United States

Department of

Agriculture

Forest Service

Northeastern

Research Station

Research Paper NE-726

Mapping Host-Species

Abundance of Three

Major Exotic Forest Pests

Randall S. Morin

Andrew M. Liebhold

Eugene R. Luzader

Andrew J. Lister

Kurt W. Gottschalk

Daniel B. Twardus
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Abstract

Periodically over the last century, forests of the Eastern United States have been devastated

by invasive pests. We used existing data to predict the geographical extent of future

damage from beech bark disease (BBD), hemlock woolly adelgid (HWA), and gypsy moth.

The distributions of host species of these alien pests were mapped in 1-km2

 cells by

interpolating host basal area/ha from 93,611 forest-inventory plots in 37 states. The

interpolated surfaces were adjusted for forest density (percent land cover) by multiplying

values by an estimate of percent forest cover derived from existing land-cover maps

(30-m2

 cells). According to our estimates, BBD currently occupies only about 27 percent of

its potential range in land area, but has invaded more than 54 percent in total host density.

HWA occupies nearly 26 percent of its potential range in land area, and about one-quarter

in total host density. Gypsy moth occupies only 23 percent of its potential range in the

Eastern United States, and only 26 percent in total host density.

The Authors

RANDALL S. MORIN, research forester, ANDREW M. LIEBHOLD, research entomologist,

EUGENE R. LUZADER, computer specialist, ANDREW J. LISTER, research forester, and

KURT W. GOTTSCHALK, research forester, are with the USDA Forest Service’s

Northeastern Research Station. Morin and Lister are located in Newtown Square,

Pennsylvania. Liebhold, Luzader, and Gottschalk are located at Morgantown, West Virginia.

DANIEL B. TWARDUS is forest health specialist with the USDA Forest Service’s

Northeastern Area, State and Private Forestry at Morgantown, West Virginia.

Manuscript received for publication 27 September 2004

Cover: Map shows percent forest cover in the Eastern United States.
1

Introduction

Invasion by exotic insects and diseases threatens the

productivity and stability of forest ecosystems around the

world (Liebhold et al. 1995a; Vitousak et al. 1996;

Pimentel et al. 2000). Forests of the Eastern United

States seem to be particularly vulnerable to these

invasions. Over the last century, they have been

devastated by forest pests such as beech bark disease,

chestnut blight, and gypsy moth (Mattson 1997), with

subsequent secondary adverse effects throughout the

invaded ecosystems (Redman and Scriber 2000).

Assessing the likelihood of future damage from specific

exotic organisms (risk assessment) is an important initial

step in developing effective strategies for managing alien

species (Reichard and Hamilton 1997; Byers et al. 2002)

both before and after their arrival in new habitats

(Liebhold et al. 1995a). A risk assessment for an invasive

species that has not yet been established typically

comprises estimates of: 1) the probability of arrival and

establishment; 2) the geographical extent of its future

range; and 3) the likely ecological, economical, and

sociological impacts.

Maps representing the geographic extent of estimated

future disturbance caused by exotic pests are an

important component of risk assessment (Rouget et al.

2002). Predictions of disturbance risk in forest

ecosystems are often limited by a lack of data and/or

predictive models for specific pest species. We developed

statistical models for predicting the abundance of host

species for beech bark disease, hemlock woolly adelgid,

and gypsy moth. The three disturbance agents have

become established in North America and currently are

expanding their range. Estimates of host-species abundance

would be useful for long-range planning to both manage

populations and implement cultural practices that minimize

damage. In addition, abundance maps can serve as base

layers that are a critical component of the USDA Forest

Service’s National Risk Map (Lewis 2002).

Beech bark disease (BBD), also known as beech scaleNectria canker, is an insect-fungus complex involving the

beech scale insect (Cryptococcus fagisuga) and the exotic

canker fungus Nectria coccinea var. faginata or the native

Nectria galligena that kills or injures American beech

(Fagus grandifolia) (Houston 1994). The disease results

when the Nectria fungus infects the bark through feeding

wounds caused by beech scale, which was introduced

accidentally into Nova Scotia from Europe around 1900.

BBD has since spread southwest into New England, New

York, Pennsylvania, West Virginia (Manion 1991), Ohio,

Michigan, North Carolina, and Tennessee. Fungal

invasion occurs about 3 to 5 years after the scale insects

are evident. Mortality often is extensive in the year that

Nectria is observed, though some trees can survive for

several years.

First observed in the United States in the 1950’s in

Virginia, hemlock woolly adelgid (HWA) is a destructive

insect pest of forest and ornamental hemlock trees. In the

Eastern United States, HWA feeds only on the foliage of

eastern hemlock (Tsuga canadensis) and Carolina hemlock

(Tsuga caroliniana), a less common species found in

isolated areas of the southern Appalachians. Heavy

infestations have killed trees in as few as 4 years, though

some trees have survived infestations for more than 10

years (McClure et al. 2001).

Widely acknowledged as the most destructive defoliating

forest insect in the United States (Davidson et al. 1999;

Sharov et al. 2002), the gypsy moth was introduced

accidentally around 1868 near Boston, Massachusetts.

Outbreaks occurred in that area about 10 years later

(Liebhold et al. 1992). The range of gypsy moth has

been expanding gradually. Defoliating populations

typically are found 5 to 6 years following establishment

in a new location (Liebhold et al. 1994; Sharov and

Liebhold 1998). In North America, gypsy moth feed on

more than 300 species of trees and shrubs. There is

considerable variation in susceptibility to defoliation

among tree species (Liebhold et al. 1995b). Species

preferred most by gypsy moth are in the genera Quercus,

Populus, and Larix.
2

Methods

The geographic distribution of suitable habitats for

BBD, HWA, and gypsy moth were mapped by

interpolation of host-species abundance estimated from

93,611 forest-inventory plots in 37 states. Interpolation

is a mathematical procedure by which values are

estimated at positions between known values. The data

were extracted from the Eastwide Database (Table 1),

which at the time the project was executed was the most

recent available source of USDA Forest Service Forest

Inventory and Analysis (FIA) data for the 37 states in

the study (Hansen et al. 1993). For our purposes,

“Eastern United States” is used to describe the 37 states

in the Eastwide Database. Host abundance for BBD,

HWA, gypsy moth was measured as basal area/ha of

American beech, basal area/ha of eastern and Carolina

hemlock, and total basal area/ha of preferred tree

species, respectively. A host species was considered

preferred by the gypsy moth if it was in the “high”

preference class developed by Liebhold et al. (1995b);

this class included 79 species.

We used the ordinary kriging procedure (Deutsch and

Journel 1998) to interpolate a surface of basal area/ha

for the host species of each disease or insect. Kriging is a

geostatistical method that provides estimates for

unsampled locations by computing weighted averages of

sampled values from nearby locations (Isaaks and

Srivistava 1989; Cressie 1993). The weights are

determined on the basis of the semivariogram, a

statistical model of the relationship between spatial

autocorrelation and distance between pairs of sampled

values. A detailed explanation of kriging is available at:

http://www.cee.vt.edu/program_areas/environmental/

teach/smprimer/kriging/kriging.html. We generated

maps from the plot data by calculating kriged estimates

on a grid of 1- by 1-km cells. The GSLIB software

library (Deutsch and Journel 1998) was used in

performing the variography and kriging.

Maps of forest susceptibility were then adjusted for

forest density (percent forest cover). During the period

of data collection, FIA plots were in effect located

Table 1.—Year and number of forest inventory plots

in each of 37 states

State Year Number of plots

Alabama 1990 3,575

Arkansas 1995 3,090

Connecticut 1985 283

Delaware 1986 134

Florida 1995 3,822

Georgia 1997 5,789

Illinois 1998 1,586

Indiana 1986 1,976

Iowa 1990 636

Kansas 1994 1,548

Kentucky 1988 1,921

Louisiana 1991 2,233

Maine 1995 2,587

Maryland 1986 672

Massachussets 1985 371

Michigan 1993 9,933

Minnesota 1990 11,553

Mississippi 1994 2,960

Missouri 1989 4,664

Nebraska 1994 428

New Hampshire 1983 586

New Jersey 1987 251

New York 1993 2,850

North Carolina 1990 4,558

North Dakota 1995 265

Ohio 1991 1,587

Oklahoma 1993 862

Pennsylvania 1989 2,964

Rhode Island 1985 117

South Carolina 1993 3,358

South Dakota 1980 46

Tennessee 1999 2,348

Texas 1992 1,925

Vermont 1989 619

Virginia 1992 3,600

West Virginia 1989 2,491

Wisconsin 1996 5,423
3

randomly, and tree data were collected only in forested

areas. As a result, we had to adjust the estimates of host

abundance in each cell for the proportion of the cell area

that was forested. This was accomplished using a regionscale land-cover dataset (National Land Cover Data)

acquired from the Multi-Resolution Land Characteristics

Consortium as a raster matrix of 30- by 30-m cells that

were placed into multiple land-cover classes (http://

www.epa.gov/mrlc/nlcd.html) (Vogelmann et al. 2001).

The data were aggregated to estimate percent forest cover

for each 1- by 1-km cell (Fig. 1). A cell was considered

forested if it was classified as deciduous, evergreen, mixed

forest, or as forested wetland. Next, the kriged maps of

host abundance were multiplied by the forest-density

map (Fig. 1) to generate maps of forest susceptibility

adjusted for percent forest cover.

When combined with current range maps, the hostdensity maps allow estimates of the current and potential

ranges of BBD, HWA, and gypsy moth (Table 2). A 1-

km2

 pixel was included in the estimates of land-area

range if it was thought to contain host basal area.

1,000 0 1,000 500 Kilometers

0 - 0.1

0.1 - 0.2

0.2 - 0.4

0.4 - 0.6

0.6 - 0.8

0.8 - 1

Estimated percent

forest cover

Figure 1.—Map of percent forest

density from National Land Cover Data.

Table 2.—Potential and current ranges and host densities of beech bark disease, hemlock woolly

adelgid, and gyspy moth

Item Beech bark Hemlock woolly Gypsy

disease adelgid moth

Potential area of host habitat (km2

) 1,589,747 714,426 3,637,478

Current area of infested host habitat (km2

) 434,548 182,942 851,753

Proportion infested (%) 27.3 25.6 23.4

Potential density of host basal area (m2

) 606,734 522,653 10,331,781

Current density of host basal area (m2

) 348,311 128,425 2,685,118

Proportion infested (%) 57.4 24.6 26.0
4

Estimated American beech

basal-area (m /ha) 2

0

0 - 0.2

0.2 - 3

3-7

Figure 2.—Kriged map of estimated basal

area of American beech.

Results

Beech Bark Disease

The basal area/ha of American beech was estimated (Fig.

2) and then multiplied by the forest-density map to

create a map of beech abundance adjusted for forest

density (Fig. 3). The greatest concentration of beech is in

the Adirondack Mountain region of New York. There are

smaller concentrations in Maine, New Hampshire,

Vermont, West Virginia, and northern Pennsylvania.

Beech also is found at low levels over a large range that

extends through most of the forested regions of the

Eastern United States. The current range of the beech

scale insect is shown in Figure 4. Our estimates indicate

that BBD currently occupies only about 27 percent of its

potential range in land area, but already has invaded

more than 54 percent of its potential area in total host

density (Table 2).

Hemlock Woolly Adelgid

The basal area/ha of hemlock was estimated (Fig. 5) and

then mulitiplied by the forest-density map to create a

map of hemlock abundance adjusted for forest density

(Fig. 6). Hemlock is distributed widely throughout the

Northeast and the northern portions of Michigan and

Wisconsin. Elsewhere, its distribution is limited to the

higher elevations of the Appalachian Mountains. The

current range of HWA is shown in Figure 7. Our

estimates indicate that HWA currently occupies nearly

26 percent of its potential range land area and nearly 25

percent of its potential range in terms of total host

density (Table 2).

Gypsy Moth

The basal area/ha of species preferred by gypsy moth was

estimated (Fig. 8) and then was mulitiplied by the forestdensity map to create a map of host species abundance

adjusted for forest density (Fig. 9). Of the three

disturbance agents, gypsy moth hosts were the most

widely distributed. Also, maximum host densities (> 20

m2

/ha) were much higher than those of BBD and HWA.

The current range of gypsy moth is shown in Figure 10.

Our estimates indicate that gypsy moth currently

occupies only 23 percent of its potential range in land

area and only 26 percent of its potential range in total

host density (Table 2).

Discussion

Planning forest pest management activities can be

improved significantly with reliable mapped estimates of

future disturbances. The methods and maps of host
5

Estimated American beech

basal-area (m /ha) adjusted

for forest density

2

0

0 - 0.2

0.2 - 3

3-7 Figure 3.—Kriged map of estimated basal

area of American beech adjusted for

forest density.



500 0 500 250 Kilometers

Figure 4.—Map of range of beech bark

disease (2003).
6

1,000 0 1,000 500 Kilometers

0

0 - 0.2

0.2 - 3

3 - 10

Estimated hemlock

basal-area (m /ha) 2

Figure 5.—Kriged map of estimated basal

area of hemlock.

1,000 0 1,000 500 Kilometers

0

0 - 0.2

0.2 - 3

3 - 10

Estimated hemlock

basal-area (m /ha)

adjusted for forest density

2

Figure 6.—Kriged map of estimated

basal area of hemlock adjusted for

forest density.
7

abundance described here should aid land managers in

deciding where pest surveys should be conducted and

silvicultural treatments should be located.

BBD has invaded areas with the greatest concentration of

beech, though vast portions of the range of beech remain

uninvaded. That beech is still abundant in many regions

where BBD has already invaded illustrates that while

significant mortality can occur, the disease generally does

not eliminate all beech from stands (Houston 1994;

Morin et al. 2001). Figure 3 highlights eastern Kentucky,

northern Ohio, and northern Indiana as areas with the

highest risk that are currently uninfested.

By contrast, HWA has invaded areas with a large

component of hemlock only recently. Estimated hemlock

abundance was highest in a large portion of New

England, New York, and northern Pennsylvania (Fig. 6).

Gypsy moth is currently occupying less than one-quarter

of its favorable habitat. Areas that have not been invaded

by gypsy moth but have the highest densities of host

species include the Ozark Mountains in Arkansas and

Missouri, central Appalachians in Virginia, West

Virginia, and North Carolina, and Wisconsin and

northern Minnesota (Fig. 9).

A limitation that might affect the accuracy of our

predictions is that while the abundance of host species is

a major factor in determining susceptibility, it is not the

only factor that determines vulnerability of a forest to a

pest. For example, local site characteristics such as

climate and soils (e.g. Houston and Valentine 1977) also

can affect forest susceptibility. Another limitation

concerns the geographic distribution of FIA plots. The

FIA inventory data include sufficient spatial information

with which to summarize these data by spatial units



500 0 500 250 Kilometers

Figure 7.—Map of range of hemlock woolly

adelgid (2003).
8

1,000 0 1,000 500 Kilometers

0

0-2

2-8

8 - 20

Estimated basal-area

(m /ha) of species

preferred by gypsy moth

2

1,000 0 1,000 500 Kilometers

0 - 0.2

0.2 - 2

2-8

8 - 18

Estimated basal-area

(m /ha) of species

preferred by gypsy moth

adjusted for forest density

2

Figure 8.—Kriged map of estimated basal

area of species preferred by gypsy moth.

Figure 9.—Kriged map of estimated

basal area of species preferred by

gypsy moth adjusted for forest density.
9

smaller than counties (Hershey and Reese 1999).

Although more than 93,000 plots were used to

interpolate host density, plot data provide only a coarse

resolution of geographic distribution of host trees. For

example, fine-scale variations in aspect, slope position,

elevation, and soil depth affect the location of a species

within a specific area. The methodologies employed here

thus represents a first approximation of the geographic

distribution of pest risk and these methods can be

elaborated in the future to generate improved estimates

of forest risk to these and other pests.

Acknowledgments

We thank the USDA Forest Service Special Technology

Development Program for funding the project. In

addition, we thank Jim Steinman, Patrick Tobin, Rachel

Reimann, and John Stanovich for reviewing an earlier

draft of this manuscript.

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Printed on Recycled Paper

Morin, Randall S.; Liebhold, Andrew M.; Luzader, Eugene R.; Lister, Andrew J.;

Gottschalk, Kurt W.; Twardus, Daniel B. 2005. Mapping host-species

abundance of three major exotic forest pests. Res. Pap. NE-726. Newtown

Square, PA: U.S. Department of Agriculture, Forest Service, Northeastern

Research Station. 11 p.

Periodically over the last century, forests of the Eastern United States devastated by

invasive pests. We used existing data to predict the geographical extent of future

damage from beech bark disease (BBD), hemlock woolly adelgid (HWA), and gypsy

moth. The distributions of host species of these alien pests were mapped in 1-km2

cells by interpolating host basal area/ha from 93,611 forest-inventory plots in 37

states. The interpolated surfaces were adjusted for forest density (percent land cover)

by multiplying values by an estimate of percent forest cover derived from existing

land-cover maps (30-m2

 cells). According to our estimates, BBD currently occupies

only about 27 percent of its potential range in land area, but has invaded more than

54 percent in total host density. HWA occupies nearly 26 percent of its potential

range in land area, and about one-quarter in total host density. Gypsy moth occupies

only 23 percent of its potential range in the Eastern United States, and only 26

percent in total host density.

Keywords: Beech bark disease; hemlock woolly adelgid; gypsy moth
Headquarters of the Northeastern Research Station is in Newtown Square,

Pennsylvania. Field laboratories are maintained at:

Amherst, Massachusetts, in cooperation with the University of Massachusetts

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

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

Warren, Pennsylvania

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