Potential Defoliation of Trees by Outbreak Populations of Gypsy Moth in the Chicago Area

Abstract

The gypsy moth, Lymantria dispar, will soon become established in much of the Midwest. If an outbreak with extremely high population levels of this serious defoliator is allowed to occur in the Chicago area, what kind of damage can be expected? A model for defoliation, refoliation and mortality was developed based on the number of trees and associated leaf biomass for each tree species classified according to its attractiveness to or probability of infestation by gypsy moth. Data for eight land uses in the Chicago metropolitan area were used in the model. Vacant lands, institutional lands dominated by vegetation (e.g., parks, cemeteries, golf courses), and residential areas have the highest tree densities and other characteristics that make them valuable as well as vulnerable to gypsy moth infestation. The highest percentage of most-preferred tree species and highest percent defoliation is predicted to occur on vacant lands, followed by institutional lands dominated by vegetation, and residential areas. 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 and vacant lands, where defoliation estimates range between 23 and 40%. 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 an outbreak.