United States 

Department of 

Agriculture Tracking Changes in the 

Forest Service usceptibility of Forest Land 

Northeastern Forest 

Experiment Station Infeste with Gypsy Moth 

Research Paper NE-690 

David A. Gansner 

John W. Quimby 

Susan L. King 

Stanford L. Arner 

David A. Drake 
Abstract 

Does forest land subjected to intensive outbreaks of gypsy moth (Lymantria 

&par L.) become less susceptible to defoliation? A model for estimating the 

likelihood of gypsy moth defoliation has been developed and validated. It was 

applied to forest-inventory plot data lo quantify trends in the susceptibility of forest 

land in south-central Pennsylvania during a period of intensive infestation. 

Results show that even though susceptibility of the region's forest apparently has 

declined, the potential for future defoliation remains relatively high. 

DAVID A. GANSNER, SUSAN L. KING, STANFORD L. ARNER, and DAVID A. 

DRAKE are resource analysts with the Forest Inventory and Analysis work unit at 

the USDA Forest Service's Northeastern Forest Experiment Station, Radnor, PA. 

JOHN W. QUlMBY is a forest pest management specialist with the Division of 

Forest Pest Management, Pennsylvania Department of Environmental 

Resources, Bureau of Forestry, Middletown, PA. 

Manuscript received for publication 22 April 1994 

- 

USDA FOREST SERVICE 

5 RADNOR CORP CTR STE 200 

PO BOX 6775 

RADNOR PA 19087-8775 

August 1994 
Introduction 

Resource and pest managers are constantly being 

challenged to show that the benefits of gypsy moth 

(Lymantria dispar L.) control outweigh the costs. To deal 

objectively with this issue, they need a better understanding 

of the consequences of doing nothing-what happens to 

infested woodlands if you don't control the pest? In this 

regard, Jim Nelson (State Forester of Pennsylvania) asked 

us to help answer a specific question: Does forest land that 

has been subjected to intensive outbreaks become less 

susceptible to gypsy moth defoliation? 

The forests of south-central Pennsylvania (Fig.l) provide 

an excellent contemporary example of a resource that 

came under heavy attack from the gypsy moth during the 

1980's. Infestation cycles were typical of the gypsy moth's 

traditional modus operandi, that is, noticeable defoliation 

during 1981-83 and again during 1985-87 with peak levels 

in 1982 and 1986. 

Defoliation combined with drought, cutting, deer browsing, 

and other stresses took a heavy toll on the region's oak 

resource. During the 1980ts, mortality and cutting removed 

about 40 percent of the original inventory of oak growing 

stock. Growth on residual oak trees offset much of the 

loss, but not enough to keep the volume of oak from 

declining between inventories (Gansner et al. 1993). 

Losses were especially noticeable in smaller size trees. 

At the same time, other species such as red maple, pine, 

hemlock, birch, blackgum, ash and yellow-poplar, which 

are less susceptible to gypsy moth, prospered. Certainly, 

these trends would suggest a change in susceptibility. 

We are fortunate that a comprehensive reinventory of 

forest land in south-central Pennsylvania was completed 

for 1989 (Alerich 1993). A model that estimates defoliation 

potential can be applied to data from 415 plots remeasured 

in that survey. This allows us to quantify and analyze shifts 

in susceptibility that have occurred in the region since the 

last inventory in 1978. 

One model for gauging the likelihood of gypsy moth 

defoliation was developed in central Pennsylvania by 

Herrick and Gansner (1986). It links defoliation potential 

(expressed as average defoliation expected during a 

3-year outbreak) to key forest-stand characteristics as 

predictor variables. The model can be used to rate the 

relative susceptibility of forest stands. For example, stands 

with the highest potential for defoliation have at least 80 

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

chestnut and black oaks, and at least 60 percent in trees 

with good crowns. Stands with the lowest rating have less 

than 20 percent basal area in oaks. 

Checking on Model Performance 

Does the guide for estimating susceptibility work? To find 

out, we ran a test of validity on the 415 remeasured 

inventory plots in south-central Pennsylvania. First, we 

used the model to predict the defoliation potential of each 

plot, employing plot characteristics recorded in 1978. That 

was before the gypsy moth got into high gear in this region. 

Only three of the four variables included in the defoliation 

potential model could be used in the analysis. Crown 

condition was not measured by forest inventory crews and 

\ WAYNE 

Figure 1.--The study area in south-central Pennsylvania. 
Abstract 

Does forest land subjected to intensive outbreaks of gypsy moth (Lymantria 

dispar L.) become less susceptible to defoliation? A model for estimating the 

likelihood of gypsy moth defoliation has been developed and validated. It was 

applied to forest-inventory plot data to quantify trends in the susceptibility of forest 

land in south-central Pennsylvania during a period of intensive infestation. 

Results show that even though susceptibility of the region's forest apparently has 

declined, the potential for future defoliation remains relatively high. 

ER, SUSAN L. KING, STANFORD L. ARNER, and DAVlD A. 

DRAKE are resource analysts with the Forest Inventory and Analysis work unit at 

the USDA Forest Service's Northeastern Forest Experiment Station, Radnor, PA. 

JOHN W. QUIMBY is a forest pest management specialist with the Division of 

Forest Pest Management, Pennsylvania Department of Environmental 

Resources, Bureau of Forestry, Middietown, PA. 

Manuscript received for publication 22 April 1994 

USDA FOREST SERVICE 

5 RADNOR CORP CTR STE 200 

PO BOX 6775 

RADNOR PA 19087-8775 

August 1994 
introduction 

Resource and pest managers are constantly being 

challenged to show that the benefits of gypsy moth 

(Lymantria dispar L.) controi outweigh the costs. To deal 

objectively with this issue, they need a better understanding 

of the consequences of doing nothing-what happens to 

infested woodlands if you don't control the pest? In this 

regard, Jim Nelson (State Forester of Pennsylvania) asked 

us to help answer a specific question: Does forest land that 

has been subjected to intensive outbreaks become less 

susceptible to gypsy moth defoliation? 

The forests of south-central Pennsylvania (Fig.1) provide 

an excellent contemporary example of a resource that 

came under heavy attack from the gypsy moth during the 

1980's. Infestation cycles were typical of the gypsy moth's 

traditional modus operandi, that is, noticeable defoliation 

during 1981 -83 and again during 1985-87 with peak levels 

in 1982 and 1986. 

Defoliation combined with drought, cutting, deer browsing, 

and other stresses took a heavy toll on the region's oak 

resource. During the 1980's, mortality and cutting removed 

about 40 percent of the original inventory of oak growing 

stock. Growth on residual oak trees offset much of the 

loss, but not enough to keep the volume of oak from 

declining between inventories (Gansner et al. 1993). 

Losses were especially noticeable in smaller size trees. 

At the same time, other species such as red maple, pine, 

hemlock, birch, blackgum, ash and yellow-poplar, which 

are less susceptible to gypsy moth, prospered. Certainly, 

these trends would suggest a change in susceptibility. 

We are fortunate that a comprehensive reinventory of 

forest land in south-central Pennsylvania was completed 

for 1989 (Alerich 1993). A model that estimates defoliation 

potential can be applied to data from 415 plots remeasured 

in that survey. This allows us to quantify and analyze shifts 

in susceptibility that have occurred in the region since the 

last inventory in 1978. 

stimating Susrce 

One model for gauging the likelihood of gypsy moth 

defoliation was developed in central Pennsylvania by 

Herrick and Gansner (1986). It links defoliation potential 

(expressed as average defoliation expected during a 

3-year outbreak) to key forest-stand characteristics as 

predictor variables. The model can be used to rate the 

relative susceptibility of forest stands. For example, stands 

with the highest potential for defoliation have at least 80 

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

chestnut and black oaks, and at least 60 percent in trees 

with good crowns. Stands with the lowest rating have iess 

than 20 percent basal area in oaks. 

Checking on Model Performance 

Does the guide for estimating susceptibility work? To find 

out, we ran a test of validity on the 415 remeasured 

inventory plots in south-central Pennsylvania. First, we 

used the model to predict the defoliation potential of each 

plot, employing plot characteristics recorded in 1978. That 

was before the gypsy moth got into high gear in this region. 

Only three of the four variables included in the defoliation 

potential model could be used in the analysis. Crown 

condition was not measured by forest inventory crews and 

\ WAYNE 

Figure 1 .--The study area in south-central Pennsylvania. 
no appropriate surrogates for the crown condition variable 

are available. This is not a serious concern because crown 

condition does not account for a large amount of the 

variation in defoliation. The 415 plots were classified and 

sorted into six distinct groups of defoliation potential (in 

percent): 

Defoliation Number of 

Group potential (1 978) plois 

Next, estimates of actual defoliation occurring from 1978 

through 1989 were recorded for plots in each of the six 

groups of defoliation potential. To obtain these estimates, 

optical bar photography and sketch maps showing 

amounts and intensity of actuai annual defotiation were 

overlayed on plot iocations. 

Finally, averages of actual defoliation for the plots in each 

defoliation potential group were compared with predicted 

values for the groups (Fig. 2). Averages of actual 

defoliation for 1985 through 1987 were used. These 3 

years encompassed a period of intensive defoliation in the 

region. Use of average defoliation for a 3-year period of 

outbreak is consistent with the procedure used to develop 

the defoliation potential model. 

1(9) 2(18) 3(24) 4127) 5(28) 6(35) 

Defoliation potential group (predicted mean defoliation) 

Figure 2.--Comparison of actual mean defoliation (1 985-87) 

with predicted potential--south-central Pennsylvania. 

Results of this test indicate that, as a measure of relative 

susceptibility, the model appears to work well. For example, 

the model assigned 140 plots to Group 1, which had a 

mean defoliation potential of only 9 percent. Actual 

defoliation recorded for the 140 plots in Group 1 averaged 

only 12 percent. At the other end of the scale, 35 plots 

were assigned to Group 6, which had a relatively high 

mean defoliation potential of 35 percent. Actual defoliation 

for these 35 plots averaged 34 percent. 

There was a significant amount of variation in actual mean 

defoliation for plots within each group. For example, actual 

defoliation for 2 of the 30 plots in Group 5 averaged 62 

percent, while no noticeable defoliation was recorded for 2 

other plots in this group. For half of the plots in Group 5, 

actual mean defoliation ranged from 15 to 47 percent. 

Actual defoliation for ail plots in Group 5 averaged 31 

percent. The predicted mean was 28 percent. 

Fosbroke and Hicks (1993) evaluated the performance of 

this same model on plots in the Ridge and Valley province of 

Maryland and in the Appalachian Plateau of Pennsylvania. 

Our findings are in basic agreement with theirs. The 

modei's best use is for rating relative susceptibility, that is, 

for separating stands at risk of heavy defoiiation from those 

where defoliation is likely to be light-and not for predicting 

actual defoliation in a given stand. 

Application of the defoliation potential model to forestinventory plot data for 1978 and 1989 allowed us to quantify 

trends in the susceptibility of south-central Pennsylvania's 

forest resource during a period of intensive infestation by 

the gypsy moth. By design, each plot represents a 

proportional share of the forest area in a county, so 

appropriate weights could be applied to susceptibility 

ratings for individual plots to derive average ratings for 

each county. Results of this analysis show that defoliation 

potential is down in all of the region's 14 counties: 

Susceptibiiity rating Percent 

County 1978 1989 change 

Bedford 

Blair 

Cambria 

Dauphin 

Fayette 

Franklin 

Fulton 

Huntingdon 

Juniata 

Mifflin 

Perry 

Somerset 

Snyder 

Union 
Figure 3.--Change in defoliation potential for plots in south-central Pennsylvania, 1978-89. 

Figure 4.-Defoliation potential of south-central Pennsylvania plots, 1989. 
This is not to say that gypsy moth is no longer a threat here. 

In fact, despite the declines in potential, susceptibility 

ratings for the region's counties remain relatively high. By 

comparison, Erie, McKean, Sullivan, and Susquehanna 

Counties, in parts of the state where oaks are not so 

plentiful, had average ratings of less than 110 in 1989. 

Locations of the 415 forest inventory piots have been 

digitized, so changes in defoliation potential for individual 

plots can be mapped. This map provides a more specific 

view of trends in susceptibility (Fig. 3). Two-thirds of the 

plots remained in the same defoliation potential class 

between inventories. However, 26 percent shifted to a 

lower ratings while 8 percent shifted to a higher one. So 

on a plot basis, losers in susceptibility outnumbered 

gainers by more than 3 to 1. In 1978, more than 40 

percent of the plots had ratings of 20 or more. By 3989, 

one-third of them were stiil in this category (Fig. 4). This 

reinforces the notion that., even though susceptibility of the 

region's forest has declined, the potential for future 

defoliation remains relatively high. 

Does forest land subjected to intensive defoliation by gypsy 

moth become less susceptible to the pest? Results of this 

analysis wou!d indicate that indeed it does. And there is 

other related news--some good and some bad, depending 

on your perspective. The species composition of southcentral Pennsylvania's forest resource is more diverse now 

than it was 15 years ago. Also, many areas ihat were 

decimated by heavy mortality and cutting are regenerating 

to provide badly needed habitat for wildlife species ihat 

require early successional ecosystems. On the down side, 

there is less oak than there used to be and much of the 

timber that died cannot be salvaged. This is bad news for 

wood-producing interests. 

Some care should be taken in extending specific results of 

this case study to other regions. Factors other than the 

gypsy moth such as drought, cutting, bark beetles, root 

rot, and deer browsing contributed to the declines in oak 

that led to reductions in the susceptibility of south-central 

Pennsylvania's forests. Effects of these other factors will 

vary from placs to place. Also, characteristics of the 

region's forests are somewhat difierent'from those on new 

frontiers of infestation. Who can say whether a post oak 

in the Ozarks of Missouri or sweetgum on the Coastal 

Plain of South Carolina will hold the same attraction for 

gypsy moth as a chestnut oak on a ridge in south-central 

Pennsylvania. 

Literature Cite 

Alerich, Carol L. 1993. Forest statistics for Penns)llvania, 

1978-1989. Resour. Bull. NE-126. Radnor, PA: U.S. 

Department of Agriculture, Forest Service, Northeastern 

Forest Experiment Station. 244 p. 

Fosbroke, David E.; Hicks, Ray R., Jr. 7993. Predi 

of gypsy moth defoliation in central hardwo 

a validation study. In: Proceedings of the 9th central 

hardwood forest conference; 1993 March 8-1 0; West 

Lafayette, IN. Gen. Tech Rep. NC-161. St Paul, MN: 

US. Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 156-170. 

Gansner, David A; Arner, Stanford 1; Widmann, Richard 

H.; Alerich, Carol L. 1993. What's happening to 

Pennsylvania's oaks? In: Finley, James C.; Jones, 

Stephen B., eds. Penn's Woods--change and 

chalienge: proceedings of the 1993 Penn State forest 

resources issues conference; 1993 April 1-2; State 

College, PA. State College, PA: Pennsylvania State 

University: 15-22. 

Herrick, Owen W.; Gansner, David A. 1986. Rating forest 

stands for gypsy moth defoliation. Res. Pap. NE-583. 

Broomall, PA: US. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 4 p. 
Gansner, David A.; Quimby, John W.; King, Susan L.; Arner, Stanford L.; 

Drake, David A. 1994. Tracking changes in the susceptibility of 

forest land infested with gypsy moth. Res. Pap. NE-690. Radnor, PA: 

US. Department of Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 4 p. 

Does forest land subject to intensive outbreaks of gypsy moth (Lymantria 

&par L.) become less susceptible to defoliation? A model for estimating 

the likelihood of gypsy moth defoliation has been developed and validated. 

It was applied to forest-inventory plot data to quantify trends in the 

susceptibility of forest land in south-central Pennsylvania during a period of 

intensive infestation. Results show that even though susceptibility of the 

region's forest apparently has deciined, the potential for future infestations 

remains relatively high. 

Keywords: Forest health, defoliation, susceptibility, hazard classification, 

iymanfria dispar L. 

em $,$ Printed on Recycled Paper 
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Parsons, West Virginia 

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College of Environmental Sciences and Forestry at Syracuse University 

University Park, Pennsylvania, in cooperation with The Pennsylvania State 

University 

Warren, Pennsylvania 

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