Illinois Natural History Survey - University of Illinois

INHS Reports Summer 2001

Slow the Spread of the Gypsy Moth

Introduced into North America in 1869, the gypsy moth has radiated west and south from its release site in the Boston area to 17 states and the District of Columbia. The current line of infestation extends over 1,200 miles from Wisconsin to North Carolina, including northern counties in Illinois and Indiana (Fig. 1). The gypsy moth is an "outbreak species"; in infested areas the populations build up over a period of years, peak in very high densities, then decline strongly in the presence of natural enemies such as parasites and diseases. In years when populations of the pest are high, oaks, willows, aspens, and other forest trees may be completely defoliated by the larvae over large areas. The resulting stress to trees due to 2-3 years of defoliation, added to other stresses such as drought or the urban environment, may kill the trees, sometimes in large tracts. Damage by gypsy moth larvae varies according to the level and extent of outbreaks but costs are millions of dollars per year. Following a successful pilot project that was initiated by the U.S. Forest Service and state and federal partners, the National Gypsy Moth


Slow the Spread (STS) Project was instituted in 1999. Illinois joined this program, which merges the efforts of states and federal agencies, to reduce damage and slow the inexorable spread of the gypsy moth. Newly invaded territory has been reduced by over 9,000 square miles annually, saving a minimum of $22 million in lost timber and recreational land use per year nationally.

The STS Project is designed to address the gypsy moth invasion at the "leading edge" of its spread while suppressing outbreaks in generally infested areas. All habitats in the U.S. that are susceptible to invasion are divided into three zones: the infested zone, the transition zone, and the uninfested zone (Fig. 2). Infested zones are typically treated under suppression program guidelines with Bacillus thuringiensis kurstaki (Btk), a bacterium specific to moth larvae, diflubenzuron, an insect growth inhibitor, or Gypcheckreg., a gypsy moth-specific virus, where outbreaks occur. In addition, natural enemies of the gypsy moth are manipulated to dampen the outbreaks in these areas.

Under the STS Project, a major management effort takes place in the transition zone. Intensive monitoring using pheromone-baited traps for adult male moths is conducted in these areas and decisions about treatment are made based on trap counts. Trapped males are counted periodically during the flight season to estimate the population density of the pest. July 10-25 is peak flight season in Illinois. Growing gypsy moth populations are treated using different methods depending on the extent and isolation of outbreaks, sensitivity of the environment, and other parameters. A general outbreak in a suburban area, for example, may be treated with Btk, an insecticide that is not harmful to vertebrate animals, including humans. A very sensitive area, such as a wildlife refuge, might be treated with pheromone flakes to disrupt the mating of gypsy moths, a method that does not affect other animals, including insects.


The gypsy moth typically does not spread in a steady progression from the generally infested areas to uninfested areas; instead, it occurs in isolated outbreaks in the transition zone. These outbreaks usually occur because of human transportation of egg masses or pupae that "hitch a ride" on transported items such as vehicles, outdoor recreational equipment, and nursery stock. One method for reducing this kind of spread is to quarantine generally infested areas with high or growing gypsy moth populations that are adjacent to transition areas. In Illinois, Lake County was quarantined in 2000. Under quarantine, nursery plant stock is inspected before shipment out of the county for presence of any life stage of the gypsy moth. Citizens moving from the area should contact officials regarding the movement of vehicles and any other property stored outdoors that may serve as egg-laying or pupation sites.

Gypsy moth numbers are increasing in Illinois, setting the stage for further spread. Outbreaks are expected to increase in 2001 and treated areas in the transition zone include areas in DuPage, McHenry, and Winnebago counties where overwintering egg masses have been located. Twenty-one counties in northern Illinois are being monitored under the STS Program and most other counties in the state are being monitored by the U.S. Forest Service, USDA Animal Plant Health Inspection Service, and state cooperators.

Scientists in the Illinois Natural History Survey (INHS) Center for Economic Entomology and Illinois Department of Agriculture are monitoring gypsy moth populations for presence of naturally occurring disease organisms and have found that a very host-specific and virulent fungal pathogen occurred in gypsy moth populations in McHenry County in 2000. Other investigations are being conducted by INHS in cooperation with the U.S. Forest Service on the effects of a group of natural pathogens, the microsporidia, that occur in European gypsy moth populations but not in North American populations.

Several Web sites are excellent sources of information about the STS Program, both advisory and technical, regarding the decision-making process for monitoring and treatment of gypsy moth infestations. Included are:

Leellen Solter and Charles Helm, Center for Economic Entomology and James Cavanaugh, Illinois Department of Agriculture

Charlie Warwick, editor

Illinois Natural History Survey

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Champaign, IL 61820

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