United States 

Department of 

Agriculture Rating Forest Stands for 

Forest Service Gypsy Moth Defoliation 

Northeastern Forest 

Experiment Station 

Research Paper 

NE-583 Owen W. Herrick 

1986 David A. Gansner 
Abstract. 

The severity of future defoliation can be estimated 

from the percentages of basal area in oaks 

black oak (Q. velutina) and chestnut oak 

in trees with good crowns, along with the average 

diameter of the stand. With information on these 

variables, the defoliation potential of any hardwood 

forest stand in an approaching gypsy moth (Lymantria 

dispar) infestation can be rated. 

Manuscript received for publication 21 February 1986 

The Authors 

Owen W. Herrick and David A. Gansner are research 

foresters, Northeastern Forest Experiment Station, 

USDA Forest Service, Broomall, PA. 
Because the levels of defoliation vary widely within 

areas infested by gypsy moth, the ability to identify 

critical locations (i.e., those most likely to suffer heavy 

defoliation during an infestation) is important to 

resource management. The purpose of this work is to 

evaluate the relationship between defoliation and forest 

stand characteristics and develop guidelines for rating 

the defoliation potential of forest stands on new frontiers of infestation. 

Approach 

Measurements have been taken each year since 1978 

to monitor and evaluate impacts of the gypsy moth on 

forest trees in central Pennsylvania. The project (a 

cooperative venture of the Pennsylvania Bureau of 

Forestry and the U. S. Forest Service) uses a network of 

some six hundred 1110-acre field plots located between 

Carlisle and State College, Pennsylvania. This was the 

leading edge of gypsy moth infestation in 1978 when the 

plots were established, before the stands were infested. 

Defoliation Data 

Ocular estimates of defoliation for ail trees 3.0 

inches dbh and larger were recorded each year at peak 

periods of defoliation (June or July). The percentage of 

foliage stripped from each tree was estimated to the 

nearest 10 percent. Ddfoliation averages (DF) for each 

plot were derived from the defoliation estimates for 

each tree (DFi), weighted by the tree's diameter 

squared (D~~) to account for differences in tree crown size: 

Average defoliation on each plot was used to assign it 

to a light (<30%), moderate (30-59%), or heavy (60+%) 

defoliation category each year (Gansner and Herrick 

1985). The peak year for defoliation was 1981; nearly 

half the plots had moderate or heavy defoliation that 

year, and plot defoliation averaged 39 percent (Fig. I). 

The next highest defoliation years were 1980 and 1982, 

with averages at 17 percent and 22 percent, 

respectively. One-fourth of the plots had heavy 

defoliation at least once during these 3 years (only 2 

percent received as much as 2 years of heavy 

defoliation). Another third of the plots received 

moderate defoliation at least once. 

The bulk of gypsy moth defoliation activity in 

central Pennsylvania occurred during the 3-year period 

from 1980 to 1982. To make allowance for the 

progressive but uneven nature of gypsy moth 

infestations we averaged each plot's defoliation 

measurements for these 3 years. Clearly, more 

locations in a sample area are likely to be defoliated 

sometime during several years of gypsy moth buildup 

than in any single year. Thus, the 3-year average 

provided a more comprehensive measure of defoliation 

at any one location during the outbreak. 

Average = 14% Average =17% Average =39% Average =22% Average =9% 

Light C<30%1 Moderate (30 to 59%) 

PI 

Heavy (60% plus) 

Figure 1.--Gypsy moth defoliation in central Pennsylvania, 1979-83. 
Predictor Variables 

Many forestland characteristics are potential 

candidates for gauging the amount of defoliation an 

area is likely sustain in an infestation. Tree species, 

size, crown condition, crown position, stocking density, 

site quality, elevation, and aspect are some prominent 

attributes that many people associate with gypsy moth 

defoliation (Bess et al. 1947, Campbell and Sloan 1977, 

Campbell 1979). More detailed data, such as the 

prevalence of bark flaps and rock outcrops and egg 

mass viability may be useful as well (Houston and 

Valentine 1977, Wilson et al. 1981). As a practical 

matter, however, only easy-to-measure timber stand 

characteristics that would be useful in prediction were 

entered into our analyses. It would be of little help in 

planning, for example, to have to rely on predicting 

weather conditions during an infestation to gauge 

potential defoliation. Also, it is assumed that some 

indicator of imminent infestation has raised enough 

concern to warrant rating an area; e.g., to identify 

forest areas that are candidates for treatment because 

of their likelihood of being defoliated to unacceptable 

levels. 

The data-plot network covers a full range of values 

representing preoutbreak conditions (1978) in the 

infested area. Selected plot variables provide a general 

description of the coverage area: 

Mean Range 

Basal area/acre (ft2) 90 15-190 

Basal area in oaks (%) 68 0-100 

Basal area in black and 

chestnut oaks (%) 

Basal area in dominant and 

codominant trees (%) 77 14-99 

Basal area in trees with 

good crown condition[(%) 55 0-100 

Average dbh (inches) 7.4 3.8-13.4 

Site index (ft., upland oaks) 61 25-100 

Stand age (years) 71 15-175 

Trees/acre (no.) 255 80-650 

Stocking (96) 81 20-158 

Slope (%I 24 0-67 

Elevation (ft.) 1535 575-2450 

1~ good crown is healthy, with few dead branches and 

normal foliage density, color, and size. 

Analysis 

The combined influence that 38 variables have on 

the 3-year defoliation average was studied by 

Automatic Interaction Detection (Sonquist et al. 1973). 

This statistical technique searched characteristics of 

the trees and timber stands for the best predictors of 

defoliation. Starting with the entire sample in one 

group, the plots were partitioned through successive 

two-way splits into a series of subgroups. The splits 

occur so that, at each step, the chosen predictor 

variable forms two new groups that cause the greatest 

reduction of variance in the dependent variable 

(average defoliation). The result is a set of best 

predictors selected by the AID algorithm, and presented 

as a branching diagram that details the configuration of 

3-year defoliation averages from forest stands 

subjected to the gypsy moth episode in central 

Pennsylvania. 

Results 

From 1980 through 1982, the 3-year defoliation 

average was 26 percent, with a range from 0.3 percent 

to 70 percent. Plots that were heavily defoliated in any 

of the 3 years averaged 43 percent. The 3-year average 

was 27 percent for plots with no more than moderate 

defoliation; for plots that had only light defoliation, it 

was 14 percent. 

Automatic interaction detection analysis of the 3- 

year defoliation averages generated 10 final 

classification groups and the average defoliation 

percentage for each group (Fig. 2). Each sample plot is a member of one of the groups. Only four of the many 

tree and timber stand characteristics analyzed as 

predictor variables were retained in this defoliation 

classification system: percentage of basal area in oaks, 

percentage of basal area in black and chestnut oaks, 

percentage of basal area in trees with good crowns, and 

average diameter of the stand. 

Forest stands represented by the sample plots in the 

group with the highest average defoliation--40 percent, 

as compared to 26 percent for the total sample--are 

identified as Group 1. They have at least 80 percent of 

their basal area in oak species,at least 70 percent of 

their basal area in black and chestnut oaks, and at least 

60 percent of basal area in trees with good crowns. 

This group comprised 10 percent of the total sample. 

The average defoliation dropped to 31 percent if a timber stand met the first two criteria for Group 1 but 

had less than 60 percent of its basal area in trees with 

good crowns (Group 2). Average defoliation was 32 

percent when 50 to 79 percent of basal area was in oak 

species, at least 40 percent of the basal area was in 

black and chestnut oaks, and at least 40 percent of the 

basal area was in trees with good crowns (Group 5). 

These three groups with the highest defoliation 

averages included 38 percent of the sample. On the 
DEFOLIATION LEVEL 

Heavy 30+% 

Moderate 20-29% 

Light ~20% 

Total t Sample (26%) % BA oaks 

% BA good crown condition 

% BA black and 

(32%) chestnut oaks 

% BA good crown condition 

% BA oaks 

condition 

Figure 2.--Guide for estimating gypsy moth defoliation potential. Any hardwood stand can be assigned to a defoliation 

group on the basis of characteristics of the stand. The Fyear average defoliation percentage is shown in parentheses 

for each group. Fractions on each line between groups show tk portion of the Pennsylvania sample that is in that 

group. 

other hand, as testimony to the strength of host species 

as a key indicator, stands with less than 20 percent of 

their basal area in oak species had the lowest 

defoliation average--9 percent (Group 10). 

This study describes a fairly simple structure that 

can be used to assign potential rates of defoliation to 

forest stands. It is likely to be most accurate when 

applied to areas with stand characteristics similar to 

those of the central Pennsylvania study area. 

Nevertheless, it is the only available model that links ' 

defoliation severity to key forest stand characteristics 

as predictor variables. Of the many elements of stand 

condition analyzed as independent variables, the four 

retained in the model make good sense. Oaks are a 

favored food of gypsy moth, and black and chestnut oak are highly preferred (Gansner and Herrick 1985); thus 

species is a strong indicator of stand attractiveness for 

gypsy moth activity: over half the group-identifying 

splits in the model were keyed to species alone. Trees 

with good crowns support abundant foliage which 

presents attractive insect feeding sites. Likewise, 

large-diameter trees often support large dominant crowns that are attractive gypsy moth feeding sites. 

Perhaps the model's most useful contribution lies in 

making it possible to separate stands at risk of heavy 

defoliation from those where defoliation is likely to be 

light. From the distribution of the 3-year averages we have arbitrarily defined heavy defoliation as above 30 

percent and light defoliation as less than 20 percent 

(Fig. 2). The model gives a basis for rating and mapping 

the'potential severity of forest defoliation. Once the 

stands are rated, management decisions can be directed 

toward protecting highly valued forest resources among 

those identified as most threatened with defoliation in 

an approaching gypsy moth infestation. 

This model's performance will be further validated 

as the gypsy moth invades new territory. Meanwhile, 

we are developing measurements that will describe tree 

mortality and changes in stand condition on this same plot network. These data will be the basis for impact 

analyses to provide further understanding of defoliation 

and subsequent tree mortality and loss relationships in 

forest stands infested by gypsy moth. As was the case 

in this study, the entire sequence of analyses.will focus on the development of more effective guides to help 

forest resource managers cope with the pest. 
Literature Cited 

Bess, H. A.; Spurr, S. H.; Littlefield, E. W. Forest site 

conditions and the gypsy moth. Harvard Forest 

Bulletin 22; 1947. 53 p. 

Campbell, R. W. Gypsy Moth: forest influence. Agric. 

Info. Bull. No. 423. Washington, DC: U.S. 

Department of Agriculture; 1979. 44 p. 

Campbell, R. W.; Sloan, R. 3. Forest stand responses to 

defoliation by the gypsy moth. Forest Science 

Monograph 19; 1977. 34 p. 

Gansner, D. A.; Herrick, 0. W. Host preferences of 

gypsy moth on a new frontier of infestation. Res. 

Note NE-330. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station; 1985. 3 p. 

QW.9. OOVERNMENT PRlNTlNQ OFFlCU 9 8 6.- 6 0 5, 0 2 6 / 4 0 0 0 7 

Houston, D. R.; Valentine, H. T. Comparing and 

predicting forest stand susceptibility to gypsy moth. 

Canadian Journal of Forest Research. 7(3):447-461; 

1977. 

Sonquist, 3. A.; Baker, E. L.; Morgan, J. N. Searching 

for structure. Ann Arbor, MI: Survey Research 

Center, Institute for Social Research, University of 

Michigan; 1973. 236 p. 

Wilson, W. W.; Ivanowsky, S. M.; Talerico, R. L. 

Methods of gypsy moth detection and evaluation: 

Estimating population quality. In Doane, C. C.; 

McManus, M. L.; eds. The gypsy moth: Research 

toward integrated pest management. Tech. Bull. 

1584. Washington, DC: U.S. Department of 

Agriculture; 1981. p. 37. 
Herrick, Owen W.; Gansner, David A. Rating forest stands for gypsy moth 

defoliation. Research Paper NE-583. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest Experiment Station; 1986. 

4 P. 

The severity of future defoliation can be estimated from the percentages of 

basal area in oaks (Quercus), black oak (Q. velutina) and chestnut oak (a 

prinus), and in trees with good crowns, along with the average diameter of the 

stand. With information on these variables, the defoliation potential of any 

hardwood forest stand in an approaching gypsy moth (Lymantria dispar) 

infestation can be rated. 

ODC 453.441: 461.1 1 

Keywords: Hazard classification; defoliation susceptibility; Lymantria dispar 
Headquarters of the Northeastern Forest Experiment Station are in 

Broomall, Pa. Field laboratories are maintained at: 

Amherst, Massachusetts, in cooperation with the University of 

Massachusetts. 

Berea, Kentucky, in cooperation with Berea College. 

Burlington, Vermont, in cooperation with the University of 

Vermont. 

Delaware, Ohio. 

Durham, New Hampshire, in cooperation with the University of 

New Hampshire. 

Hamden, Connecticut, in cooperation with Yale University. 

Morgantown, West Virginia, in cooperation with West Virginia 

University, Morgantown. 

Orono, Maine, in cooperation with the University of Maine, 

Orono. 

@ Parsons, West Virginia. 

@ Princeton, West Virginia. 

Syracuse, New York, in cooperation with the State University of 

New York College of Environmental Sciences and Forestry at 

Syracuse University, Syracuse. 

University Park, Pennsylvania, in cooperation with the 

Pennsylvania State University. 

Warren, Pennsylvania.