Illinois Natural History Survey - University of Illinois

Gypsy Moth in the Chicago Area: Disaster for All of Moderate Problem for Some?

The gypsy moth, Lymantria dispar, has a long history of defoliating forests and city trees in the northeastern part of the U.S. In cities, this pest can adversely influence landscape aesthetics, property values, recreation, microclimate, and wildlife. In addition, repeated defoliation by this pest can kill trees. The westward invasion of the gypsy moth is now threatening Indiana and Illinois, with the official front of established populations now in eastern Wisconsin. We wanted to predict potential defoliation to trees in the Chicago area that might occur after the establishment and possible major outbreak of this pest.

Our procedure for estimating the impact of a gypsy moth outbreak on urban trees involved three major steps. First, the number of trees and amount of leaves for each tree species were determined. Second, each tree species was classified according to its attractiveness to or probability of infestation by gypsy moth. Third, a model for defoliation, summer refoliation, and mortality was developed based on published information and the data described above.

According to our calculations, the potential defoliation by gypsy moth in the Chicago area is relatively modest, ranging from 14% in Chicago and suburban Cook County to 26% in DuPage County. Localized defoliation can be higher, however, particularly on institutional lands dominated by vegetation (e.g., parks, cemeteries, golf courses) and on vacant lands where defoliation estimates range between 23% and 40%. On these lands there is a high proportion of tree species that are highly vulnerable to gypsy moth. Note, however, that less than one-tenth of one percent of the total number of trees in the entire Chicago area are predicted to die because of gypsy moth defoliation during a two-year outbreak.

Another area of particular concern is residential lands, which contain most of the tree leaves in the Chicago area. Estimated defoliation in residential areas is highest in the most rural area, DuPage County (20%), and relatively low in Chicago (6%) and suburban Cook County (3%). Potential for gypsy moth defoliation is generally highest in the more rural areas and on land where trees remain in a more natural setting.

We concluded that when the gypsy moth becomes established in the Chicago area, outbreaks are not likely to cause large-scale problems requiring city- or countywide action. However, local controls may be necessary on vacant land, institutional lands (e.g., parks, cemeteries, golf courses), and residential lands, particularly in DuPage County, and in forest stands with high usage or visibility. Park and forest preserve districts may want to develop plans for managing gypsy moth in areas with high percentages of preferred hosts. In addition, because individual residential trees can be defoliated, education programs should be developed to inform homeowners about trunk banding and other cultural, microbial, or chemical techniques to control gypsy moth larvae.

David W. Onstad and Michael R. Jeffords, Center for Economic Entomology

Gypsy moth larvae congregating during the day on a tree.

Pupal skin that has been cast off next to part of egg mass.

Mature gypsy moth larva.


Charlie Warwick, editor

Illinois Natural History Survey

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