become infested, routine traffic between bordering states and Minnesota will further increase the probability of infestations in Minnesota.

Once the gypsy moth becomes established, the approach will switch from eradication to population management. The purpose of population management is to maintain or suppress a pest population below the point where damage becomes significant or unbearable. Gypsy moth damage consists of the direct impact on host tree health from defoliation. Indirect damage includes unsightly, aesthetically unappealing trees and the nuisance of larvae on lawns, shrubs, homes, sidewalks, picnic tables and other objects. Thus, people will want to reduce both the direct and indirect damage caused by the gypsy moth.

Control strategies will include foliage protection and population reduction. It will likely require routine attention by land managers, homeowners, resort operators and municipalities in areas where it is established.

Agency and Citizen Responsibilities

Agencies of federal, state and local governments are cooperating in efforts to detect, monitor, control and provide information about the gypsy moth in Minnesota. It is helpful to know the responsibilities of the agencies. In addition, citizens' groups in Minnesota can contribute to a successful program.

Citizen Input

Early detection is the key to success of the current gypsy moth program. Therefore, an informed citizenry able to identify the various life stages of gypsy moth can be helpful. People can look for gypsy moth and then contact an appropriate agency (listed below) if gypsy moths are suspected in an area.

People moving to Minnesota from gypsy moth infested areas as well as human traffic between Minnesota and infested nearby states (for example, Michigan) may result in persons unknowingly introducing gypsy moth here. Public awareness of how the gypsy moth is transported by people can help prevent this type of introduction. Organized groups can assist in the detection and monitoring of gypsy moth. Input is most effective when supervised by the two agencies responsible for detection (MDA and APHIS). The publication "Don't Move Gypsy Moth", Program Aid #1329, is available through USDA-APHIS.

Minnesota Department of Agriculture

Under Chapter 18 of the Minnesota Statutes, and specifically the Plant Pest Act, the Commissioner of Agriculture is directed to "prevent the introduction into and the propagation and dissemination within this state of plant pests and to provide for their suppression and control." This may be done in consultation or cooperation with the United States Department of Agriculture, Animal and Plant Health Inspection Service (USDA-APHIS). Final decisions on survey or control rest with the Commissioner making MDA the lead agency in the detection, suppression and eradication of serious plant pests. USDA is not authorized to engage in these activities without MDA approval. The Commissioner of Agriculture makes final decisions based on input from USDA, local government, experts and citizens. The cost of eradication programs is shared equally between MDA and USDA when USDA funds are available for that purpose.

United States Department of Agriculture, Animal and Plant Health Inspection Service

The United States Department of Agriculture, Animal and Plant Health Inspection Service's (USDA-APHIS) major concern is with the interstate spread of this federally quarantined pest. It also provides funding and technical assistance for detecting and eliminating infestations in states such as Minnesota that are outside of the generally infested regions of the United States. USDA-APHIS may provide up to 50 percent of the funding for gypsy moth eradication programs in Minnesota.

Agricultural Extension Service, University of Minnesota

Each county has an extension educator who can be contacted for educational materials or information about gypsy moth. An extension specialist housed on the St. Paul campus of the University of Minnesota contributes insect information to aid in decision making, develops educational materials on gypsy moth and provides information backup for the extension educator.

Minnesota Department of Natural Resources

The Department of Natural Resources (DNR), Division of Forestry is responsible for developing a long-range management plan for gypsy moth on state, county and private forest lands. When gypsy moth becomes established and is no longer a candidate for eradication, the DNR will be responsible for management programs in forested areas. DNR personnel also assist MDA in determining control strategies for gypsy moth.

Forest Service, United States Department of Agriculture

When an introduced forest pest such as gypsy moth is declared established, the Forest Service becomes the lead federal government agency for cooperating with states. Through a memorandum of understanding with USDA-APHIS, the Forest Service also becomes responsible for aiding states on eradication projects in excess of 640 contiguous acres. The Forest Service is also responsible for gypsy moth management operations on federal lands.

Municipal Foresters and Local Governments

Local governments and municipal foresters often cooperate with federal, state and local authorities. Cooperation could include supplying personnel for inspection, distributing information flyers to affected neighborhoods and assisting in any control efforts deemed necessary. Once the gypsy moth becomes established in a community, local ordinances for inspection, education and control would be considered.

Acknowledgements

We wish to thank the many people who provided helpful suggestions concerning this publication, especially staff, Minnesota Department of Agriculture, Minnesota Department of Natural Resources, the United States Department of Agriculture Forest Service, and the United States Department of Agriculture Animal and Plant Health Inspection Service. Existing publications on gypsy moth from varied sources also were used in developing this folder.

Photographs were obtained from the following sources: Dr. Michael R. Jeffords, Illinois Natural History Survey—Section of Economic Entomology (photos 4 and 7); United States Department of Agriculture Forest Service (photos 1-3, 8 and 9); and James B. Hanson, USDA Forest Service (photo 5). Minnesota trap catch data and eradication information were provided by the MDA and USDA-APHIS.

We thank them for the use of this material.

Publication of this brochure was funded in part by the USDA Forest Service, Northeastern Area, State and Private Forestry in cooperation with the Minnesota Department of Natural Resources, Division of Forestry, Urban and Community Forestry Program and the Minnesota Shade Tree Advisory Committee.

Produced by Communication and Educational Technology Services, University of Minnesota Extension.

In accordance with the Americans with Disabilities Act, this material is available in alternative formats upon request. Please contact your University of Minnesota Extension office or the Extension Store at (800) 876-8636.