United States

Department of

Agriculture

Forest

Service

Northern

Research Station

Research Paper 

NRS-18

The Use of Bacillus thuringiensis 

kurstaki for Managing Gypsy 

Moth Populations under the Slow 

the Spread Program, 1996-2010, 

Relative to the Distributional 

Range of Threatened and 

Endangered Species

Laura M. Blackburn

Donna S. Leonard

Patrick C. Tobin
Abstract

The Slow the Spread (STS) Program operates along the expanding population front of the gypsy moth, 

from Minnesota to North Carolina. The primary objective of the program is to eliminate newly-founded 

colonies that form ahead of the leading edge to reduce the gypsy moth’s rate of spread and delay the 

costs associated with infestation and outbreaks. Although the majority of areas under the STS Program are 

treated with control methods specifi c to the gypsy moth, commercial formulations of Bacillus thuringiensis

var. kurstaki (Btk) are the second most used tactic. Bacillus thuringiensis kurstaki can directly affect other 

Lepidoptera, as well as indirectly affect species that depend on Lepidoptera for pollination services or as 

a food source. Because of these nontarget effects, proposed treatment areas are always reviewed by the 

U.S. Department of Interior – Fish and Wildlife Service as well as state agencies that are responsible for 

the conservation of threatened and endangered species to ensure that government programs to control 

gypsy moth are not likely to have an adverse effect. In this report, we used a variety of sources to compile a 

spatial database of the historical distributional ranges of 21 threatened and endangered species that occur 

within the STS management area. We then quantifi ed the area of overlap between areas treated with Btk

under the STS Program from 1996 to 2010 and the distributional ranges of these species to evaluate the 

use of Btk with regard to federal and state management guidelines. The percentage of overlap between the 

distributional ranges for each of the 21 nontarget species was <1 percent in any year, while the cumulative 

percent overlap (1996 to 2010) was generally <3.34 percent. Species with the greatest overlap between 

their respective range and Btk treated areas were most often those species for which distributional data 

were rare. Although Btk can affect nontarget species, its prudent use in combination with the existing review 

process reduces the adverse effects on threatened or endangered species.

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Manuscript received for publication June 2011

Cover:

(A) Eastern prairie fringed orchid (Platanthera leucophaea). 

Paul Pratt, Ojibway Nature Centre (www.ojibway.ca/orchids.htm)

(B) Karner blue butterfl y (Lycaeides melissa samuelis) adult. 

Catherine Herms, The Ohio State University, Bugwood.org

(C) Indiana bat (Myotis sodalis). Jerry A. Payne, 

USDA Agricultural Research Service, Bugwood.org

(D) Unexpected cycnia (Cycnia inopinatus) larva. Sam Houston, 

BugGuide.net

Cover design by Laura Blackburn (USDA Forest Service, Northern Research Station).

The use of trade, fi rm, or corporation names in this publication is for the information and convenience 

of the reader. Such use does not constitute an offi cial endorsement or approval by the U.S. Department 

of Agriculture or the Forest Service of any product or exclude others that may be suitable.
INTRODUCTION

The gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), 

was introduced into North America in 1869 near Boston, Massachusetts 

(Liebhold et al. 1989). Gypsy moth larvae are polyphagous folivores 

that can exploit more than 300 species of deciduous and coniferous host 

trees; oak, aspen, willow, apple, and larch species are among the highly 

preferred hosts (Elkinton and Liebhold 1990, Liebhold et al. 1995). 

Larvae hatch from overwintering egg masses in spring, proceed through 

fi ve (male) or six (female) instars, pupate, and emerge as adults in midsummer. In North America, gypsy moth adult females cannot fl y and 

they emit a sex pheromone to attract males, which are capable of fl ight. 

Since its introduction into North America, the gypsy moth has generally 

spread at mean rates of 6-18 km yr-1 (Tobin et al. 2007) and is considered 

to be established over 863,334 km2

 in North America (Fig. 1, U.S. Code 

of Federal Regulations, Title 7, Chapter III, Section 301.45), an area that 

represents only one-fourth of the habitat considered to be susceptible to 

gypsy moth infestation (Morin et al. 2005).

The Authors

LAURA BLACKBURN, a 

certifi ed geographic information 

systems professional, is an 

operations research analyst 

with the U.S. Forest Service, 

Northern Research Station, 

Morgantown, WV.

DONNA LEONARD is Slow the 

Spread Program Manager for 

the U.S. Forest Service, Forest 

Health Protection, Asheville, 

NC.

PATRICK TOBIN is a research 

entomologist with the U.S. 

Forest Service, Northern 

Research Station, Morgantown, 

WV.

1

Figure 1.—Gypsy moth generally infested area in North America, 2010 (U.S. 

Code of Federal Regulations, Title 7, Chapter III, Section 301.45-3; Canadian 

Food Inspection Agency, Plant Health Division, Policy Directive D-98-09).
2

Management strategies to reduce gypsy moth damage 

and spread fall into one of three programs: (1) 

outbreak suppression in areas where gypsy moth is 

established; (2) eradication in areas where populations 

are not established, such as the west coast of North 

America; and (3) barrier zone management along 

the leading population front in an effort to slow 

the rate of gypsy moth spread and delay the costs 

associated with infestation and outbreaks (Leuschner 

et al. 1996, Sharov and Liebhold 1998a, Sharov et 

al. 2002). Barrier zone management is accomplished 

through the gypsy moth Slow the Spread (STS) 

Program, undertaken jointly by the U.S. Department of 

Agriculture (USDA), Forest Service, and cooperating 

states (Gypsy Moth Slow the Spread Foundation, Inc. 

2011, Tobin and Blackburn 2007). Across all three 

management programs, several treatment options are 

deployed against gypsy moth populations, depending 

on the population density, spatial extent of the target 

population, and management objective. One treatment 

tactic that is used in all three management programs 

is the biopesticide Bacillus thuringiensis var. kurstaki

Berliner (Btk) (Reardon et al. 1994).

Bacillus thuringiensis is a gram positive, naturally 

occurring soil microbe that has been used in 

management programs against multiple insect species 

across several taxa, including Coleoptera, Diptera, 

Hymenoptera, and Lepidoptera (Schnepf et al. 1998). 

More than 360 products manufactured from B. 

thuringiensis spores and toxins or toxins alone are 

registered for use in the United States (Garczynski and 

Siegel 2007). In recent decades, several agricultural 

crops have been genetically modified by insertion of 

B. thuringiensis toxin genes into plants as a means 

of pest control (Gould 1998, Hutchison et al. 2010, 

Shelton et al. 2000). Sporangia of B. thuringiensis

contain a spore and a toxin crystal that must be 

ingested by the insect for infection to occur. The 

toxin produces lesions in the gut cells of a susceptible 

host, usually causing death within 2 days (Broderick 

et al. 2006). Different subspecies and strains of B. 

thuringiensis carry different toxins that are specific 

to different insect hosts; the kurstaki strain is specific 

to Lepidoptera. The use of Btk against gypsy moth 

populations has been studied extensively (reviewed 

by Hajek and Tobin 2010, Solter and Hajek 2009). 

Aerial and ground applications of Btk are targeted 

against early gypsy moth instars, which are the most 

susceptible life stages (Reardon et al. 1994).

Although Btk is more host-specific than chemicalbased insecticides, its application can deleteriously 

affect many species of Lepidoptera (Peacock et 

al. 1998, Wagner et al. 1996), one of the largest 

insect orders, with more than 11,000 species in the 

United States and Canada alone (Borror et al. 1992). 

Studies that have considered the nontarget effects 

of B. thuringiensis, including the use of Btk against 

gypsy moth populations, number in the hundreds 

(see Garczynski and Siegel 2007 for a review). 

Nontarget effects from the use of Btk in gypsy moth 

management programs are broadly considered to be 

direct and indirect. Direct nontarget effects are caused 

by direct exposure to applications of Btk; the toxin 

is ingested and lepidopteran larvae consequently 

die or show sub-lethal effects that often have fitness 

consequences. Indirect effects occur when direct 

effects on some species affect other trophic levels; 

for example, decreases in abundance of Lepidoptera 

can negatively affect plant populations that depend on 

lepidopteran pollinators, as well as species that depend 

on Lepidoptera as a food source.

To ensure that adverse impacts to federally 

protected species are not likely to occur as a result 

of government programs to manage gypsy moth 

populations, proposed treatment areas are reviewed by 

the U.S. Department of the Interior – Fish and Wildlife 

Service as well as state agencies that are responsible 

for the conservation of protected species (U.S. 

Department of Agriculture 1995). The STS Program 

is thus unlikely to use Btk in areas where a threatened 

or endangered species, or critical habitat, is known to 

occur and therefore be impacted by Btk.

In this report, we examined the extent to which Btk

treatments against gypsy moth populations managed 

under the STS Program were applied within the 

ranges of nontarget species from 1996 to 2010 to 

evaluate the use of Btk with regard to federal and state 

management guidelines. We specifically addressed 

threatened and endangered species that are putatively 

both directly and indirectly affected. Although Btk is 
3

used in all three gypsy moth management programs, 

we focused on its use in the STS Program for three 

reasons. First, spatially referenced Btk treatment areas 

are archived by the STS Program and thus are readily 

available from 1996 to 2010, which facilitates a 

robust exploratory analysis through time. Second, Btk

treatments implemented in the STS management area 

encompass a large geographic area (>190,000 km2

) 

across 10 states, from North Carolina to Minnesota 

(Fig. 2), and thus potentially overlap with the 

distributional range of several nontarget species. Third, 

unlike in eradication and suppression programs, Btk

treatments are consistently used in the STS Program 

each year, enabling exploration of year-to-year patterns 

in the use of this biopesticide.

Figure 2.—Area (orange and yellow) managed under the gypsy moth Slow the Spread Program, 2010. The red area is 

considered to be generally infested by gypsy moth, the orange area represents a monitoring zone where trapping is used to 

estimate spread rates, and the yellow area represents the active management zone where treatments are deployed (Tobin 

and Blackburn 2007).

Btk Treatment Areas under the 

Gypsy Moth Slow the Spread Program

The framework, scientifi c rationale, objectives, 

organization, and implementation of the gypsy moth 

STS Program are described in Tobin and Blackburn 

(2007). Briefl y, the STS Program was a pilot program 

from 1992-1999, with the bulk of management 

activities beginning in 1996. In 2000, the program 

became fully operational as a principle component 

of a USDA strategy to manage the gypsy moth. 

The management plan includes the deployment of 

pheromone-baited traps to detect incipient colonies 

that form ahead of the leading edge of the expanding 

gypsy moth population front. Upon detection and 
4

delimitation, one of several treatment tactics is 

used to eliminate smaller populations before they 

expand and coalesce with the expanding front. This 

approach thereby reduces the rate of gypsy moth range 

expansion (Sharov and Liebhold 1998b).

The biopesticide Btk was the most commonly used 

STS tactic from 1996 to 2000, and is currently the 

second most used tactic after mating disruption 

(Roberts et al. 2010, Tobin and Blackburn 2007). 

Mating disruption is the use of products impregnated 

with synthetic pheromone that are aerially applied 

to foliage to chemically interfere with male moths’ 

ability to locate females (Thorpe et al. 2006). At higher 

gypsy moth population densities, mating disruption 

is not effective because some males are still able to 

locate females. In this case, Btk, which is densityindependent, is often used. It is generally applied 

aerially to foliage when early instars are feeding 

(Reardon et al. 1994). The treatment of choice against 

higher density populations is Btk unless the treatment 

area occurs in an environmentally sensitive habitat 

where the gypsy moth-specific nucleopolyhedrovirus, 

LdNPV, commercially produced as Gypchek®

(Reardon et al. 1996), is used. However, LdNPV is 

not easily mass-produced because it must be produced 

in vivo (Hajek and Tobin 2010). Spatially-referenced 

areas treated with Btk are maintained by and available 

from the STS Program (Roberts et al. 2010). The area 

treated annually from 1996 to 2010 is listed in Table 1, 

and the general location of treated areas is presented in 

Figure 3.

METHODS

Compilation of Distributional Ranges 

of Threatened and Endangered 

Species

Records of the historical and current distributional 

ranges of threatened and endangered species that could 

be potentially affected by the use of Btk were obtained 

from a variety of sources, including the gray and 

peer-reviewed literature, websites, universities, field 

guides, and state and federal agencies (Agriculture 

and Agri-food Canada 2009; Bartlett 2010; Bess 

2005; Bowles 1999; Butchkoski 2010; Crosson et 

al. 1999; Cuthrell et al. 1999; Garner and Gardner 

1992; Hilty 2010; Iowa Department of Agriculture 

and Land Stewardship 2010; Johnson et al. 1995; 

Lienk et al. 1991; NatureServe 2010; New York State 

Department of Environmental Conservation 2010; 

North Carolina Natural Heritage Program 2010; 

Opler et al. 2010; Peacock et al. 1998; Penskar and 

Higman 2000; Pruitt and TeWinkel 2007; Pyle 1992; 

Rings et al. 1992; Scott 1986; Sheviak and Bowles 

1986; U.S. Fish and Wildlife Service 2009, 2010; 

University of Alberta 2010; Virginia Department 

of Game and Inland Fisheries 2004; Wagner 2005; 

Wagner et al. 1997, 2001, 2011; Warren et al. 2010). 

We focused specifically on 18 Lepidopteran species 

with larval stages that are reasonably synchronized 

with the larval stage of the gypsy moth and would 

therefore be exposed to Btk. We included species 

that are active as feeding larval stages from April 

to June, the period over which Btk is applied under 

STS. We also considered three species that could 

be indirectly affected through the application of 

Btk: two species of threatened and endangered 

bats that feed on Lepidoptera, and one species of 

a threatened and endangered plant that depends on 

lepidopteran pollinators in the family Sphingidae. 

Table 1.—Area treated with the biopesticide 

Bacillus thuringiensis kurstaki (Btk), and the 

percentage of area treated with Btk relative to the 

entire area treated under the gypsy moth Slow the 

Spread Program, 1996-2010

Year Area treated (km2) Percent of all area treated

1996 97.6 64.2

1997 270.0 86.1

1998 283.7 74.4

1999 216.0 53.3

2000 344.0 47.8

2001 268.7 23.7

2002 142.3 5.9

2003 305.6 11.7

2004 499.9 25.8

2005 439.5 26.2

2006 518.4 17.7

2007 303.0 13.5

2008 219.7 10.9

2009 168.0 24.1

2010 267.4 12.4
5

Figure 3.—Spatial representation of Btk treatment areas (not drawn to scale) deployed under the Slow the Spread Program, 

1996-2010.

All species considered in this report are listed in 

Table 2. The distributional ranges for these species 

should be considered as a conservative estimate for 

several reasons. First, we considered the historical 

distribution of threatened and endangered species, as 

opposed to currently known distributions or critical 

habitats to assess the overlap between applications of 

Btk and ranges. Second, in cases for which specifi c 

point locations of threatened and endangered species 

were available from our sources, we included the 

entire county when compiling distributional ranges. 

Distributional ranges were spatially compiled in 

ArcGIS 9.3 (ESRI, Redlands, CA), and are presented 

for each species in Figure 4.
6

Use of Btk within Ranges of 

Threatened and Endangered Species

For each year, from 1996 to 2010, we considered the 

spatial extent of the area treated using Btk relative 

to the distributional ranges of the threatened and 

endangered species we evaluated to estimate the area 

of overlap for each year and species. We also used a 

more conservative approach to quantify the potential 

overlap between Btk applications and the range of 

each species. In this case, we used a grid of 1 × 1-km 

Table 2.—List of threatened and endangered species considered in this report

Species (common name; Family) Distributional range (km2)

Species Potentially Directly Affected

Atrytone arogos (Boisduval & Leconte) (Arogos skipper; Hesperiidae) 516,215

Boloria selene (Denis & Schiffermüller) (silver-bordered fritillary; Nymphalidae) 757,620

Catocala antinympha (Hübner) (sweetfern underwing; Noctuidae) 8,518a

Catocala gracilis Edwards (graceful underwing; Noctuidae) 9,435a

Cycnia inopinatus (Hy. Edwards) (unexpected cycnia; Arctiidae) 61,619a

Erynnis persius (Scudder) (Persius duskywing; Hesperiidae) 318,948

Fagitana littera (Guenée) (marsh fern moth; Noctuidae) 4,052a

Hesperia metea Scudder (cobweb skipper; Hesperiidae) 645,553

Hesperia ottoe Edwards (ottoe skipper; Hesperiidae) 360,594

Incisalia irus (Godart) (frosted elfin; Lycaenidae) 415,409

Incisalia polios Cook & Watson (hoary elfin; Lycaenidae) 345,730

Lycaeides melissa samuelis Nabokov (Karner blue butterfly; Lycaenidae) 254,479

Lycaena helloides (Boisduval) (purplish copper; Lycaenidae) 566,437

Neonympha mitchellii (French) (Mitchell’s satyr; Nymphalidae) 59,580

Photedes enervata (Guenée) (many-lined cordgrass moth; Noctuidae) 2,703a

Pyrgus wyandot (Edwards) (Appalachian grizzled skipper; Hesperiidae) 228,369

Speyeria idalia (Drury) (regal fritillary; Nymphalidae) 715,220

Tricholita notata Strecker (marked noctuid; Noctuidae) 8,048a

Species Potentially Indirectly Affected

Corynorhinus townsendii virginianus Handley (Virginia big-eared bat; Vespertilionidae) 65,351

Myotis sodalis Miller & Allen (Indiana bat; Vespertilionidae) 558,551

Platanthera leucophaea (Nuttall) (Eastern prairie fringed orchid; Orchidaceae) 351,811

a

Distributional data were rare and uncertain.

cells and selected any cells that overlapped with the 

Btk treatment area. We then considered this enlarged 

area as the Btk treatment area when assessing the 

overlap with the distributional range of each species. 

An example of this process is shown in Figure 5. The 

grid of 1 × 1-km cells buffers the treated areas so that 

the area potentially affected by the application of Btk

would include, for example, application overspray 

or drift.
7

Figure 4.—Known distributional ranges of threatened and endangered species considered in this report. (Figure 4 continues 

on the next page.)
8

Figure 4 (continued).—Known distributional ranges of threatened and endangered species considered in this report. (Figure 4 

continues on the next page.)
9

Figure 4 (continued).—Known distributional ranges of threatened and endangered species considered in this report. 
10

Figure 5.—Two spatial scales were used to quantify the overlap between Btk treatment areas and the distribution ranges of 

threatened and endangered species: the actual treatment area (red), and the treated area plus any 1 × 1-km cells overlapping 

the treatment area (yellow).

RESULTS

The amount of distributional range of each species 

that overlaps areas treated with Btk under STS is listed 

by year in Table 3. Overlap areas vary considerably, 

in part due to the irregularity in the extent of the 

historical distributional ranges for each species 

(Fig. 4). The total amount of overlapping area, from 

1996 to 2010, ranged from 2.6 km2

 (many-lined 

cordgrass moth, Photedes enervata, a species for 

which distribution data were rare and uncertain; see 

Table 2) to 3,946.7 km2

 (cobweb skipper, Hesperia 

metea Scudder). 

Year-to-year trends in the overlap of distributional 

ranges and areas treated with Btk are presented in 

Figure 6. In this case, the area of overlap is presented 

as a proportion of the total known distributional 

range for each species (Table 2). We also included 

the overlapped area when considering the Btk treated 

area expanded over a network of 1 × 1-km cells 

(Fig. 6b). The percentage of overlap in any year, from 

1996 to 2010, and across all species is <1 percent, 

even when the Btk treatment areas are expanded 

(Fig. 6). The overall mean (95-percent confidence 

intervals) of the percent overlap across species and 

years is 0.05 percent (0.04-0.05 percent) for the actual 

treated areas and 0.11 percent (0.10-0.13 percent) for 

the 1 × 1-km scale.
11

Table 3.—Area (km2) of the known distributional range of species overlapping Btk treatment areas deployed under the Slow the Spread ProgramSpecies 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010A. arogos 41.9 121.2 11.4 134.4 221.7 161.2 74.4 219.9 276.4 238.1 112.0 109.3 96.5 103.8 172.7B. selene 57.0 188.3 148.2 173.4 326.2 255.7 123.2 231.1 475.2 556.2 376.9 264.6 204.7 162.4 249.4Catocala antinympha 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 1.2 2.6 0.0 0.0 0.0 0.0 0.0Catocala gracilis 0.0 0.0 0.0 8.1 2.9 0.0 3.3 1.2 2.4 0.3 5.7 3.0 8.4 4.5 0.0Corynorhinus townsenddii virginianus 56.0 22.3 0.0 3.5 71.3 0.0 0.0 0.0 18.0 127.3 86.8 43.0 3.8 0.0 0.0

Cycnia inopinatus 0.0 23.2 88.6 109.2 162.2 153.2 98.0 172.7 286.4 356.9 95.4 84.1 30.2 44.3 48.1E. persius 7.5 175.3 117.0 76.6 144.0 189.8 99.6 170.1 238.2 443.8 278.7 157.2 167.6 129.5 202.8F. littera 0.0 0.0 0.0 8.1 2.9 0.0 3.3 1.2 2.4 0.3 5.7 3.0 8.4 4.5 0.0H. metea 98.9 259.6 148.2 188.6 345.6 258.0 93.0 229.8 446.3 650.3 448.8 255.3 174.9 133.4 216.0H. ottoe 0.0 143.4 141.6 161.1 241.2 171.0 117.0 209.5 355.8 423.1 135.2 120.5 122.4 118.8 174.4I. irus 98.9 142.1 80.1 155.9 313.2 161.6 91.9 280.3 470.1 621.7 270.2 182.9 104.9 111.3 136.7I. polios 50.5 188.3 145.5 133.8 264.3 220.8 104.2 209.0 229.1 256.3 143.3 77.2 162.0 78.9 110.6Lycaeides melissa samuelis 0.0 167.2 110.5 72.4 141.3 151.3 111.2 150.7 139.0 245.2 91.4 92.7 124.7 119.3 161.8Lycaena helloides 0.0 176.1 148.2 169.9 254.9 255.7 117.7 231.1 453.7 505.1 224.9 189.8 204.3 154.3 230.0M. sodalis 76.2 57.6 100.5 75.6 140.2 51.4 39.9 52.4 222.4 445.7 414.0 220.5 66.1 56.3 89.4N. mitchellii 0.0 0.0 0.0 15.3 3.5 0.1 4.0 2.8 20.7 41.9 78.0 28.7 23.3 29.0 20.8Photedes enervata 0.0 0.7 0.0 0.0 0.0 0.0 0.0 0.6 1.2 0.0 0.0 0.0 0.0 0.0 0.0Platanthera leucophaea 1.0 54.5 80.1 130.1 214.8 145.5 79.1 207.2 342.9 466.2 213.3 162.8 63.5 51.2 83.1Pyrgus wyandot 57.0 22.3 0.0 3.5 71.3 0.0 8.0 7.1 21.6 177.5 294.7 87.3 32.4 11.0 25.5S. idalia 57.0 198.4 145.5 169.2 323.6 249.0 123.2 211.8 403.1 653.4 434.5 268.9 147.8 152.4 221.5T. notata 0.0 0.0 0.0 8.1 2.9 0.0 5.7 6.3 3.6 4.3 14.7 8.1 16.5 4.5 8.1
12

0

0.25

0.5

0.75

1

0

0.25

0.5

0.75

1

19961998200020022004200620082010

(A)

(B)

Year

Percent overlap

Figure 6.—The percentage of overlap between each species’ 

known distributional range and (a) the actual area treated 

with Btk and (b) the actual treated area including 1 × 1-km 

overlapping cells for each year, 1996 to 2010. Within each 

year, each symbol represents a species. The general trend 

line over time is shown, and was estimated using locally 

weighted polynomial regression in R (R Development Core 

Team 2011).

(a)

(b)

The cumulative percentage of overlap across all years 

(1996 to 2010) between the historical distributional 

range of each species, and the actual area treated with 

Btk and the area of treatment expanded to a 1 × 1-km 

scale, is shown in Figure 7. The cumulative percentage 

of overlap is <1 percent for the actual treatment area 

and <3.5 percent at the 1 × 1-km scale for all species 

except for Cycnia inopinatus (unexpected cycnia), for 

which distributional data were scarce (Table 2). The 

percentages for this species were 2.84 percent for the 

actual treated area (Fig. 7a) and 6.04 percent for the 

1 × 1-km scale (Fig. 7b).

The relationship between distributional ranges of 

nontarget species and the use of Btk to manage 

gypsy moth populations under the STS Program 

is highlighted for three species: Lycaeides melissa 

samuelis (Karner blue butterfl y, Fig. 8), Cycnia 

(A)

(B)

3

2

1

0

6

4

2

0

Cumulative percent overlap (1996 -2010)

A. arogos

Catocala antinympha

Corynorhinus townsenddii virginianus

E. persius

H. metea

Photedes enervata

I. polios

Lycaena helloides

N. mitchellii

Pyrgus wyandot

B. selene

T. notata

Catocala gracilis

Cycnia inopinatus

F. littera

H. ottoe

I. irus

Lycaeides melissa samuelis

M. sodalis

Platanthera leucophaea

S. idalia

Figure 7.—Cumulative percentage of overlap between each 

species’ known distributional range and (a) the actual area 

treated with Btk and (b) the actual treated area including 

1 x 1-km overlapping cells.

(a)

(b)

inopinatus (unexpected cycnia, Fig. 9), and 

Platanthera leucophaea (eastern prairie fringed 

orchid, Fig. 10). Karner blue butterfl y and eastern 

prairie fringed orchid represent nontarget species that 

generate much concern in the STS Program due to the 

number of treatments in the midwestern United States, 

where these two species are distributed. They also 

represent species that are directly (L. melissa samuelis) 

and indirectly (P. leucophaea) affected by the use of 

Btk. Unexpected cycnia is also highlighted because 

it represents a species with scarce distributional data 

(Table 2) and the highest cumulative area of overlap 

between its range and Btk applications when actual 

treatment areas and areas extrapolated at the 1 × 1-km 

scale are used (Fig. 7).
13

Figure 9.—The distributional range of Cycnia inopinatus and the locations of Btk treatment blocks under the Slow the Spread 

Program, 1996-2010.

Figure 8.—The historical distributional range of Lycaeides melissa samuelis and the locations of Btk treatment blocks under 

the Slow the Spread Program, 1996-2010.
14

Figure 10.—The distributional range of Platanthera leucophaea and the locations of Btk treatment blocks under the Slow the 

Spread Program, 1996-2010.

In all cases, we relied on available information 

regarding the distributional ranges of the threatened 

and endangered species used in this report. Current 

data on the distributional ranges of threatened and 

endangered species are particularly rare, and thus the 

distributional ranges considered here represent the 

historical ranges, which are a conservative estimate of 

the currently known ranges. In the case of L. melissa 

samuelis, however, the Wisconsin Department of 

Natural Resources, in cooperation with the U.S. Fish 

and Wildlife Service and several public and private 

land managers, has developed a Karner blue butterfl y 

Habitat Conservation Plan (Wisconsin Department of 

Natural Resources 2010). As part of this plan, state 

and federal agencies have identifi ed specifi c areas 

for conservation of L. melissa samuelis. This 

managed distributional range in Wisconsin is 

7,527.9 km2

, considerably smaller than the historical 

range of 96,245.5 km2

. Thus, we compared the 

overlap of Btk applications with both the historical 

and currently managed range of L. melissa samuelis

in Wisconsin (Fig. 11) and summarize the area of 

overlap in Table 4. The maximum percent overlap of 

the currently managed range of L. melissa samuelis in 

Wisconsin and Btk treatments for any year from 1996 

to 2010 was 0.46 percent when the actual treated areas 

were used, while the mean across all years was 0.05 

percent. When the treatment areas were expanded to 

the 1×1-km scale, the maximum percent overlap for 

any year was 0.83 percent while the mean across all 

years was 0.12 percent. Thus, the use of Btk in areas 

actively managed for the conservation of Karner blue 

butterfl y in Wisconsin is rare and limited to the edges 

of the conservation area (Fig. 11).
15

Figure 11.—Lycaeides melissa samuelis in Wisconsin: (a) the historical distribution and (b) its currently managed range under 

the Karner blue butterfl y habitat conservation plan (Wisconsin Department of Natural Resources 2010). Areas treated with Btk

under the Slow the Spread Program are shown in both (a) and (b).

(a) (b)

Table 4.—Area of overlap (km2) between actual Btk treated areas or areas when extrapolated to a 1 × 1-km 

scale, with the historical and currently managed distributional range of L. melissa samuelis in Wisconsin

 Actual treated area 1×1-km Scale

Year Historical range Current range Historical range Current range

1996 0.0 0.0 0.0 0.0

1997 167.2 34.5 327.8 62.7

1998 142.2 2.6 282.3 8.1

1999 57.2 1.7 129.5 5.1

2000 136.1 0.0 297.4 0.0

2001 148.6 0.0 316.7 0.0

2002 71.5 0.0 134.6 0.0

2003 133.9 0.0 207.4 0.0

2004 100.9 0.0 229.7 0.0

2005 196.6 0.0 334.9 0.0

2006 29.4 0.0 69.8 0.0

2007 52.8 11.2 117.4 19.2

2008 88.7 7.1 189.2 15.2

2009 87.6 3.4 211.4 18.2

2010 129.9 0.3 294.4 2.0
16

SUMMARY

We found that the percentage of overlap between the 

distributional ranges of 21 nontarget species likely 

to be affected by Btk and the area of Btk treatments 

was <1 percent for all species in any year, whether 

calculations included the actual Btk treated area 

(Fig. 6a) or treated areas expanded to a 1×1-km scale 

(Fig. 6b). Furthermore, the cumulative percent overlap 

from 1996 to 2010 was <1.03 percent for the actual 

treated areas (Fig. 7a) and <3.34 percent at the 

1×1-km scale (Fig. 7b) for all but one species. Species 

with the greatest overlap between their respective 

range and Btk-treated areas, F. littera, T. notata, and 

C. inopinatus (Fig. 7), were most often those for which 

distributional data were particularly rare (Table 2). The 

low cumulative percent overlap of the managed range 

of L. melissa samuelis, for which there is a state-level 

management plan in Wisconsin (Fig. 11), highlights 

the rarity of Btk use in areas that are actively managed 

for species conservation. Moreover, these estimates for 

L. melissa samuelis underscore how conservative the 

estimates are for the other threatened and endangered 

species considered in this report for which only 

historical—as opposed to the currently known—

distributions were available.

The increased use of mating disruption against gypsy 

moth populations managed under the STS Program, 

particularly since 2000 (Table 1), has relegated Btk to 

a secondary tactic. However, Btk will continue to be 

an important tool to manage gypsy moth populations 

in the STS Program because mating disruption is not 

effective against higher density populations (Thorpe et 

al. 2006), and because Gypchek®, which specifi cally 

targets gypsy moth, can be manufactured only in 

limited quantities (Hajek and Tobin 2010). Although 

Btk can affect nontarget species, its prudent use in 

combination with the existing review process reduces 

the adverse effects on threatened or endangered 

species. 

ACKNOWLEDGMENTS

We thank Chris Lettau and Nick Clemens (Wisconsin 

Department of Agriculture, Trade and Consumer 

Protection) for georeferenced data on the Karner blue 

butterfl y Habitat Conservation Plan for Wisconsin. 

We also thank Ksenia Onufrieva (Virginia Polytechnic 

Institute and State University, Department of 

Entomology), Amy Hill (USDA Forest Service, Forest 

Health Protection, Northeastern Area), and Leellen 

Solter (Illinois Natural History Survey) for their 

helpful comments in preparation of this report.

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Bacillus thuringiensis kurstaki for managing gypsy moth populations 

under the Slow the Spread Program, 1996-2010, relative to the 

distributional range of threatened and endangered species. Res. Pap. 

NRS-18. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, 

Northern Research Station. 20 p.

The Slow the Spread Program operates along the expanding population front of 

the gypsy moth, from Minnesota to North Carolina. The primary objective of the 

program is to eliminate newly-founded colonies that form ahead of the leading 

edge to reduce the gypsy moth’s rate of spread and delay the costs associated 

with infestation and outbreaks. Although the majority of areas under the STS 

Program are treated with control methods specifi c to the gypsy moth, commercial 

formulations of Bacillus thuringiensis var. kurstaki (Btk) are the second most used 

tactic. Bacillus thuringiensis kurstaki can directly affect other Lepidoptera, as well 

as indirectly affect species that depend on Lepidoptera for pollination services or 

as a food source. Because of these nontarget effects, proposed treatment areas 

are always reviewed by the U.S. Department of Interior – Fish and Wildlife Service 

as well as state agencies that are responsible for the conservation of threatened 

and endangered species to ensure that government programs to control gypsy 

moth are not likely to have an adverse effect. In this report, we used a variety of 

sources to compile a spatial database of the historical distributional ranges of 21 

threatened and endangered species that occur within the STS management area. 

We then quantifi ed the area of overlap between areas treated with Btk under the 

STS Program from 1996 to 2010 and the distributional ranges of these species to 

evaluate the use of Btk with regard to federal and state management guidelines. 

The percentage of overlap between the distributional ranges for each of the 21 

nontarget species was <1 percent in any year, while the cumulative percent overlap 

(1996 to 2010) was generally <3.34 percent. Species with the greatest overlap 

between their respective range and Btk treated areas were most often those species 

for which distributional data were rare. Although Btk can affect nontarget species, 

its prudent use in combination with the existing review process reduces the adverse 

effects on threatened or endangered species. 

KEY WORDS: Bacillus thuringiensis kurstaki, biological invasions, biopesticide, 

 invasive species management, gypsy moth, Lymantria dispar, 

 nontarget effects, threatened and endangered species

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