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

Methods

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

Results

Tree crowns cover an average of 11% of the land area in Chicago, 23% in suburban Cook County, 19% in DuPage County, and 19% of the entire study area. The study area tree population is 50,830,000 with 31,810,000 trees in suburban Cook County, 14,900,000 trees in DuPage County, and 4,130,000 trees in Chicago. Tree densities average 68 trees/ hectare in Chicago, 169 trees/ ha in suburban Cook County, and 173 trees/ha in DuPage County (12). The greatest leaf biomass is in suburban Cook County (195,620 metric tons), followed by DuPage County (93,280t) and Chicago (38,190t). Institutional lands dominated by vegetation had the highest tree density, followed by vacant land, residential and other land uses (Table 1). The greatest number of trees and most leaf biomass is found on institutional lands dominated by vegetation and residential lands (Table 1).

Figure 1 presents the percentage of preferred tree species (by number not leaf biomass) in each of seven land-use categories in each of the three regions of the Chicago area. Although there is much variability across the three regions, the percentages tend to decrease, on average, going from the parks and preserves on the left to transportation areas on the right in Figure 1. DuPage County lands tend to be much different from the lands in the other two regions in Cook County. The threshold value of PREF=25% is not exceeded in the transportation areas and in several of the residential, commercial, and institutional-building categories for certain regions (Figure 1). Because PREF<25% for the agriculture category in all three areas studied, we omitted this category from the figures.

Figure 1.

Percentage of preferred tree species in each land-use category in each of the three regions: Chicago, Cook County except the city of Chicago, and DuPage County. The land-use categories are apartments (Apt); vacant land; parks, preserves and golf courses (Inst-Veg); residential (Resid); commercial (Comm); institutional lands dominated by buildings (IB); and transportation areas and corridors (Trans)

Table 2 shows tree data and predicted defoliation for major land uses in the three regions of the study area. As Table 1 demonstrated, 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 are predicted to occur on vacant lands, followed by institutional lands dominated by vegetation, and residential (Table 2). DuPage Co. is predicted to suffer the highest level of defoliation (26%) for an entire region with the othertwo areas only incurring half as much. Although trees on vacant land are predicted to receive the highest damage of any category, the trees on the institutional lands dominated by vegetation are likely valued more by citizens and managers. These parks, cemeteries, and golf courses are predicted to be defoliated by 26%, 23%, and 32% in the City of Chicago, suburban Cook Co., and DuPage Co., respectively, during a gypsy moth outbreak. Note that the residential and other land use categories for DuPage Co. have much higher levels of predicted defoliation compared to those same land uses in Chicago and suburban Cook Co. (Table 2).

Figure 2a-c shows the percentages of intermediate and preferred hosts predicted to be defoliated by a gypsy moth outbreak by the main model. Least-preferred hosts are not defoliated. The percent defoliation exceeded 50% in only three cases involving preferred hosts: apartments (APT) in Chicago (Fig. 2a) and institutional-BLDG (IB) and vacant lands in DuPage County (Fig. 2c).

Figure 2a–c.

The percentages of intermediate and preferred hosts predicted to be defoliated by a gypsy moth outbreak in the city of Chicago (a), suburban Cook County (b), and DuPage County (c). The land-use categories are apartments (Apt); vacant land; parks, preserves and golf courses (Inst-Veg); residential (Resid); commercial (Comm); institutional lands dominated by buildings (IB); and transportation areas and corridors (Trans). No trees are found in the IB and Trans categories for Cook County. No preferred host trees are found in the Trans category for Chicago and the Comm category for DuPage County

Because defoliation is much lower in intermediate species, only preferred species are predicted to refoliate with this database and model. Refoliation occurs on only three land uses, affecting 145,000 trees in Chicago’s multi-family land use (apartments); 57,000 trees on institutional lands dominated by buildings in DuPage County; and 5,566,000 trees on vacant lands in DuPage County.

Mortality is predicted to occur only on institutional areas dominated by buildings in DuPage County, the only land use where defoliation exceeded 60% (Fig. 2c). This value corresponds to the highest percentage of preferred hosts in Figure 1. We predict that 8,600 (20%) and 21,500 (50%) trees will die after the first and second years of a gypsy moth outbreak in this area. These numbers are much less than one-tenth of one percent of the total number of trees in the entire Chicago area. The actual level of mortality may be less if our assumption about more stressful urban conditions is wrong.

The results for the other two versions of the model are not much different. Table 3 presents the final or August leaf biomass losses as percentages of normal levels for the three regions and the entire Chicago study area. The second version of the model, compared to the main version, produced similar or slightly higher defoliation values without the use of a threshold value for PREF (Table 3). In addition, because the highest defoliation levels are not changed by the second model, neither are the refoliation and mortality estimates described above. As expected from the lower parameter values in the third version of the model, the final defoliation levels are slightly smaller than those produced by the second version (Table 3). Generally, DuPage Co. is predicted to incur the highest levels of defoliation.

Discussion

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 on vacant lands where defoliation estimates range between 23 and 40%. On these lands there is a high proportion of preferred tree species (35-58% of the population) and the trees are often aggregated in stand structures.

Another area of particular concern is residential lands, which contain over half of the leaf biomass 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 uses where trees remain in a more natural stand structure. As urbanization and human influences increase, tree structure, which was dominated before human settlement by most preferred and intermediately preferred species of oak and hickory (14), will tend to shift toward less preferred tree species (12).

When the gypsy moth becomes established in the Chicago area, outbreaks are not likely to cause large-scale problems requiring city- or county-wide action. However, local controls may be necessary on vacant land, institutional land dominated by vegetation, 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, as individual residential trees can be defoliated, education programs could be developed to inform homeowners regarding trunk banding and other cultural, microbial, or chemical techniques to control gypsy moth larvae (4,16).

Many entomologists believe that low-diversity tree stands may be more susceptible to pest outbreaks than are higher-diversity stands (5). For the highly polyphagous gypsy moth, species diversity is not as important as preference-class diversity. Our models attempt to account for this aspect of insect outbreaks by making defoliation a function of the percentage of the trees in the preferred-host class.

Our models may underestimate defoliation on intermediates and least-preferred hosts when the percentage of preferred hosts exceeds a high value, such as 40%. Data reported by Gottschalk (9), however, indicate that when preferred hosts lose 40-60% of their foliage in forest stands, intermediate and least-preferred hosts lose, respectively, less than one-half and one-tenth as much as preferred species.

The most important assumption in our analysis is the relative attack rates for each class of trees. Therefore, more studies are needed to determine the attractiveness of more tree species to feeding by gypsy moth. Feeding by both small and large larvae should be studied, because large larvae are typically less inhibited by host quality than first and second instar larvae.

Results from this study are likely to be similar for other Midwestern cities developed in a mixed prairie â· oak-hickory forest. These results, however, may differ from results expected for cities within different forest or vegetation types. It is probable that for cities built in forested areas with many highly preferred hosts, that urbanization will tend to decrease potential defoliation levels. However, for cities developed in non-forest vegetation types or built in forests dominated with least-preferred species, urbanization through tree planting and species diversification, will tend to increase potential defoliation by the gypsy moth.

We believe that our main model can be used for other cities or for areas of particular land use smaller than a city. Users would need a tree database consisting of numbers of trees and an estimate for leaf biomass by species. Leaf biomass can be calculated from information on number and sizes of trees (13).