cc:

Stephen Owens

James Jones

Vicki Dellarco 

William Jordan

Margie Fehrenbach

Donald Brady

William Diamond

Joan Harrigan-Farrelly

Jack Housenger

Tina Levine

Richard Keigwin

Keith Matthews

Robert McNally

Lois Rossi

Douglas Parsons

Enesta Jones

Vanessa Vu 

Jeannette Martinez

Sheryl Reilly

Alan Reynolds

OPP Docket

FIFRA SAP Members

Steven G. Heeringa, Ph.D. (FIFRA SAP Chair)

Janice E. Chambers, Ph.D., D.A.B.T., Fellow A.T.S.

Kenneth M. Portier, Ph.D.

Daniel Schlenk, Ph.D.

FQPA Science Review Board Members

David Andow, Ph.D.

Fred Gould, Ph.D.

Anthony Ives, Ph.D.

Jeremy Kroemer, Ph.D.

Steven Peck, Ph.D.

Susan Ratcliffe, Ph.D.

Richard Roush, Ph.D.

Mark Sears, Ph.D.

Jon Tollefson, Ph.D.

John Westbrook, Ph.D.
SAP Minutes No. 2011-02

A Set of Scientific Issues Being Considered by the 

Environmental Protection Agency Regarding:

Insect Resistance Management for SmartStaxTM 

Refuge-in-the-Bag, a Plant-Incorporated Protectant 

(PIP) Corn Seed Blend

December 8-9, 2010

FIFRA Scientific Advisory Panel Meeting

held at

One Potomac Yard

Arlington, Virginia
2

NOTICE

These meeting minutes have been written as part of the activities of the Federal Insecticide, 

Fungicide, and Rodenticide Act (FIFRA), Scientific Advisory Panel (SAP). The meeting 

minutes represent the views and recommendations of the FIFRA SAP, not the United States 

Environmental Protection Agency (Agency). The content of the meeting minutes does not 

represent information approved by the Agency. The meeting minutes have not been reviewed 

for approval by the Agency and, hence, the contents of these meeting minutes do not necessarily 

represent the views and policies of the Agency, nor of other agencies in the Executive Branch of 

the Federal government, nor does mention of trade names or commercial products constitute a 

recommendation for use.

The FIFRA SAP is a Federal advisory committee operating in accordance with the Federal 

Advisory Committee Act and established under the provisions of FIFRA as amended by the 

Food Quality Protection Act (FQPA) of 1996. The FIFRA SAP provides advice, information, 

and recommendations to the Agency Administrator on pesticides and pesticide-related issues 

regarding the impact of regulatory actions on health and the environment. The Panel serves as 

the primary scientific peer review mechanism of the Environmental Protection Agency (EPA), 

Office of Pesticide Programs (OPP), and is structured to provide balanced expert assessment of 

pesticide and pesticide-related matters facing the Agency. FQPA Science Review Board 

members serve the FIFRA SAP on an ad hoc basis to assist in reviews conducted by the FIFRA 

SAP. Further information about FIFRA SAP reports and activities can be obtained from its 

website at http://www.epa.gov/scipoly/sap/ or the OPP Docket at (703) 305-5805. Interested 

persons are invited to contact Sharlene R. Matten, Ph.D., SAP Designated Federal Official, via email at matten.sharlene@epa.gov.

In preparing these meeting minutes, the Panel carefully considered all information provided and 

presented by EPA, as well as information presented in public comment. This document 

addresses the information provided and presented by EPA within the structure of the charge.
3

TABLE OF CONTENTS

NOTICE.......................................................................................................................................................2

PARTICIPANTS.........................................................................................................................................5

INTRODUCTION.......................................................................................................................................8

PUBLIC COMMENTERS.......................................................................................................................10

SUMMARY OF PANEL DISCUSSION AND RECOMMENDATIONS ...........................................12

DETAILED RESPONSES TO CHARGE QUESTIONS......................................................................23

PART A: BIOLOGY OF EUROPEAN CORN BORER, SOUTHWESTERN CORN BORER, CORN EARWORM,

AND CORN ROOTWORM ................................................................................................................23

CHARGE QUESTION 1 ..............................................................................................................................23

CHARGE QUESTION 2 ..............................................................................................................................29

CHARGE QUESTION 3 ..............................................................................................................................38

CHARGE QUESTION 4 ..............................................................................................................................40

CHARGE QUESTION 5 ..............................................................................................................................44

CHARGE QUESTION 6 ..............................................................................................................................49

PART B: MODELING OF RESISTANCE EVOLUTION..............................................................................51

CHARGE QUESTION 7 ..............................................................................................................................51

CHARGE QUESTION 8 ..............................................................................................................................65

CHARGE QUESTION 9 ..............................................................................................................................75

ADDITIONAL DISCUSSION .................................................................................................................92

IDENTIFICATION OF RESEARCH NEEDS CONCERNING MAJOR CORN PESTS AND FUTURE USE .................

OF BT CORN PRODUCTS ..........................................................................................................................92

REFERENCES..........................................................................................................................................93

APPENDIX 1. EMPIRICAL METHODS FOR ESTIMATING DOSE AND EFFICACY............106

APPENDIX 2: TWO-LOCUS MODEL OF RESISTANCE EVOLUTION ....................................119

5

Panel Members for the Meeting of the Federal Insecticide, Fungicide and 

Rodenticide Act Scientific Advisory Panel (FIFRA SAP)

to Consider and Review

Scientific Issues Related to Insect Resistance Management for SmartStaxTM 

Refuge-in-the-Bag, a Plant-Incorporated Protectant (PIP) Corn Seed Blend

December 8-9, 2010

PARTICIPANTS

FIFRA SAP Chair

Steven G. Heeringa, Ph.D.

Research Scientist & Director for Statistical Design

University of Michigan Institute for Social Research

Ann Arbor, Michigan

Designated Federal Official

Sharlene R. Matten, Ph.D.

U.S. Environmental Protection Agency

Office of Science Coordination & Policy

FIFRA Scientific Advisory Panel

EPA East Building, MC 7201M

1200 Pennsylvania Avenue, N.W.

Washington, D.C. 20460

Tel: 202-564-8450, Fax: 202-564-8382, matten.sharlene@epa.gov

FIFRA Scientific Advisory Panel Members

Janice E. Chambers, Ph.D., DABT, ATS 

William L. Giles Distinguished Professor 

Director, Center for Environmental Health Sciences 

College of Veterinary Medicine 

Mississippi State University 

Mississippi State, Mississippi

Kenneth M. Portier, Ph.D.

Program Director, Statistics

American Cancer Society

National Home Office

Atlanta, Georgia 

 
6

Daniel Schlenk, Ph.D.

Professor of Aquatic Ecotoxicology & Environmental Toxicology 

Department of Environmental Sciences 

University of California, Riverside 

Riverside, California

 

FQPA Science Review Board Members

David A. Andow, Ph.D. 

Distinguished McKnight University Professor, Insect Ecology

Department of Entomology 

University of Minnesota 

St. Paul, Minnesota

Fred L. Gould, Ph.D. 

William Neal Reynolds Professor of Agriculture

Department of Entomology &

Adjunct Professor

Department of Genetics

North Carolina State University

Raleigh, North Carolina 

Anthony R. Ives, Ph.D. 

Professor

Department of Zoology

University of Wisconsin

Madison, Wisconsin 

Jeremy A. Kroemer, Ph.D. 

Research Entomologist

USDA-ARS, CICGRU

Genetics Laboratory c/o Insectary

Iowa State University

Ames, Iowa 

Steven L. Peck, Ph.D. 

Associate Professor

Department of Integrative Biology

Brigham Young University

Provo, Utah

Susan T. Ratcliffe, Ph.D.

Director, USDA North Central Region Integrated Pest Management Center

Department of Crop Sciences

University of Illinois at Urbana-Champaign

Urbana, Illinois 
7

Richard T. Roush, Ph.D. 

Professor & Dean

Melbourne School of Land & Environment

University of Melbourne

Victoria, Australia 

Mark Sears, Ph.D. 

Professor Emeritus

School of Environmental Sciences

University of Guelph

Guelph, Ontario

Jon J. Tollefson, Ph.D.

Professor

Department of Entomology

Iowa State University

Ames, Iowa 

John K. Westbrook, Ph.D.

Research Meteorologist

Areawide Pest Management Research Unit

USDA, ARS, SPARC

College Station, Texas
8

INTRODUCTION

The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Scientific Advisory Panel 

(SAP) has completed its report of the SAP meeting regarding scientific issues associated with

“Insect Resistance Management for SmartStaxTM Refuge-in-the-Bag, a Bt PlantIncorporated Protectant.” Advance notice of the SAP meeting was published in the Federal 

Register on October 27, 2010. The review was conducted in an open Panel meeting on 

December 8-9, 2010 at One Potomac Yard, Arlington, Virginia. Materials for this meeting are 

available in the Office of Pesticide Programs (OPP) public docket or via Regulations.gov, 

Docket No. EPA-HQ-OPP-2010-0772. Steven Heeringa, Ph.D., chaired the meeting. Sharlene 

Matten, Ph.D., served as the Designated Federal Official. Steven Bradbury, Ph.D., Director, 

Office of Pesticide Programs (OPP), and Keith A. Matthews, J.D., Director, Biopesticides and 

Pollution Prevention Division (BPPD) provided opening remarks at the meeting. Presentations 

of technical background materials were provided by Jeannette Martinez, BPPD and Alan 

Reynolds, BPPD.

The Agency is currently evaluating the SmartStaxTM 1 Refuge-in-the-Bag (RIB) product, a multitrait plant-incorporated protectant (PIP) corn seed blend consisting of a mixture of 95% Bacillus 

thuringiensis (Bt) corn seed and 5% refuge corn seed for insect resistance management (IRM) of 

Ostrinia nubilalis (Hübner) (European corn borer, ECB), Diatraea grandiosella (Dyar) 

(southwestern corn borer, SWCB), and Diabrotica sp. (corn rootworm, CRW), the primary target 

pests. SmartStax RIB was developed jointly by the Monsanto Company and Dow AgroSciences, 

LLC. In July 2009, Monsanto Company and Dow AgroSciences, LLC obtained registrations for 

SmartStax with a 5% reduced structured refuge requirement in the U.S. Corn Belt based on the 

multiple modes of insect control. SmartStax was brought to the market in 2010. In December 

2009, Monsanto and DAS applied for additional registrations of SmartStax to allow a 5% seed 

mixture refuge option (‘Refuge in the Bag’) in the U.S. Corn Belt. 

The focus of this FIFRA SAP was on IRM considerations associated with SmartStax RIB for

control of ECB, SWCB,` and corn rootworm. IRM considerations associated with another Bt

PIP corn seed mixture targeting corn rootworm Optimum® AcreMaxTM12

http://www.epa.gov/scipoly/sap/meetings/2009/february/232009finalreport.pdf

Corn RootwormProtected Corn (Pioneer Hi-Bred) were addressed in the February 2009 FIFRA SAP meeting 

( ).

During a February 1998 FIFRA SAP meeting (see 

http://www.epa.gov/scipoly/sap/meetings/1998/february/finalfeb.pdf), the Panel concluded that 

seed mixtures should not be considered as a viable IRM refuge option for ECB and corn 

earworm in Bt corn. The concern was that ECB larvae can move from plant to plant within corn 

fields (including from refuge plants to Bt plants and vice-versa) which could reduce the 

effectiveness of the seed mixture at preventing pest resistance. 

 1 SmartStax® is a registered trademark of Monsanto Technology LLC. SmartStax (MON 89034 × TC1507 × MON 

88017 × DAS-59122-7) is a combined trait corn product with multiple effective modes of insect control for the key 

above-ground and below-ground corn pests. 2 Optimum® and AcreMaxTM 1 are trademarks of Pioneer Hi-Bred.
9

Subsequent to the 1998 SAP, new biological data and simulation modeling were developed to 

support the potential use of a seed mixture IRM strategy in Bt corn. The Office of Pesticide 

Programs considered these data and utilized the Office of Research and Development’s (ORD) 

Population Genetics (POPGEN) model to evaluate the risk of ECB resistance developing in a 

seed mixture environment. The Agency requested that the FIFRA SAP address scientific issues 

associated with the SmartStax RIB IRM strategy relative to the effectiveness of block refuges 

currently required for lepidopteran pests of Bt corn. 
10

PUBLIC COMMENTERS

Oral statements were presented by:

1) J. Lindsey Flexner, Ph.D., Pioneer Hi-Bred International, Inc. 

2) Graham Head, Ph.D., Monsanto Company

3) Nicholas Storer, Ph.D., Dow AgroSciences, LLC

4) Robert Bowman, past-President of the Iowa Corn Growers Association, National Corn 

Growers Association 

5) Gregory Jaffe, J.D., The Center for Science in the Public Interest 

Written statements were provided by:

1) David Onstad, Ph.D., University of Illinois

2) Mike Caprio, Ph.D., Mississippi State University

3) Bruce Hibbard, Ph.D., United States Department of Agriculture, Agricultural Research 

Service, University of Missouri

4) Lawrent Buschman, Ph.D., Kansas State University

5) James Reed, President, Illinois Corn Growers Association

6) Dean Taylor, President, Iowa Corn Growers Association

7) Gregory Ruehle, CEO, Independent Professional Seed Association

8) Rob Korff, Korff Farms, Inc., Norborne, Missouri

9) Leon Corzine, Corn Grower, Assumption, Illinois

10) Kenneth McCauley, Corn Grower, White Cloud, Kansas

11) Terry Elsbernd, Corn Grower, Decorah, Iowa

12) Randy Schertz, Corn Grower, Eureka Illinois

13) Gary Duffy, President, South Dakota Corn Growers Association

14) Steve Hudson, Corn Grower, Indiana
11

15) Ron Litterer, Corn Grower, Iowa

16) Graham Head, Ph.D., Monsanto Company and Nicholas Storer, Ph.D., Dow AgroSciences, 

LLC

17) Laura Higgins, Ph.D., Pioneer Hi-Bred, a DuPont Business

18) David Morgan, President, Syngenta Seeds, Inc.
12

SUMMARY OF PANEL DISCUSSION AND RECOMMENDATIONS

Overall Panel Summary 

The Panel was requested to consider the durability of the 5% SmartStax seed mixture given the 

available biological, ecological, and genetic information, and IRM modeling. SmartStax corn is 

a multi-toxin double pyramid in which there are three Bt toxins targeting lepidopteran stalkboring (and ear feeding) pests (Cry1A.105, Cry2Ab2, and Cry1Fa) and two Bt toxins targeting 

corn rootworm (Cry34/35Ab1 and Cry3Bb1). The Panel concluded that a 5% SmartStax seed 

mixture would have comparable durability to SmartStax planted with a 5% structured refuge for 

CRW resistance management. However, the Panel stated that resistance management for a 

pyramid should focus on the pest(s) with the greatest likelihood of resistance in a seed mixture

compared to a structured refuge. In this case, the Panel was more concerned about the evolution 

of resistance by the European corn borer (ECB) and southwestern corn borer (SWCB) than corn 

rootworm (CRW) because of the difference in selection intensity to a high-dose versus a lowdose of Bt toxins expressed in SmartStax corn. As stated in the 1998 SAP report, for high-dose 

cases when toxicity of the cultivar causes low survival of heterozygous pest individuals, seed 

mixtures will have lower durability than structured refuges with the same percentage of Bt

plants. This point was illustrated by the modeling exercise performed by the Panel during the 

meeting (see Appendix 2). There are also greater uncertainties for seed mixtures than for 

structured refuges due to the lack of information about larval movement for ECB and SWCB and 

how larval movement affects the survival of heterozygotes. The Panel identified many 

uncertainties associated with the Monsanto, Dow and EPA/ORD modeling efforts and stated the 

models contained assumptions that will lead to overestimates of durability for the 5% SmartStax 

seed mixture. The Panel also considered CEW resistance associated with corn to be a serious 

resistance risk to both corn and cotton, but could not quantify the role of selection associated 

with 5% SmartStax seed mixtures in the Midwest and migration between the Midwest and the 

South on the rate of evolution for Bt resistance in CEW. The overall conclusion of the Panel is 

that the 5% SmartStax seed mixture will be substantially less durable than SmartStax planted 

with a 5% structured refuge for resistance management of the lepidopteran pests, ECB and

SWCB. 

Part A: Biology of European Corn Borer, Southwestern Corn Borer, Corn Earworm, and 

Corn Rootworm 

Charge Question 1 

European corn borer (ECB) has both local and long distance dispersal capability. Currently, the 

proportion and frequency of individuals in a population engaging in dispersal before or after 

mating is unclear. While it has been established that ECB mate in aggregation sites near 

cornfields, mark-release-recapture studies in the U.S. have typically had a low recapture success 

(<1%). Recently it was suggested that 1-day old female ECB may engage in obligate pre-mating 

dispersal (Dorhout et al. 2008). 
13

Please comment on the uncertainties regarding ECB movement including mating sites, premating dispersal, and the proportion of the population engaging in long-distance dispersal. How 

might these aspects of ECB movement affect a potential seed blend strategy?

Panel Response Summary

The Panel concluded that a comparison with empirically fitted parameters cannot be made to 

estimate the effect of seed mixtures on resistance evolution. This is because both known 

biological variation and uncertainty concerning ECB adult dispersal make it a challenge to 

construct a reasonable “worst-case” scenario for between-field refuges. Major sources of 

uncertainty about adult dispersal include: dispersal by susceptible and resistant ECB, sex-specific 

movement, pre-mating vs. post-mating movement, mating biology and sexual selection, wind 

and weather events. Known factors generating variation in dispersal include: sex pheromones, 

humidity, geographic and seasonal variation, diurnal cycle, and age of the adult. 

Theoretical considerations indicate that a seed mixture strategy will increase the rate of 

resistance evolution compared to the present structured refuges. In general, greater male 

movement will tend to delay and greater female movement will tend to speed up the rate of 

resistance evolution, and that for ECB and SWCB, intermediate rates of adult dispersal will have 

the slowest rate of resistance evolution. The effect of a seed mixture would be to increase the 

rate of adult dispersal between Bt and refuge plants, thereby increasing the rate of resistance 

evolution.

The quantitative increase in evolutionary rate can be evaluated theoretically. The Panel 

suggested that to compare the rate of resistance evolution between seed mixtures and structured 

refuges, resistance evolution for the structured refuges should be determined for adult movement 

rates that minimize the rate of resistance evolution. Additional investigations to reduce 

uncertainties are also required and research on mating biology and sexual selection should be 

encouraged.

Charge Question 2

Scientific Advisory Panels (1998 and 2000) discouraged the Agency from the use of Bt seed 

mixtures to control lepidopteran target pests because substantial larval movement could be 

expected between Bt and non-Bt plants which could lead to more rapid selection of resistance. 

BPPD has reviewed new data developed by Dow and Monsanto simulating the effects of 

SmartStax on various instars of potentially mobile Lepidoptera. These data provide evidence that 

SmartStax is highly toxic to 1st, 2nd, and 3rd instars. But, there was greater survivability among 

4th and 5th instars. While it has been established that ECB disperse as neonates, there is some 

uncertainty with respect to lepidopteran propensity for dispersal off non-Bt plants as later 

instars. BPPD notes that simulation models incorporating data on high larval mortality on 

SmartStax plants have (in some cases) predicted that seed blends may be as durable as 

structured refuges.

Please comment on ECB larval plant-to-plant movement including uncertainties about lateinstar movement and the potential effect on the durability of a seed blend strategy.
14

Panel Response Summary

The Panel considered the ecological and evolutionary context of ECB larval movement in 

answering this question. Based on the known behavior and ecology associated with ECB larval 

movement, the Panel developed several plausible ECB larval movement hypotheses that would 

be expected to increase the rate of resistance evolution (by increasing the fitness of 

heterozygotes) via one of the four general larval movement scenarios applicable to many species. 

The uncertainty about late-instar movement has been overstated by Dow and Monsanto as late 

instar ECB exhibit considerable movement. 

The Panel concluded that there were insufficient data to parameterize these specific ECB 

hypotheses and evaluate the four scenarios empirically to quantify the durability of the seed 

mixture strategy. Remaining significant uncertainties include: genotype-specific mortality 

(especially heterozygote mortality) of moving and sedentary larvae, rate (or percent) of larval 

moment, and other aspects of fitness of survivors moving from non-Bt plant to Bt plants and 

vice-versa. 

Because of these uncertainties, the Panel recommended that the ECB larval movement 

hypotheses and the general larval movement scenarios be evaluated using specifically designed 

theoretical evolution models. The Panel examined in detail the structure and results of the 

Monsanto model and the EPA/ORD model, and found that these models did not evaluate or did 

not clearly evaluate any of the four general larval movement scenarios or any of the detailed 

ECB hypotheses that could lead to reduced durability of the seed mixture. The durability of a 

5% SmartStax seed mixture as projected by both the Monsanto and EPA/ORD models is 

overstated, and may be overestimated considerably because these plausible ECB larval 

movement hypotheses have not been considered. 

Charge Question 3

It is typically assumed that, since European corn borer (ECB) and southwestern corn borer 

(SWCB) are similar in many ways, ECB can serve as a surrogate for SWCB to address 

uncertainties regarding biology and genetics. The applicants’ efficacy data, however, suggest 

that SmartStax is somewhat less toxic to older instars of SWCB. Results of a larval exposure 

study by Monsanto showed that SWCB survival was higher than ECB on SmartStax. Should some 

SWCB larvae disperse as older instars, the rate of adaptation to SmartStax could increase in a 

seed blend deployment. BPPD concluded that simulation models should incorporate such 

information in their analyses. There is currently a lack of data on the propensity of SWCB larval 

plant-to plant movement and on how ECB and SWCB differ in this respect, if at all.

Please comment on the assumption that ECB is a suitable biological surrogate for SWCB and 

BPPD’s concerns that a SmartStax seed blend may affect SWCB differently than ECB.
15

Panel Response Summary

The Panel agreed that the results from the Monsanto model and EPA/ORD model are inadequate 

to make any scientifically sound conclusions about resistance evolution in SWCB because they 

do not include density-dependent mortality, which is an important feature in SWCB population 

ecology. However, on the broader issue of whether ECB is a reasonable surrogate for SWCB, 

the Panel expressed different views in the degree to which ECB information could be used for 

SWCB. Some members of the Panel concluded that ECB is a poor surrogate because of the 

many ways the two species differ biologically and therefore different models would have to be 

developed for SWCB. Others suggested that at a structural level the same resistance evolution 

model can be used for the two species, but that the parameter values must be different. 

Charge Question 4

Corn earworm (CEW) was not considered in the applicants’ and EPA/ORD’s analyses for a 5% 

SmartStax seed blend based on the assumption that the insect does not overwinter in the Corn 

Belt where the blend has been proposed. BPPD is concerned, however, that there could be areas 

in the southern portion of the Corn Belt where CEW may be able to successfully overwinter, 

particularly in less severe winters. Such areas may need to be identified because they could 

contribute to increased selection for CEW resistance to Bt corn (including the proposed 5% 

SmartStax seed blend).

Please comment on the assumption that corn earworm does not successfully overwinter in the 

Corn Belt and poses less of a risk for resistance. If CEW can potentially overwinter in parts of 

the Corn Belt, should the insect be considered in the analysis of the proposed 5% SmartStax seed 

blend?

Panel Response Summary

The Panel discussed CEW long-distance migration from the South to the Midwest in the spring, 

CEW reverse migration from the Midwest to the South in the late summer, and whether these 

migratory patterns have an effect on the selection for CEW resistance in the Midwestern Corn 

Belt. There are several studies demonstrating that the major CEW infestations in the Midwest 

are caused by CEW moths that originate in southern states. There is also evidence for reverse 

migration of CEW from the Midwest to southern states. The Panel concluded that recent studies 

demonstrate that moths have evolved sophisticated mechanisms that enable them to move

adaptively from one geographic location to another. 

The Panel considered CEW resistance associated with corn to be a serious resistance risk to both 

corn and cotton. Considering all of the evidence, the Panel could not quantify the role of 

selection associated with 5% SmartStax seed mixtures in the Midwest and migration between the 

Midwest and the South on the rate of evolution for Bt resistance in CEW. Some Panel members 

suggested that high adoption of seed mixtures in the Corn Belt might have an effect on selection 

for CEW resistance on Bt cotton later in the season. In a worst-case scenario, there might not be 

any refuge for CEW within Midwestern corn that is planted as a seed mixture. Other crops and 

wild plants in the Midwest might be good hosts for CEW, but most of these crops are sprayed 
16

with insecticide and would not be good sources for production of susceptible insects. The Panel 

suggested that a detailed survey of alternate crops and more information regarding Bt pollen 

exposure scenarios would be useful.

Charge Question 5

To assess dose expression for corn rootworm (CRW) Bt toxins, the level of survival (adult 

emergence) is typically compared between artificially infested Bt and non-Bt corn plots. 

However, density-dependent mortality in non-Bt plots can potentially confound the comparison 

by reducing overall survival and adult emergence. (Density-dependent mortality is not expected 

in Bt plots due to effects of the toxin on young larvae.) To account for this effect, the dose 

calculation can be adjusted by removing density-dependent mortality from the control plots. This 

effectively increases the dose mortality estimate by raising the number of larvae present in nonBt plots relative to the surviving larvae in Bt plots.

For the SmartStax toxins, Dow/Monsanto made a density-dependent adjustment to their dose 

estimates based on density/survival relationships developed by Onstad et al. (2006). The 

resulting dose mortality profile was: Cry34/35Ab1 (99.75%), Cry3Bb1 (99.75%), and 

Cry34/35Ab1/Cry3Bb1 pyramid (99.95%). On the other hand, BPPD has also considered 

separate work by Hibbard et al. (2010), which suggests that density-dependent mortality occurs 

at higher egg density levels than those assumed by Dow/Monsanto. In light of this research, 

BPPD recommended in its 2009 risk assessment of SmartStax that dose should also be evaluated 

without a density-dependent adjustment. The non-adjusted dose profile for the SmartStax toxin 

is: Cry34/35Ab1 (94.2%), Cry3Bb1 (97.5%), and Cry34/35Ab1/Cry3Bb1 pyramid (98.2%).

Please comment on dose estimates for the SmartStax toxins (Cry34/35Ab1 and Cry3Bb1) 

targeting corn rootworm given the different interpretations of density-dependent mortality.

Panel Response Summary

The Panel agreed that a density-dependent mortality adjustment is merited theoretically; but 

questioned the accuracy and precision of the compensatory adjustment values used in the Dow 

model. The Panel agreed with BPPD’s recommendation in the 2009 risk assessment of 

SmartStax to evaluate dose without a density-dependent adjustment and use the non-adjusted 

dose profile for each Bt toxin: Cry34/35Ab1 (94.2%), Cry3Bb1 (97.5%), and 

Cry34/35Ab1/Cry3Bb1 pyramid (98.2%). 

Density-dependent mortality estimates were calculated from field studies conducted under 

optimal field conditions that the Panel considered unrealistic and therefore the densitydependence mortality was overestimated. The Panel concluded there is no statistically credible 

method to distinguish between 94% and 99% dose mortality because the dose mortality profiles 

are poorly estimated. Both the unadjusted and the density-dependent adjusted dose mortality 

profiles have very high variances. The Panel recommended that the CRW models be rerun using 

the non-adjusted dose profiles (and lower values) and that the re-analysis consider the variance 

of dose mortality profiles using first order Monte Carlo simulations. 
17

Charge Question 6 

Northern and western corn rootworm studies have shown that male emergence in 5% seed 

blends can be variable and may be up to 60 times lower compared to emergence in non-Bt plots 

(Data submitted by Monsanto). This information was not included in any of the models used in 

the SmartStax seed blend analysis. The SAP (2009) concluded that a reduction in the number of 

males from Bt seed blends could have a negative impact on the effective refuge. BPPD is 

concerned with the potentially negative effects a reduction in male emergence might have on 

product durability.

Please comment on the potential effects of lowered male emergence of Northern and Western 

corn rootworm on the durability of the seed blend and whether this information should be 

incorporated into the risk assessment.

Panel Response Summary

The Panel expressed concern for the potential effects of lowered male emergence and 

recommended inclusion of these data in risk assessment. Existing data regarding adult male 

emergence are limited and highly variable. The Panel recommended further studies to gather 

additional data on male and female emergence ratios and consideration of the impacts of beetle 

fitness and emergence timing on mate selection and overall mating. 

Part B: Modeling of Resistance Evolution

Charge Question 7

The durability of the proposed 5% SmartStax seed blend strategy was compared to the durability 

of a 5% structured refuge for lepidopteran and corn rootworm target pests. Monsanto developed 

a deterministic three locus model for ECB/SWCB and Dow created a stochastic two locus model 

for CRW. Separate analyses were conducted using EPA/ORD’s two locus and three locus 

deterministic, probabilistic model to estimate the risk of resistance evolution with a 5% seed 

blend and structured refuge. The applicants and EPA/ORD each made conservative 

assumptions, though of differing degrees, for parameters determined to be sensitive in the 

models. For example, more conservative initial resistance allele frequencies and fitness 

assumptions significantly lowered the time to resistance in EPA/ORD’s model for ECB and 

SWCB. In Monsanto’s modeling of ECB and SWCB, a greater degree of dispersal between 

compliant and non-compliant fields significantly affected the estimated time to resistance.

Please comment on the appropriateness of the assumptions and inputs used for the following 

parameters in the Monsanto, Dow, and EPA/ORD models:

• Initial resistance allele frequency for single traits Cry1A.105, Cry2Ab2, Cry1F, 

Cry34/35Ab1, and Cry3Bb1 for all modeled pests;

• Survival/fitness for all modeled pests; and

• Dispersal for ECB and SWCB as modeled by Monsanto and EPA/ORD. 
18

Panel Response Summary

The Panel pointed out the importance of being mindful of the basic differences in model 

structure among the Monsanto, Dow, and EPA/ORD models because these differences influence 

whether certain parameters are present in the model and the range of potential durabilities that 

can be model outcomes. Assumptions about model structure are in many respects more critical 

than the parameter values themselves. Model structure involves the equations that characterize 

dynamic processes, embody assumptions about causality, and specify the parameters that can be 

quantified. The assumptions and data used to estimate parameter values are subsequent to 

choices about model structure. In relation to the discussion of parameter values specified in the 

Charge Question, the Panel addressed structural issues related to the parameters as well as the 

estimation of the parameter values themselves. An additional discussion about model structure is 

also available in the response to Charge Question 9. 

1) Initial Resistance Allele Frequency for the Single Traits (Cry1A.105, Cry2Ab2, 

Cry1Fa, Cry34/35Ab1 and Cry3Bb1) for all Modeled Pests (all models). The Panel 

concluded that the initial resistance allele frequencies chosen for Cry1A.105, Cry2Ab2, 

and Cry1Fa were appropriate for both ECB and SWCB as “best guesses” in the face of 

limited information. Therefore, some are possibly overestimates and hence may

underestimate durability. For WCR and NCR, the initial resistance allele frequencies 

for Cry34/35Ab1 and Cry3Bb1 were probably underestimates of the actual values and 

probably overestimate durability. For WCR and NCR the Panel suggested that a 

different model structure may be more appropriate for assessing resistance development

because resistance will likely be determined by multiple loci acting with small effects, 

not a single major locus. In this situation a quantitative genetics model might be a 

better model structure. The Panel noted that there is likely to be substantial 

geographical variation in allele frequencies, and therefore initial resistance allele 

frequency should not be treated as a single value for a species. Because resistance 

could develop locally from initially high local allele frequencies and then spread 

geographically, worst-case scenarios must be used that assume high initial allele 

frequencies.

2) Cross-Resistance Potential (Monsanto and EPA/ORD). Cross-resistance is a special 

case of the more general problem of estimating survival/fitness values, and the Panel 

focused on the lepidopteran-active toxins because cross-resistance may greatly reduce 

durability of SmartStax for targeted lepidopterans. In general, there is considerable 

evidence to support hypotheses of cross-resistance, especially between Cry1A.105 and 

Cry1Fa in many insect species. The Panel concluded that there is some evidence of 

partial cross-resistance of Cry1A.105 and Cry2Ab2 with a Cry1Fa resistance allele in 

ECB. The Monsanto and EPA/ORD models assumed that there would be no crossresistance and consistently treated Cry1A.105 as a unique protein involving a novel 

mode of action with no cross-resistance between Cry1A.105 and each of its component 

toxins, Cry1Fa and Cry1Ac. The assumption of no cross-resistance would overestimate 

the durability of SmartStax corn. The Panel agreed that the potential for crossresistance should be considered in any model. 
19

3) Survival/Fitness for all Modeled Pests. The Panel concluded that the fitness values 

used in the ECB, SWCB, WCR, and NCR models were underestimated for the 

heterozygotes. For ECB and SWCB potential epistasis among resistance loci was

insufficiently examined. The Monsanto and EPA/ORD models ignored densitydependent mortality and complex selection associated with corn kernels. In addition, 

the Monsanto model did not model between-plant movement of larvae in a way that 

addresses the risks to resistance evolution. The Panel noted that none of the parameters 

for a quantitative genetics model for WCR or NCR have been estimated. All of these 

factors will overestimate the durability of SmartStax.

4) Dispersal for ECB and SWCB as Modeled by Monsanto and EPA/ORD. The 

Panel concluded that adult dispersal for ECB and SWCB would likely randomly mix 

the adults within fields of SmartStax seed mixtures. Nonetheless, this does not rule out 

the possibility of non-random mating; for example, timing of adult emergence could 

lead to resistant individuals having a greater chance of mating with each other. 

Furthermore, there could be non-random oviposition in seed mixtures; for example, 

damage to non-Bt plants could increase relative oviposition rates on Bt plants. In these 

examples, non-random mating and oviposition would speed resistance evolution. 

Therefore, seed mixtures do not rule out the possibility that non-random mating and 

non-random oviposition decrease durability.

Charge Question 8

EPA/ORD encountered challenges in the lepidopteran modeling with partitioning nonmultiplicative interactions that occurred between more than two resistance genes since the 

mortality caused by each locus was not independent. With two gene pyramids this non-additivity 

can be assigned to the single two locus interaction, but in a three gene pyramid there are three 

possible two locus interactions. In the absence of data, this non-additivity was partitioned 

equally among the three two locus interactions. As more than two Bt genes are pyramided, this 

problem will have to be addressed so that resistance evolution in the target pests to these 

products can be more accurately simulated.

Does the Panel have any recommendations for distributing non-multiplicative interactions in 

models to evaluate multi-gene pyramided products?

Panel Response Summary

The Panel expressed concern with the way in which survivals to different single toxins are 

combined to calculate genotype-specific survival on the pyramided plants in the models of 

resistance evolution. The Panel emphasized that the way in which survival of different 

genotypes are combined has a large impact on the predictions that all models make about the 

durability of pyramided crops. It appeared to the Panel that the Monsanto and EPA/ORD models 

combined survival rates in ways that generated low heterozygote survival, thereby 

overestimating durability of SmartStax in all simulations. Information on the survival of the 

genotypes conferring resistance to one or multiple Bt toxins for the target pests of SmartStax is 

not available, causing large uncertainties in the predictions of any model of resistance evolution. 
20

While theory can suggest ways in which survival to single toxins might be combined to generate 

survival to multiple toxins, empirical information is sparse (Appendix 2: High and Low Dose 

Scenarios and Larval Movement in Seed Mixtures). 

When there is larval movement among plants in seed mixtures, it is necessary to know the 

survival rates of all insect genotypes both before and after movement. Thus, more information is 

needed than just the survival of different genotypes on Bt plants, as emphasized in the previous 

paragraph. The concern from the previous paragraph must be addressed for the two or more 

larval stages before the larvae move and after they move (stage-specific survival rates). In 

addition to generating genotypic survival rates to multiple toxins for each larval stage, the stagespecific survival rates must be combined across stages. These stage-specific survival rates may 

be combined multiplicatively if these survival events associated with each stage are independent. 

All of the models the Panel examined combine the stage-specific survival rates multiplicatively. 

However, while this might be a reasonable assumption in the absence of empirical evidence, 

other possibilities should be explored, and to the knowledge of the Panel, this topic has not been 

investigated theoretically.

Non-multiplicative ways in which the resistance of different loci can combine to determine the 

stage-specific or total immature survival of all genotypes to multiple toxins are forms of 

epistasis. Specific mechanisms by which epistasis can occur include, but are not limited to: 1) 

constitutive, low-level expression of Cry-proteases, 2) developmentally restricted expression of 

low levels of Cry-protease, 3) genes regulating expression of receptor genes, and 4) crossresistance. These and other forms of epistasis should be investigated as mechanisms that cause 

non-multiplicative survival rates of the multiple locus genotypes. The consequences of different 

forms of epistasis on resistance evolution should be evaluated.

Charge Question 9

Based on a review of the submitted simulation modeling, the preliminary conclusions are:

1) For CRW, a 5% seed mixture and a 5% structured refuge had comparable durability in 

both the EPA and Dow models;

2) For ECB, a 5% seed mixture was less durable than a 5% structured refuge in simulations 

with EPA’s model. However, ECB resistance did not evolve within 158 generations in 

any of the simulations with the 5% seed mixture, similar to the level of durability 

predicted by Monsanto’s model. There was no difference in durability between the 5% 

seed mixture and the 5% structured refuge in Monsanto’s model. Resistance did not 

evolve to either refuge option within 100 generations (the extent to which the model was 

run); 

3) For SWCB, a 5% seed mixture was less durable (78 generations) than a 5% structured 

refuge (118 generations) in EPA’s model simulations. Conversely, with Monsanto’s 

model there was no difference in the prediction for durability between the 5% seed

mixture and the 5% structured refuge. Resistance did not evolve to either refuge option 

within 100 generations (the limit of the model simulations).
21

Please comment on the reliability of the estimates of resistance evolution by each of the 

three models in light of the biological and parameter uncertainties identified by BPPD. 

Panel Response Summary

The Panel considered the durability of the 5% SmartStax seed mixture given the available 

biological, ecological, and genetic information, and IRM modeling for ECB, SWCB, and CRW. 

Due to the uncertainties associated with the Monsanto, Dow, and EPA/ORD modeling efforts, 

the Panel concluded that there was an insufficient scientific basis for supporting the SmartStax 

RIB 5% seed mixture as an effective IRM strategy for all pests of concern. For ECB and SWCB, 

the 5% SmartStax seed mixture may lead to more rapid resistance evolution than 5% structured 

refuges. 

SmartStax corn is a multi-toxin double pyramid in which there are three Bt toxins targeting 

lepidopteran stalk-boring (and ear feeding) pests (Cry1A.105, Cry2Ab2, and Cry1Fa) and two Bt 

toxins targeting corn rootworm (Cry34/35Ab1 and Cry3Bb1). The Panel concluded that a 5% 

SmartStax seed mixture would have comparable durability to SmartStax planted with a 5% 

structured refuge for CRW resistance management. However, the Panel stated that resistance 

management for a pyramid should focus on the pest(s) with the greatest likelihood of resistance 

in a seed mixture compared to a structured refuge. In this case, the Panel was more concerned 

for the evolution of resistance by ECB and SWCB than CRW because of the difference in 

selection intensity to a high-dose versus a low-dose of Bt toxins expressed in SmartStax corn. 

As stated in the 1998 EPA SAP report on Bacillus thuringiensis (Bt) Plant-Pesticides and 

Resistance Management (EPA SAP 1998) and in the scientific literature (e.g., Mallet and Porter 

1992; Davis and Onstad 2000), for high-dose cases when toxicity of the cultivar causes low 

survival of heterozygous pest individuals, seed mixtures will have lower durability than 

structured refuges with the same percentage of Bt plants. This point was illustrated by the 

modeling exercise performed during the meeting (see Appendix 2). There are also greater 

uncertainties for seed mixtures than for structured refuges due to the lack of information about 

larval movement for ECB, and SWCB, and how larval movement affects the survival of 

heterozygotes. 

The Panel was also concerned about maintaining susceptible populations if seed mixtures are 

planted over a wide geographical area. This problem is much greater for 5% SmartStax seed 

mixtures than for 5% structured refuges due to the mortality caused when larvae move from nonBt to Bt plants in seed mixtures. Simply, IRM depends on the persistence of susceptible insects, 

and if broad adoption of seed mixtures threatens the regional persistence of the pest, then the risk 

of resistance evolution may increase greatly. 

The Panel concluded that the Monsanto, Dow and EPA/ORD models all contain attributes that 

could lead to overestimates of the durability of the 5% SmartStax seed mixture, especially for 

SWCB. The Panel noted that modeling resistance durability involves high levels of uncertainty, 

and interpreting model results must be done in light of this uncertainty. Several areas of 

uncertainty were highlighted: decision model uncertainty, model completeness, and parameter 

uncertainty. 
22

The Panel recommended four areas that needed further theoretical investigation to assess and 

design IRM strategies for SmartStax seed mixtures. 

1) Modify the Monsanto, Dow, and EPA/ORD models to remove the attributes that likely 

lead to overestimates of durability, or develop new models that more accurately assess 

durability; 

2) Investigate the integration of IRM strategies with adaptive IRM management approaches 

that include well-defined triggers for taking remedial actions and clearly identify the most 

appropriate remedial actions;

3) Model IRM at the regional scale to account for risks of resistance evolution that may 

result from long-term, region-wide suppression of pest population densities; and

4) Investigate the incremental introduction of products such as the 5% SmartStax seed 

mixture, which have high levels of risk uncertainty, especially for lepidopteran pests.

This incremental approach would decrease the percent of non-Bt in a seed mixture based 

on coupling monitoring for resistance and population density. 
23

DETAILED RESPONSES TO CHARGE QUESTIONS

Part A: Biology of European Corn Borer, Southwestern Corn Borer, Corn Earworm, and 

Corn Rootworm

Charge Question 1 

European corn borer (ECB) has both local and long distance dispersal capability. Currently, 

the proportion and frequency of individuals in a population engaging in dispersal before or after 

mating is unclear. While it has been established that ECB mate in aggregation sites near 

cornfields, mark-release-recapture studies in the U.S. have typically had a low recapture success 

(<1%). Recently it was suggested that 1-day old female ECB may engage in obligate pre-mating 

dispersal (Dorhout et al. 2008). 

Please comment on the uncertainties regarding ECB movement including mating sites, premating dispersal, and the proportion of the population engaging in long-distance dispersal. How 

might these aspects of ECB movement affect a potential seed blend strategy?

Panel Response

The Panel concluded that a comparison with empirically fitted parameters cannot be made to 

estimate the effect of seed mixtures on resistance evolution because of the challenges involved in 

filtering through the known causes of variation and sources of uncertainty concerning ECB adult 

dispersal to construct a reasonable “worst-case” scenario for between-field refuges. Major 

sources of uncertainty about adult dispersal include: dispersal by susceptible and resistant ECB, 

sex-specific movement, pre-mating vs. post-mating movement, mating biology and sexual 

selection, wind and weather events. Known factors generating variation in dispersal include: sex 

pheromones, humidity, geographic and seasonal variation, diurnal cycle, and age of the adult. 

Theoretical considerations indicate that a seed mixture strategy will increase the rate of 

resistance evolution compared to the present between-field refuges. In general, greater male 

movement will tend to delay and greater female movement will tend to speed up the rate of 

resistance evolution, and that for ECB and SWCB, intermediate rates of adult dispersal will have 

the slowest rate of resistance evolution. The effect of a seed mixture would be to increase the 

rate of adult dispersal between Bt and refuge plants, thereby increasing the rate of resistance 

evolution.

The quantitative increase in evolutionary rate can be evaluated theoretically. The Panel 

suggested that to compare the rate of resistance evolution between seed mixtures and structured 

refuges, resistance evolution for the structured refuges should be determined for adult movement 

rates that minimize the rate of resistance evolution. Additional investigations to reduce 

uncertainties are also required and research on mating biology and sexual selection should be 

encouraged.
24

Theoretical Expectations 

Models of resistance evolution for high dose-refuge events generally find that the rate of 

evolution is slowest for intermediate levels of adult movement (Comins 1977; Caprio 2001; Ives 

and Andow 2002; Storer et al. 2003; Sisterson et al. 2004). Evolution is fastest when adults do 

not move very much from their natal habitat or when they almost always move from their natal 

habitat, but it is slowest at intermediate rates of leaving. 

The reasons for this are not fully understood, but the most compelling explanation is based on 

the contrasting effect of male and female movement (Ives and Andow 2002; Hu 2008). The 

Panel discussed how simple patch models illustrate the contrasting effects of female and male 

movement out of the natal field and its impact on the rate of resistance. When females move 

more, resistance evolution speeds up. When males move more, resistance evolution is slower. 

Thus, when both sexes move at intermediate rates, a balance is struck between the speeding up 

caused by females and the slowing down of resistance evolution caused by males. Female 

movement speeds up resistance because females produce the offspring that can be selected for 

resistance. Under the high-dose assumption, most of the adults emerge from refuge habitats. 

Females that stay in the refuge will have offspring in the refuge, and these offspring cannot be 

selected for resistance. Females that leave the refuge may return to a refuge, but many will end 

up in a Bt habitat. Offspring produced in the Bt habitat will be selected for resistance. Hence the 

more females move, the greater the proportion of offspring that will be selected for resistance, 

and the faster the rate of evolution. 

Male movement slows down resistance because males disrupt RR homozygote genotypes in Bt

fields. Under the high dose assumption, most of the (few) adult RR genotypes will occur in the 

Bt fields. As indicated previously, most males will emerge from refuges. Males emerging from 

refuges will typically be SS genotypes. When these males leave, many will end up in Bt fields. 

These males compete with the (few) RR males emerging from the Bt fields for the (few) RR 

females. If there are sufficient males from the refuge and they have equal or superior intrasexual selection coefficients, then most offspring produced from RR females will be RS 

heterozygotes, which are susceptible to Bt. 

At extremely low rates of adult movement these relationships break down, and resistance 

evolution becomes very fast. This happens because there is so little movement that there is 

virtually no dispersal between the refuge and Bt habitats. Thus, evolution in the Bt habitats 

occurs as if there was no refuge.

In contrast to patch models, spatially-explicit models include movement distance. In other 

words, movement distance is ignored in patch models, and these models assume that adults 

disperse far enough that they can reach any field type in proportion to the area of the field type. 

Spatially-explicit models produce results somewhat different from patch models when all adults 

have highly restricted movement distances, such as movement only to the adjacent field (nearest 

neighbor dispersal). Additional theoretical results with these spatially-explicit models 

demonstrate that if only 2% of the adults move a long distance (and all others move only to the

nearest neighbor dispersal), results are identical to those obtained from the patch models. In 

other words, if only a small proportion of ECB and SWCB adults move several kilometers, then 
25

movement distance is an insignificant factor in resistance evolution, and can be ignored. This 

will be addressed in the next section of the response to this question.

One of the key assumptions for the high-dose refuge strategy was that there must be random 

mating for IRM to be successful. This is not entirely true. A distinction must be made between 

local mating and global mating. Local mating occurs in a specified area that is a part of the 

geographic extent of the whole population. Such an area could be a crop field for ECB or 

SWCB. Global mating occurs over the entire geographic extent of the population, and comprises 

many local areas (crop fields). 

Nearly all of the IRM models assume that there is local random mating and this is a critical 

assumption. If there is local (non-random) assortative mating, resistance evolution can be quite 

fast. Local random mating means that in the local environment (crop field), each female mates 

with any of the locally available males with equal probability, and each male mates with any of 

the locally available females with equal probability. Obviously mate choice and sexual selection 

violate local random mating and could generate novel evolutionary dynamics

Global non-random mating is typically associated with slower rates of resistance evolution than 

global random mating, which is a counter-intuitive result. Global random mating means that 

each female mates with any male in the entire population with equal probability and each male 

mates with any female in the entire population with equal probability. This can occur if all 

adults disperse from their natal sites and settle randomly in fields (including their original natal 

field) before mating. If some adults do not disperse from their natal sites, there will be global 

non-random mating. When there is some global non-random mating, there will be some 

assortative mating among fields, with RR x RR matings more likely in Bt fields than in the 

population at random. This should result in faster resistance evolution. However, because 

intermediate levels of movement produce the slowest rates of resistance evolution and some 

global non-random mating must be associated with intermediate levels of movement, the effect 

on resistance evolution will be determined by the balance between these forces (intermediate 

dispersal slowing it down and global non-random mating speeding it up). Ives and Andow 

(2002) showed that for high-dose events, the effect of global non-random mating is much weaker 

than the effect of intermediate levels of dispersal, and in all cases, the effect of global nonrandom mating had insignificant effects on the rate of resistance evolution. Thus, some may 

(wrongly) conclude that any factor (e.g., seed mixes) that increases dispersal to 100% will reduce 

the rate of resistance evolution because global mating becomes random. In actuality, the 

opposite is true, and seed mixes are expected to speed up resistance evolution compared to 

refuges in different fields. 

Movement Distances of ECB and SWCB

In general, Panel members agreed that sufficient ECB and SWCB disperse far enough that 

spatially-explicit models were not necessary for understanding resistance evolution for high 

dose-refuge systems. This suggests that for these two species, intermediate rates of adult 

dispersal will have the slowest rate of resistance evolution. If there are sex-specific differences 

in adult movement, high male dispersal and low female dispersal would delay resistance 
26

evolution the most. Conversely, low male dispersal and high female dispersal would result in the 

fastest resistance evolution.

The Panel discussed research studies concerning dispersal distances of ECB and SWCB. 

Research on dispersal of ECB provides somewhat conflicting evidence related to the spatial scale 

of movement. Chiang (1972) showed that when ECB first invaded Minnesota, it spread across 

the state at a rate of 100 km/yr, implying a high rate of dispersal of 50 km per generation. 

Showers (1993) examined the spread of a persistently introduced deleterious genotype, and 

found that it occurred as far away as 32 km from the release area. Chiang et al. (1965) found that 

during the spring large numbers of adults migrate from central Iowa to southern Minnesota, a 

distance of 150 km. These dispersal estimates indicate that a small proportion of adults disperse 

long distances. This long distance movement may be insufficient to delay resistance evolution 

on its own, but it is sufficient to imply that patch models are adequate for understanding 

resistance evolution and explicitly spatial models are not essential. 

Mark release recapture (MRR) studies by a number of researchers (Legg 1983; Showers et al. 

2001; Bailey et al. 2007; Reardon et al. 2006) have been used as evidence for long-distance 

dispersal of ECB. The results of these studies indicated that the actual distances marked 

individuals moved were typically much less than 7 km. Based on many studies concerning low 

recapture rates of marked individuals, it can be inferred that low recapture rates are common.

However, the reported rates of recapture for ECB are not low compared to other insect species. 

Many factors influence recapture rates. For example, capturing or rearing individuals, handling 

and marking them, and release procedures may have considerable influence on the subsequent 

behavior of marked individuals such that their normal behavioral sequence is disrupted and their 

recapture probability is reduced. Thus, low recapture rates do not indicate that long-range 

dispersal occurred. MRR results are consistent with long-range dispersal of ECB, but the 

observations cited in the previous paragraph provide stronger evidence for small amounts of 

long-range dispersal of ECB.

Compared to ECB, there is considerably less research on movement of SWCB. Where the 

species co-exist, there are anecdotal observations that SWCB is equal to or less dispersive than 

ECB (Guse et al. 2002; McCauley et al. 1995). SWCB is also an invasive species, originating in 

Mexico. It spread eastward at rates between 20-55 km/yr (Fairchild et al. 1965) or 10-27 km/ 

generation, which is considerably slower than ECB. Additional evidence of long-range dispersal 

comes from pheromone traps, which are highly attractive to males from considerable distances 

(Goodenough et al. 1989). Thus, it is likely that there is sufficient long distance dispersal that 

spatially-explicit models are not essential for understanding resistance evolution in this species.

Rates of Evolution for Within-Field vs. Structured Refuges

Compared to between-field refuges, the effect of within-field refuges is to increase the dispersal 

rates of adults from both Bt and non-Bt habitats. This is because the probability of leaving a 

habitat is inversely related to the size of the habitat. A within-field refuge is essentially a fine 

scale mixture of very small units of Bt and non-Bt habitat. Block refuges within fields would 

require adults to move across a field before they had dispersed from their natal sites (perhaps as 

much as 1 km). Strip refuges would require adults to move a few rows before they had dispersed 
27

from their natal sites (perhaps as much as 100 meters). Seed mixtures would require adults to 

move only a few meters before they had dispersed from their natal sites (perhaps less than 10 

meters). Female ECB always move more than 10 meters after eclosion and before mating and 

oviposition, so for seed-mixtures, dispersal rates will be 100%, and greater than for between field 

refuges. 

The main consequence is that seed mixtures will have faster resistance evolution than between 

field refuges, ceteris paribus.

How Much Faster is Resistance Evolution?

The Panel concluded that no direct empirical comparison can be made to estimate the effect of 

seed mixtures on resistance evolution because of the challenges involved in filtering through the 

known causes of variation and sources of uncertainty concerning ECB adult dispersal (see lists 

below) to construct a reasonable “worst-case” scenario for between-field refuges. Lacking this 

information, the Panel suggested that the relative rate of resistance evolution between seed 

mixtures and between-field refuges could be estimated by specifying intermediate adult 

movement rates for between-field refuges that minimize the rate of resistance evolution and then 

comparing the projected evolutionary rates to those of 100% adult dispersal (seed mixtures).

Although this worst-case scenario is equivalent to a case where Bt and non-Bt crops are planted 

in the same fields every year (not rotated), in the absence of additional information, it is an 

appropriate baseline comparison. First, even when fields are rotated, there are two sequential 

generations each year without rotation. Thus, the worst-case scenario is at least half realistic. 

Second, when fields are rotated there may still be spatial autocorrelation with fields near a 

previous Bt field more likely to be Bt the following year. This spatial autocorrelation will give 

lower dispersal between field types. Finally, the appropriate physical scale of the relevant spatial 

autocorrelation is not known. This will depend quantitatively on the adult dispersal kernels for 

males and females. These dispersal kernels will be costly to acquire, so investigating a worstcase scenario may alleviate the need for additional expensive experiments. 

Factors Generating Variation in Dispersal

1) Sex Pheromone. Multiple males can be attracted from hundreds of meters away to 

single calling females (based on MRR data and dispersal from known sources). 

Pheromone sources within 80-90 meters of each other probably compete for males.

2) Humidity. An ECB adult that finds itself in a habitat with low relative humidity (RH)

will leave that habitat in search for a habitat with higher RH. This movement probably 

occurs during the night or early morning because during the day, it is difficult to find 

adults in drier habitats (during the day they are in humid, so-called aggregation sites). 

Adults have been observed to disperse hundreds of meters to locate a suitable moist 

habitat to stay in during the day.

3) Geographic and Seasonal Variation. Known geographical and seasonal differences 

in adult dispersal have been linked to variation in daytime RH in the habitats in the 
28

landscape. There are probably other factors causing geographic and seasonal variation 

in dispersal, but these are poorly known. For example, there is a rainfall gradient that 

results in wetter summers in the eastern part of the North American range of ECB and 

drier summers in the western part of the range. In addition, the northeastern Corn Belt

is wetter than the southwestern Corn Belt. Nebraska, Kansas and South Dakota and 

parts of Iowa are dry and ECB is typically found during the day only in the so-called 

aggregation sites throughout the entire growing season. There is some evidence that 

ECB will move to the irrigated parts of fields in these dry areas. ECB are typically 

found in daytime aggregation sites during the first generation throughout its geographic 

range. This is because corn is too short to provide sufficient moisture during the day. It 

is not clear how far adults will move to find aggregation sites, i.e., do they pass over 

suitable sites and disperse long distances or do they tend to go the closest site. 

Dispersal of later generations appears to vary considerably by geographic region. For 

example, in Pennsylvania, North Carolina, and Maryland adults are typically found in 

corn fields during the day. This is probably also true for Minnesota. At these locations, 

the RH inside corn fields during the day can be quite high providing a suitable daytime 

habitat for ECB. 

4) Diurnal Cycle. Adults can be observed dispersing from daytime resting sites at dusk. 

During the early part of the evening, females have been observed drinking water and 

ovipositing. Ovipositing females lay 1-3 egg masses in a night. Later in the night, 

often starting around midnight, females will release sex pheromone to call for males. It 

is not known how frequently an individual will call in her lifetime, and it is not known 

if the same individual will oviposit and call in the same night. Sometime later and as 

necessary, adults will seek high humidity habitats to stay during the day. While mating 

has been observed in the aggregation sites, it is not clear where most of the mating 

occurs.

5) Age of Adult. Adults emerge from pupae either during the early evening (typically 

before midnight) or the early morning (typically after dawn). About 50% emerge at 

each of these times, and the sex ratio of emerging adults is similar at both times. Males 

emerge several days before females. After emergence, adults spend about 2 hours 

hardening up their cuticle and wings. Females will disperse from their emergence site 

typically after midnight and do not release pheromone prior to this dispersal (for night 

emerging moths). Day emerging moths presumably disperse soon after hardening their 

wings, but this is not clearly known. Dorhout et al. (2008) suggested that it is possible 

that ECB has an obligate pre-mating long-distance dispersal. Flight mill data tend to 

over-estimate willingness to fly, flight durations and flight distances; therefore, 

interpretation of these data should be confirmed using independent means. 

Major Sources of Uncertainty about Adult Dispersal

1) Dispersal by Susceptible and Resistant ECB. There is no information whether 

susceptible and resistant ECB emerging from non-Bt plants would exhibit similar 

dispersal, pheromone calling and response, and mating behavior. Until resistant ECB 

are found, it will not be possible to evaluate this.
29

2) Sex-Specific Movement. Rather little data exists on sex-specific movement of ECB. 

Although some data may be said to indicate that males are more dispersive than 

females, a sounder conclusion is that the two sexes are equally dispersive, and perhaps 

males are more dispersive than females. Because sex-specific dispersal is likely to 

depend on the environment, such as relative male and female density, and nearby food 

and oviposition resources, it may take some effort to understand the conditions under 

which sex-specific dispersal could disrupt resistance management.

3) Pre-mating vs. Post-Mating Movement. Relatively little theoretical work has been 

done on the influence of pre- and post-mating movement, so there are no clear 

expectations for how these affect resistance evolution. Some recent theoretical 

investigations suggest that there is little difference in the effect of pre- and post-mating 

female movement, but other investigations have reported significant effects. The 

reasons for the different theoretical results are not yet clear. However, little data are 

available to characterize pre- and post-mating dispersal in ECB. It is clear, however, 

that both sexes disperse before mating and females disperse after mating. The relative 

movement rates have not been quantified.

4) Mating Biology and Sexual Selection. Although technically not a dispersal factor, the 

interaction between mate selection, dispersal and survival selection may have 

significant effects on the evolution of resistance. Sexual selection (including intrasexual and inter-sexual selection, also known as mate competition and epigamic 

selection) is known to have significant effects on evolution. In the resistance evolution 

literature, mating has been incorporated by assumed random or non-random mating. 

However, a behavioral evolution perspective has been missing. This perspective can 

generate conditional assortative and disassortative mating that is not possible to 

incorporate simply through assumptions of non-random mating. 

5) Wind and Dispersal. Sustained periods of prevailing winds and acute wind events 

(e.g., passage of cold fronts) might disperse ECB and SWCB long distances. Although 

the dispersal of ECB in the spring from Iowa to Minnesota probably was assisted by the 

warm prevailing spring winds (Chiang et al. 1965), the role of winds in long distance 

dispersal is poorly understood, and its potential influence on resistance evolution 

remains to be evaluated. 

Charge Question 2

Scientific Advisory Panels (1998 and 2000) discouraged the Agency from the use of Bt seed 

mixtures to control lepidopteran target pests because substantial larval movement could be 

expected between Bt and non-Bt plants which could lead to more rapid selection of resistance. 

BPPD has reviewed new data developed by Dow and Monsanto simulating the effects of 

SmartStax on various instars of potentially mobile Lepidoptera. These data provide evidence that 

SmartStax is highly toxic to 1st, 2nd, and 3rd instars. But, there was greater survivability among 

4th and 5th instars. While it has been established that ECB disperse as neonates, there is some 

uncertainty with respect to lepidopteran propensity for dispersal off non-Bt plants as later 
30

instars. BPPD notes that simulation models incorporating data on high larval mortality on 

SmartStax plants have (in some cases) predicted that seed blends may be as durable as 

structured refuges.

Please comment on ECB larval plant-to-plant movement including uncertainties about lateinstar movement and the potential effect on the durability of a seed blend strategy.

Panel Response

The Panel considered the ecological and evolutionary context of ECB larval movement in 

answering this question. Based on the known behavior and ecology associated with ECB larval 

movement, the Panel developed several plausible ECB larval movement hypotheses that would 

be expected to increase the rate of resistance evolution (by increasing the fitness of 

heterozygotes) via one of the four general larval movement scenarios applicable to many species. 

The uncertainty about late-instar movement has been overstated by Dow and Monsanto as late 

instar ECB exhibit considerable movement. 

The Panel concluded that there were insufficient data to parameterize these specific ECB 

hypotheses and evaluate the four scenarios empirically to quantify the durability of the seed 

mixture strategy. Remaining significant uncertainties include: genotype-specific mortality 

(especially heterozygote mortality) of moving and sedentary larvae, rate (or percent) of larval 

moment, and other aspects of fitness of survivors moving from non-Bt plant to Bt plants and vice 

versa. 

Because of these uncertainties, the Panel recommended that the ECB larval movement 

hypotheses and the general larval movement scenarios be evaluated using specifically designed 

theoretical evolution models. The Panel examined in detail the structure and results of the 

Monsanto model and the EPA/ORD model, and found that these models did not evaluate or did 

not clearly evaluate any of the four general larval movement scenarios or any of the detailed 

ECB hypotheses that could lead to reduced durability of the seed mixture. The durability of a 

5% SmartStax seed mixture as projected by both the Monsanto and EPA/ORD models is 

overstated, and may be overestimated considerably because these plausible ECB larval 

movement hypotheses have not been considered. 

General Larval Movement Scenarios Applicable to Many Species

A seed mix may cause faster resistance evolution if larvae move among plants in such a way that 

RS heterozygotes have higher survival than when they are restricted to feeding only on Bt plants. 

This increases the heterozygosity of resistance so that resistance is less recessive (or more 

dominant). Four larval movement scenarios are identified that could increase heterozygosity and 

thereby speed up resistance evolution. These four scenarios are not mutually exclusive, but they 

could be readily incorporated into resistance evolution models, such as the EPA/ORD model.

1) The BNI Scenario. This scenario proceeds as follows: the RS heterozygote hatches on a 

Bt plant, feeds a little bit, but before it dies, it moves to a non-Bt plant, where it completes 

development. Individual probability of survival of an RS heterozygote must be higher 
31

than for an SS susceptible larva undergoing the same scenario. With a higher survival, 

heterozygosity increases. This is labeled the BNI scenario because a larva moves from a 

Bt plant to non-Bt plant with a difference in individual probability of survival. 

2) The NBI Scenario. This scenario proceeds as follows: the RS heterozygote spends its 

early life on a non-Bt plant, and late in life it moves to a Bt plant, where it completes 

development. Individual probability of survival of an RS heterozygote must be higher 

than for an SS susceptible larva undergoing the same scenario. With a higher survival 

probability, heterozygosity increases. This is labeled the NBI scenario because a larva 

moves from a non-Bt plant to a Bt plant with a difference in individual probability of 

survival. 

3) The BNP and 4) NBP Scenarios. Two additional scenarios focus on average population 

survival rates under the assumption that there is no difference in individual survival 

probability between RS and SS larvae. The BNP scenario proceeds as follows: the RS 

heterozygote larvae have a greater propensity to move from a Bt plant to a non-Bt plant 

than the SS susceptible larvae. Thus even though the RS and SS larvae that move have 

the same individual survival rate, because more RS larvae move, the average survival rate 

of the RS larvae is higher than the average survival rate of the SS larvae. This is because 

the average is taken over both the larvae that move and the larvae that do not move. 

Hence, and heterozygosity increases. This is labeled the BNP scenario because a larva 

moves from a Bt plant to non-Bt plant with a difference in average population survival. 

Analogously, the NBP scenario requires that the probability of moving from a non-Bt

plant to a Bt plant is lower for a late instar RS heterozygote than an SS larva. In this case 

a greater proportion of SS larvae become exposed to a Bt plant, which reduces their 

average survival compared to the RS heterozygote larvae, and heterozygosity increases. 

This is labeled the NBP scenario because a larva moves from a non-Bt plant to a Bt plant 

with a difference in average population survival.

Other General Larval Movement Scenarios

There are also many scenarios by which larval movement may have limited effect on resistance 

evolution (Glaum et al. 2011). Movement of SS larvae from Bt plants can reduce the selective 

pressure on the population, sufficiently to counteract the effects of increased heterozygosity. In 

addition, genotype specific feeding behavior can equalize the probability of survival of RS and 

SS larvae. For example even if the RS heterozygote has higher individual probability of survival 

than an SS larva on a fixed dose of Cry toxin, the RS heterozygote may consume more Cry toxin, 

resulting in a higher dose that causes the same mortality as the smaller amount consumed by the 

SS larva.

Larval Movement: Empirical Observations

The Panel made several general points concerning larval movement, and specific points 

concerning ECB movement that are relevant to the four general modeling scenarios described 

above.
32

1) Movement by neonate larvae will occur from both Bt and non-Bt plants. 

2) Movement by late instars will be primarily from non-Bt to Bt plants. This is because 

early stages will have already died on or dispersed from the Bt plants.

3) For ECB and SWCB, density-dependent dispersal and density-dependent mortality may 

occur on non-Bt plants, but are unlikely to occur on Bt plants. The number of eggs in a 

typical egg mass laid on a single plant is greater than the number of larvae that can 

mature to adults on a plant. These density-dependent effects may mitigate the effect of 

the BNI and BNP scenarios if enough larvae move to non-Bt plants to trigger these 

effects, although significant mitigation is probably unlikely.

4) There is considerable larval movement of ECB on a plant after the 3rd instar (White and 

Andow 2007; Schmidt et al. unpublished data). In general, movement appears to be 

associated with molting.

5) Plant morphology and larval food resources differ substantially between the first and 

second generation on corn. The first generation is associated with whorl-stage corn, and 

larvae can feed on young leaves and young stalks. The second generation is associated 

with anthesis, silking and ear development. During the second generation, larvae have 

available many food resources, including young leaf blades (in tillers), mature leaf 

midribs, collars and sheaths, young and mature stalk, tassel, stamens, pollen, ear buds and 

developing ears, including, silks, kernels, husks, and cobs.

6) The results presented in the docket suggest survival of susceptible 4th and 5th instars of 

ECB and SWCB is high when they moved from non-Bt to Bt plants. The results for ECB 

(RPN-09-075, Head et al. 2009) may underestimate this effect because control larvae had 

a low rate of survival (42%). This compresses the range of possible survival of treatment 

larvae to 0-42%, which increases type 2 statistical error. In addition, the low control 

survival may be indicative of a weak ECB colony (e.g., pathogen infected). Weakened 

larvae might be more susceptible to the experimental treatments, resulting in artificially 

low survival. The SWCB dataset may be more reliable because of the high survival rate

of control larvae (86%). Thus, some larval stage after the 1st instar is likely to fit the NBI 

and NBP scenarios.

7) ECB overwinters as diapausing 5th instar larvae in corn stalks. Pre-diapausal activities 

are indicated by cessation of feeding, voiding the gut and initiating a wandering stage. 

After they wander they locate a suitable overwintering site (1st generation ECB locate a 

suitable pupation site after the wandering stage) and become quiescent. Morphologically, 

diapausing larvae have large fat bodies and appear plump and whiter especially along the 

dorsal midline. It is possible that a significant portion of 5th instar larvae found surviving 

on Bt plants at the end of the year are in diapause and not feeding. Although their present 

and future exposure to Bt toxin will be virtually nil, each larva does choose a suitable 

overwintering site in the corn stalk and passes this stalk material through its gut. Thus, 

all diapausing larvae found in Bt plants must be able to survive consumption of some 

level of Bt toxin at the end of the season. In the spring, overwintered larvae become 
33

active. Most must feed on water, and some proportion will feed on the old corn tissue 

before pupating. Relative survival of RR, RS and SS diapausing larvae on Bt and non-Bt

plants and tissues is not known. In a noctuid pest of cotton (Trichoplusia ni), >50% of 

susceptible 5th instars survived to pupate and ~88% of these pupae eclosed whether the

5th instars were starved or exposed to Cry1Ac/Cry2Ab cotton tissue (Li et al. 2007). The 

role of movement of diapausing larvae of resistance evolution is not well understood.

Detailed ECB Movement Hypotheses: First Generation Larvae

The Panel described how the movement of first-generation larvae may speed up the evolution of 

resistance by fitting one of the four scenarios described above. There are a few circumstances 

under which known aspects of ECB larval movement would likely result in faster resistance 

evolution.

Egg masses are typically laid on the undersurface of expanded leaf blades, between the arch and 

the collar. When eggs hatch, larvae hang out near the egg mass to harden cuticle. Some larvae 

may cannibalize or eat the unhatched eggs or remaining egg chorion. Some larvae commence 

feeding, probably on leaf hairs, while others start to walk either up or down the undersurface of 

the leaf blade (most of the behavioral observations described here and subsequent sections are 

reported in Lamb 1992). Larvae prefer the hairless undersurface to the richly pubescent upper 

surface of the leaves because the hairs interfere with their walking. If a larva ends up on the 

upper surface of a leaf blade, it will tend to migrate to the hairless midrib, where it can quickly 

move. About half of the larvae move up the blade and the other half move down the blade. 

Those moving down the leaf blade end up crawling into the leaf whorl, where they may initiate 

feeding. Most larvae will move around inside the whorl, but it is not known how many will 

leave the whorl for another plant. It is likely that larvae are relatively sedentary during this 

period, and the BNI and BNP scenarios are not likely to be initiated at this point in larval 

development. Typically, larvae feed in the whorl through the 3rd instar. Most later instars move 

out of the whorl, in some cases because the whorl disappears as the plant develops, and these 4th

and 5th instars bore into the young stalk. Larvae may move between plants at this time, but rates 

of movement are not known. If larvae move between plants at this time, the NBI and NBP 

scenarios could occur.

The neonates moving up the blade end up at the leaf tip, and there they silk off, and dangle to be 

dispersed in the wind. A significant fraction of these will land on a lower leaf on the same plant. 

In humid environments, larvae can walk for more than 12 hours without feeding. More larvae 

will move up and down rows than across rows (Ross and Ostlie 1990), but when considering the 

distance moved across rows is farther than the distance moved to an adjacent plant in a row, the 

probability of moving a given distance is similar within and across rows. These larvae cannot be 

involved in the BNI or BNP scenarios unless they feed before they move. The amount of 

feeding by neonates before moving is not known.

Recent studies on ECB larval movement with partial Cry1Ab resistance have documented 

enhanced dispersal of neonate larvae from Bt plants and Bt diet (Prasifka et al. 2009a, 2009b; 

Goldstein et al 2010; Moser et al. unpublished data). Partial resistance provides some tolerance 

to Cry toxins, but resistant homozygotes cannot complete development on a Bt plant. Hence, all 
34

genotypes are phenotypically susceptible on a Bt plant. Thus, to avoid confusion with major 

resistance alleles, which is the focus of the response to this question, the Panel designates the 

partial resistance genotypes with lower case letters: rr is a partial resistant homozygote, rs is a 

partial resistant heterozygote, and ss is a susceptible homozygote. Goldstein et al. (2010) found 

that ss neonates left Bt plants within 24 h ~1.8 times more frequently than non-Bt plants. 

Prasifka et al. (2009a) observed a higher rate of dispersal of rr neonates from Cry1Ab plants than 

ss ones, possibly due to heightened mortality of ss compared to rr neonates following toxin 

exposure. On Bt diets containing Bt corn leaf tissue, rr neonates moved shorter durations and 

distances than ss ones, and importantly, heterozygous rs neonates moved intermediate durations 

and distances. In addition, rr neonates remained on the Bt diet longer than rs or ss ones (Prasifka 

et al. 2009b). At low concentrations of Cry1Ab, rs neonates remained on Bt diet longer than ss

ones. These results have two implications for resistance evolution in seed mixtures. First, they 

may indicate that the BNP scenario is unlikely because more ss larvae may leave a Bt plant than 

rs ones. Second they may indicate that the BNI scenario is possible, because rs larvae can feed 

on Bt diets when ss ones cannot. Both indications, however, must be considered provisional 

until major resistance can be evaluated.

Major resistance to Cry1Fa has been recovered from a Cry1Fa-selected laboratory ECB colony 

at the USDA-ARS in Ames, IA. In unpublished results on larval neonate recovery, resistant 

(RR) and susceptible neonate larvae (SS) dispersed at enhanced rates (95-100%) from pyramided 

Bt plants; whereas, Cry1Fa (RR) resistant individuals had more natural dispersal rates (around 

50% of recovered individuals) from single-toxin Cry1Fa expressing plants (Moser et al. 

unpublished data). Cry1Fa (RS) heterozygotes dispersed at an intermediate rate (around 67% of 

recovered individuals) off of single-toxin Cry1Fa expressing plants, whereas the recovered 

susceptible (SS) larvae dispersed at rates near 100% off of the single-toxin Cry1Fa maize (Moser 

et al. unpublished data). 

Detailed Movement Hypotheses: Second Generation Larvae

The Panel stated that the consequences of movement of second generation larvae are more 

complex than movement of first-generation larvae because of the multiple food sources the 

larvae have available. This complexity generates a richer behavioral context and there are many 

more ways, compared to the first generation, by which larval movement can speed up resistance 

evolution via one of the four scenarios

Egg masses tend to be laid on the undersurface of the leaf blade within 20cm of the leaf collar on 

mature leaves at or slightly above the primary ear. Egg masses occur less frequently at other 

sites, but egg masses can be found on lower leaves, on the leaf sheath, on the upper surface of the 

leaf blade, on tillers, on ear husks and so on. Newly hatched larvae will walk either up or down 

the leaf blade. Those moving up the blade will disperse by silking off the leaf tip. It is not 

known if these larvae eat before dispersing. Those moving down the blade and those settling on 

new plants will encounter the undersurface of the collar and continue walking down the outside 

of the leaf sheath. This will lead them to the upper surface of the collar of the leaf below their 

original leaf. 
35

The fate of the neonate larva depends on what awaits them in this collar. If the leaf is an older 

leaf on the bottom part of the plant, the collar loosely encircles the stem, and the larva tends to 

crawl inside the collar and down into the leaf sheath, were it will feed on the inside of the leaf 

sheath. On a slightly higher leaf, but still below the primary ear, the collar is tighter, but an ear 

bud may force the sheath to expand, creating a small gap that the larva enters. The larva can be 

found feeding on the tips of the ear bud leaves, which is the morphological homolog of the leaf 

blade, or on the leaf sheath. Rarely do they feed directly on the developing ear bud. At an ear 

leaf, the larva will ignore the collar and leaf sheath and either enter the ear from the ear tip or 

from the ear shank. Rarely does it bore through the husks on the side of the ear. When it enters 

the ear tip, it will feed on silks until they become too dry, or on developing kernels. A larva

enters the silk channel and eats or makes its way to the ear where it feeds on the developing 

kernels. A neonate and a 2nd instar can be found feeding on a single kernel, often inside the 

kernel. Older larvae bore between the kernel rows, feeding on multiple kernels. When the 

kernels become dry, they are much less suitable food for the neonates so older larvae are found 

in the cob. On leaves above the ear, the neonate larva will typically find a cache of corn pollen 

in the collar. If the pollen has been moistened, such as from dew, it supports a community of 

yeasts and other microbes that make it an enticing stew. Larvae feeding on this mixture are 

typically larger and more active than those without this food. If the pollen is not moistened, it 

will be blown out of the leaf collars and will be more evenly distributed on the upper surface of 

the leaf blade. Here tiny clumps of pollen, as small as 0.5 mm in diameter, will support a 

remarkably diverse group of fungi, which the larvae will eat. The yeasty pollen caches in the 

collars also attract a diverse group of insects, including minute pirate bugs, which will consume 

large numbers of neonate ECB larvae in minutes. If the larva makes it to the pollen cache, it 

buries itself in the cache where it is safe from predation by pirate bugs. 

Third instar larvae may be found at these same sites, but many larvae leave these sites and 

colonize the midrib of a leaf blade. Although it has not been quantified, there is considerable 

movement between plants, because midrib damage is common on plants showing no signs of 

early larval feeding. The midrib appears to be a suitable site only for 3rd instars, because it is 

rare to find other instars in midribs. 

Fourth and fifth instars are typically found in tunnels that they bored into the stalk or cob. Some 

of these tunnels were initiated from the collars, sheaths and ears where the larvae occurred 

during their younger stages. Many, however, are initiated from new locations and larvae must 

have moved to make these tunnels. Unlike bark beetles, ECB larvae often abandon their tunnels 

and move to new sites where they start a new one. 

The BNI and BNP scenarios (described above) can occur only if the younger instars feed on the 

Bt plant before they disperse. Collar, sheath, and ear husks and shanks are toxic and the BNI and 

BNP scenarios could occur from these locations by older neonates and 2nd instars. These tissues 

typically express high doses of Cry toxin. The pollen caches and silks may have lower 

concentrations of Cry toxin, which could discriminate between RS and SS individuals (killing 

the SS and allowing the RS to survive) and would be one BNI scenario. 

The kernel is a complex tissue. The pericarp is maternal tissue, the germ is a combination of 

maternal and paternal genomes, and the endosperm is two parts maternal and one part paternal. 
36

Part of the endosperm is composed of starch and/or sugar, which contains little protein. A larva 

feeding on a kernel will encounter the pericarp first, followed by the endosperm and lastly the 

germ. The effect on the larva will depend on the maternal and paternal plant genotypes. If the 

maternal plant is a Bt plant, then all of the kernel tissues will express Cry toxin, although the 

concentrations may differ among the tissues depending on whether the paternal plant was a Bt

plant or not. If the maternal plant was a non-Bt plant, then the pericarp will not express Cry 

toxin. If the paternal plant was a Bt plant, the endosperm and germ will express Cry toxin, 

although the concentrations may be less in the endosperm than the germ. If the paternal plant 

was a non-Bt plant, none of the kernel tissues would express Cry toxin. Thus, when cross 

pollination between Bt and non-Bt plants occurs, the ear is a mosaic of expressing and nonexpressing tissues. Cross pollination will be common in seed mixtures, and uncommon in 

between field refuges. This mosaic may facilitate the BNI and NBI scenarios (described above), 

and it may also reduce the effectiveness of the seed-mix refuge.

The NBI scenario is likely to occur during some stages of larval development. Older larvae 

move several times, so this is not a barrier to the scenario. The high survival rates reported for 

5th instar SS larvae on Bt plant tissue compared to the low survival of neonates indicates that 

survival rates increase as the larva matures. Increased survival as development occurs will also 

likely take place for RS and RR genotypes. These results indicate that the “killing power” of Cry 

toxin attenuates with larval development. Given the known toxicity mechanisms, this 

attenuation is likely to be genotype specific, and there will be some stages where RS survival is 

higher than SS survival on Bt plants.

Unlike the other three scenarios discussed above, there is little evidence about ECB larval 

movement supporting or refuting the NBP scenario (described above). Perhaps it is unlikely or

perhaps there needs to be some experiments conducted to determine if ECB larval movement 

could fit this scenario.

Uncertainty about Larval Movement and Evaluation of Monsanto and EPA/ORD Models

The Panel examined in detail, the structure, parameterization and results of the Monsanto and 

EPA/ORD models in relation to the potential effect of larval movement on the evolution of 

resistance. The Monsanto model did not incorporate structural equations to model larval 

movement and the approximations used in that model are structured to minimize the effect of 

larval movement on the rate of resistance evolution. Hence this model has overestimated the 

durability of seed mix refuges (and overestimated the difference between seed mixes and 

between-field refuges). 

The EPA/ORD model did incorporate structural equations to model larval movement, however, 

none of the scenarios or hypotheses described above were evaluated. Hence, this model has also 

overestimated the durability of seed mix refuges (and overestimated the difference between seed 

mixes and between-field refuges). The larval movement scenarios that were assessed with the 

EPA/ORD model were substantially more sophisticated, realistic, and easy to understand than 

the assessment provided by the Monsanto model. The Panel concluded that elaboration of the 

EPA/ORD model would be superior to generalizing the Monsanto modeling approach for 
37

evaluating the evolutionary consequences of the general larval movement scenarios and specific 

ECB larval movement hypotheses described above. 

There are several sources of uncertainty about ECB larval movement. At the highest level of 

abstraction, there is uncertainty about model structure. Although there is a considerable amount 

known about ECB larval movement, investigations have not systematically examined larval 

movement behavior to have some degree of confidence that all movement hypotheses likely to 

increase the rate of resistance evolution have been identified. For example, it is not known if 

neonate larvae feed on leaf hairs shortly after ecolosion and prior to dispersal from the plant they 

hatched out on. Without this information, it is not possible to know if the immediate movement 

of neonate larvae affects the rate of resistance evolution. Even less is known about movement of 

SWCB larvae.

At an intermediate level of abstraction, it is not known if any of the processes and parameter 

values is correlated with each other or if they are independent. For example, the neurological 

basis of insect behavior suggests that larvae with a greater propensity to move may also have 

corresponding increases in adult movement behaviors (i.e., higher activity in larvae translates to 

increased activity in adults, as seen for Drosophila foraging and shaker genes; Engel et al. 2000; 

Pereira and Sokolowski 1993; Ueda and Wu 2006; Ishimoto et al. 2005). If movement is 

governed by a genetic component (such as the homolog of the foraging locus) in ECB, selection 

for enhanced larval locomotion may give rise to correlated movement responses that speed up 

resistance evolution either by larval or adult movement. 

These kinds of correlational uncertainties are difficult to analyze because of there are so many 

possibilities (there are n(n-1)/2 correlations that need to be evaluated, where n is the number of 

parameters). The Panel suggested one way to evaluate correlation uncertainty is to identify the 

correlations that may increase the rate of resistance evolution the most. One way to do this is to 

parameterize the model with the extreme values and look for parameter combinations that give 

rapid resistance evolution. This has an advantage of requiring only 2n simulations, which is the 

same order as the number of parameter correlations, and provides more informative results than 

random draws from a parameter distribution, which will rarely evaluate extreme values. Once a 

short list of parameter combinations are identified, a smaller number of correlations can be 

evaluated more thoroughly, and once the key correlations have been identified, then it will be 

possible to randomly sample parameters from a joint probability distribution and to determine if 

there is a biological basis for the identified correlations.

Finally, there is considerable parameter uncertainty associated with larval movement. In other 

words, the quantitative value of the larval movement parameters is poorly known for either the 

first or second generation ECB. This kind of uncertainty can be evaluated through Monte Carlo 

simulation, such as drawing on known or suspected probability distributions of the parameters as 

was done with the EPA model. However, it is critical that the models are structured to evaluate 

the four general larval movement scenarios, and this has not been done.
38

Charge Question 3

It is typically assumed that, since European corn borer (ECB) and southwestern corn borer 

(SWCB) are similar in many ways, ECB can serve as a surrogate for SWCB to address 

uncertainties regarding biology and genetics. The applicants’ efficacy data, however, suggest 

that SmartStax is somewhat less toxic to older instars of SWCB. Results of a larval exposure 

study by Monsanto showed that SWCB survival was higher than ECB on SmartStax. Should some 

SWCB larvae disperse as older instars, the rate of adaptation to SmartStax could increase in a 

seed blend deployment. BPPD concluded that simulation models should incorporate such 

information in their analyses. There is currently a lack of data on the propensity of SWCB larval 

plant-to plant movement and on how ECB and SWCB differ in this respect, if at all.

Please comment on the assumption that ECB is a suitable biological surrogate for SWCB and 

BPPD’s concerns that a SmartStax seed blend may affect SWCB differently than ECB.

Panel Response

The Panel agreed that the results from the Monsanto model and EPA/ORD model are inadequate 

to make any scientifically sound conclusions about resistance evolution in SWCB because they 

do not include density-dependent mortality, which is an important feature in SWCB population 

ecology. However, on the broader issue of whether ECB is a reasonable surrogate for SWCB, 

the Panel expressed different views in the degree to which ECB information could be used for 

SWCB. Some members of the Panel concluded that ECB is a poor surrogate because of the 

many ways the two species differ biologically and therefore different models would have to be 

developed for SWCB. Others suggested that at a structural level the same resistance evolution 

model can be used for the two species, but that the parameter values must be different. 

ECB and SWCB are phylogenetically and taxonomically related in the Crambidae, 

morphologically similar, share similar life cycles, corn is their major host, feed on corn stalks in 

similar ways, and have a similar toxicity spectrum to Cry toxins, among a long list of 

similarities. In many ways, ECB is a better surrogate for SWCB than its congener, sugarcane 

borer (Diatraea saccharalis), which has a wider functional host range and is considerably less 

sensitive to many of the Cry toxins that kill both ECB and SWCB. However, there are many 

differences between ECB and SWCB (Guse et al. 2002; Onstad et al. 2002, see discussion 

below). Some of these differences have no apparent relation to resistance evolution, but others 

are extremely important, especially density-dependent mortality. Because the Monsanto and 

EPA models did not include a density-dependent mortality function, the Panel agreed that the 

results from the Monsanto and EPA/ORD model were inadequate to make any scientifically 

sound conclusions about resistance evolution in SWCB.

A functional difference between these species, for example, is that SWCB is more tolerant of 

low humidity than ECB. This is evident from the geographic distribution of the species: ECB 

inhabits a moister climatic zone than SWCB. This difference, however, may result in differences 

in adult movement. As summarized in the answer to charge question 1 above, many aspects of 

adult ECB movement are driven by their constant search for sufficiently moist daytime habitats. 

Hence, perhaps adult SWCB are less dispersive than adult ECB. If this were true, the effect of a 
39

seed mix would accelerate resistance even more in SWCB than it would for ECB. Onstad et al. 

(2002) have suggested that SWCB adult are less dispersive than ECB adults, especially before 

mating.

Modeling efforts by Ives and Andow (2002) indicated that higher net reproduction fecundity 

speeds up the rate of resistance evolution for high-dose events. Research suggests that SWCB 

probably has a higher fecundity and higher larval survival than ECB (Knutson and Gilstrap 

1990). Therefore, SWCB may evolve resistance faster than ECB. However, it is not clear how a 

seed mix refuge might accentuate such a difference.

SWCB larvae are reported to be less sensitive to Cry toxins than ECB larvae (registrant 

submitted study, RPN-09-075, Head et al. 2009). An important implication is that RS 

heterozygote survival may exceed 5%, for one or more of the toxins and this neutralizes the 

advantages of the high-dose strategy. The high-dose strategy may be further undermined with 

increased interplant movement of SWCB larvae via the BNI and NBI scenarios described in the 

response to Charge Question 2.

A very significant difference between the species involves the way the larvae use the plant and 

associated intra-specific larval interactions. ECB larvae can be found in many locations on a 

corn plant. In contrast, SWCB larvae end up concentrated in tunnels at the base of the stalk, near 

ground level (Knutson 1987; Knutson and Gilstrap 1990; Chippendale and Sorenson, accessed 

Dec. 2010). SWCB is cannibalistic, and when the larvae concentrate in this way, typically only 

one survives. ECB occasionally cannibalizes, but this is incidental and in most cases many 

larvae can coexist even when they come in close contact with each other. As such, densitydependent mortality is an important consideration for SWCB.

The Panel recommended that the Monsanto and EPA/ORD resistance evolution models include a 

strong density dependent mortality function in modeling the evolution of SWCB resistance. Ives 

et al. (2011) demonstrated that models without density-dependent mortality, gene “frequency 

only” models exhibit qualitatively and quantitatively different evolutionary trajectories than 

models with even mild density-dependent larval mortality (see also Neuhauser et al. 2003). The

Monsanto modeling and EPA/ORD modeling did not include density dependent mortality for 

SWCB. This means that predictions from these modeling efforts are not reliable in estimating 

the evolution of SWCB resistance. The Panel indicated that density-dependent selection (soft 

selection, Wallace (1981)) might be operating in this system in contrast to the traditional 

concepts of viability selection (hard selection, Wallace (1981)). The larvae that win out in 

cannibalistic encounters in the presence of Bt are likely to be those that have a slight fitness 

advantage from being more resistant to Bt and therefore healthier. Such selection can be very 

strong even when a similar number of insects successfully develop from a seed mix as in a pure 

stand of non-Bt corn. Examples of such selection have been documented for the sheep blowfly 

(McKenzie and Whitten 1982, 1984; McKenzie 1996) and the potential for this type of densitydependent advantage has been noted in several studies involving corn earworm larvae (Stinner et 

al. 1997; Dial and Adler 1990). Periodic, locally high densities of SWCB may drive resistance 

via density-dependent selection if a higher level of fitness is evident in heterozygotes or resistant 

homozygotes. Neither the Monsanto nor the EPA model allows for density-dependent selection. 

This may also be important for CRW resistance evolution.
40

Charge Question 4

Corn earworm (CEW) was not considered in the applicants’ and EPA/ORD’s analyses for a 5% 

SmartStax seed blend based on the assumption that the insect does not overwinter in the Corn 

Belt where the blend has been proposed. BPPD is concerned, however, that there could be areas 

in the southern portion of the Corn Belt where CEW may be able to successfully overwinter, 

particularly in less severe winters. Such areas may need to be identified because they could 

contribute to increased selection for CEW resistance to Bt corn (including the proposed 5% 

SmartStax seed blend).

Please comment on the assumption that corn earworm does not successfully overwinter in the 

Corn Belt and poses less of a risk for resistance. If CEW can potentially overwinter in parts of 

the Corn Belt, should the insect be considered in the analysis of the proposed 5% SmartStax seed 

blend?

Panel Response

The Panel discussed CEW long-distance migration from the South to the Midwest in the spring, 

CEW reverse migration from the Midwest to the South in the late summer, and whether these 

migratory patterns have an effect on the selection for CEW resistance in the Midwestern Corn 

Belt. There are several studies demonstrating that the major CEW infestations in the Midwest 

are caused by CEW moths that originate in Southern states. There is also evidence for reverse 

migration of CEW from the Midwest to Southern states. The Panel concluded that recent studies 

demonstrate that moths have evolved sophisticated mechanisms that enable them to move 

adaptively from one geographic location to another. 

The Panel considered CEW resistance associated with corn to be a serious resistance risk to both 

corn and cotton. Considering all of the evidence, the Panel could not quantify the role of 

selection associated with 5% SmartStax seed mixtures in the Midwest and migration between the 

Midwest and the South on the rate of evolution for Bt resistance in CEW. Some Panel members 

suggested that high adoption of seed mixtures in the Corn Belt might have an effect on selection 

for CEW resistance on Bt cotton later in the season. In a worst-case scenario, there might not be 

any refuge for CEW within Midwestern corn that is planted as a seed mixture. Other crops and 

wild plants in the Midwest might be good hosts for CEW, but most of these crops are sprayed 

with insecticide and would not be good sources for production of susceptible insects. The Panel 

suggested that a detailed survey of alternate crops and more information regarding Bt pollen 

exposure scenarios would be useful.

Corn Earworm Overwintering

The CEW is thought to overwinter as far north as 40° North latitude that approximately 

corresponds to the area of the last spring freeze being before April 30 (Snow and Copeland 1971; 

Hardwick 1965). Climatic data reveal that the area of last spring freeze being before April 30 

has recently (1971 – 2000) extended far northward (go to http://cdo.ncdc.noaa.gov/cgibin/climatenormals/climatenormals.pl?directive=prod_select2&prodtype=CLIM2001&subrnum

%20to%20Freeze/Frost%20Data%20from%20the%20U.S.%20Climate%20Normals). Therefore, 

overwinter survival of CEW is plausible in several states within the Corn Belt, including all of 
41

Kansas, Missouri, Iowa, Illinois, eastern Nebraska, southern regions of South Dakota, 

Minnesota, Wisconsin, Michigan, and most of Ohio. The geographic extent of the proposed 

allowable areas for use of a 5% seed mixture also overlaps with at least one area where cotton is 

currently planted (40,000 acres were planted in Kansas in 2010). The important questions 

concern the origin of the CEW adults that produce larvae infesting Midwest corn.

Long-Distance Migration from the South to the Midwest

Although some of the CEW larvae infesting Midwest corn may come from a local overwintering 

population, there are a number of studies that demonstrate that the major infestations in most of 

the Midwest are caused by CEW moths that are flying up to the Midwest from Southern states. 

Several studies indicate that CEW moths and larvae appear in areas of the Midwest after periods 

when the wind is appropriate for enhancing the area moths can traverse in one to a few nights 

(Pair et al. 1987). Airborne radar tracked a dense cluster of CEW moths from infested corn 

fields in the Lower Rio Grande Valley of Texas and Mexico for a distance of 400 km in a single 

night (Wolf et al. 1990). The long-distance flight of CEW moths have been closely correlated 

with a strong wind feature in the lowest km of the atmosphere (i.e., low-level wind jet) that 

provides frequent migration opportunities northward from the Southern Plains to the Northern 

Plains (Westbrook et al. 1998). The meteorological data are reinforced by data indicating the 

presence of citrus pollen on CEW found in the Midwest (Lingren et al. 1993, 1994). Based on 

these data, CEW moths must have flown to the Midwest from much more southern areas.

Reverse Migration

If moths simply fly from the South to the Midwest and die there in the winter as pupae then this 

is an evolutionary dead-end and would not contribute to resistance development (McNeil 1987). 

The idea that noctuid moths moved north following the availability of lush vegetation has been 

considered likely (Rabb and Stinner 1978; Kennedy and Storer 2000), but the mechanism 

triggering this northward movement does not appear to be proximate recognition by the moths of 

lush crops from long distances. The prevailing perspective in the 1970’s through much of the 

1980’s was that there was no reverse migration back to the South later in the season. However, 

more data on CEW indicates that they also migrate back south (e.g. Pair et al. 1987; Gould et al. 

2002). Pair et al. (1987) found that wind direction is often favorable for southward migrations in 

the fall. Radar observations have identified abrupt peak concentrations of CEW and other 

similar-size insects flying southward in the wake of passing cold fronts in the fall in Texas 

(Beerwinkle et al. 1994). Gould et al. (2002) found that a large fraction of late season CEW 

moths captured in Louisiana and Texas have C13 signatures indicating that they came from larvae 

that fed on C4 plants even though there are very limited local C4 hosts available for CEW moth 

production at that time. These results support other findings of reverse migration by the CEW 

moths. For example, Dowd (2001) (cited in Gould et al. 2002) estimated that there can be 

16,000 large CEW larvae per hectare of Midwest field corn in the late summer, and that only 

50% of these late season larvae in Wisconsin and Pennsylvania go into diapause (unpubl. data). 

These two results combine to indicate that there are a large number of potential southward 

migrant moths in the Midwest late in the season. More recently, southward migration from the 

Corn Belt associated with the passage of a cold front was simulated using an atmospheric 

transport model (Westbrook 2008).
42

Helicoverpa armigera is very closely related to Helicoverpa zea (i.e., CEW). The data from 

China on late summer southward migration of H. armigera is more extensive (Feng et al. 2009 

and references within) than the studies on H. zea late summer migrations in the U.S. The Feng et 

al. (2009) study is the most comprehensive and demonstrates that in the late summer H. 

armigera moths are found in appropriate air layers for correct movement and survival, and that 

when air movement is not in the correct direction, the H. armigera moths correct for that by their 

flight orientation as if guided by a compass.

Beyond the data on CEW and H. armigera, studies on a number of other noctuids provide strong 

evidence of northward migration in summer and southward migration in the fall (Showers et al. 

1993; Showers1997; McNeil 1987). The question arises as to whether this reverse migration 

results from a behavioral adaptation or whether some moths in the North simply are flying 

during a time when the wind is moving south and they get swept up in these winds. Recent, high

profile articles demonstrate that moths have behavioral systems that seem to be as sophisticated 

as those of migratory birds in determining long distance flight paths that are adaptive (Chapman 

et al. 2008, 2010; Reynolds et al. 2010). 

The Panel concluded that recent studies demonstrate that moths have evolved sophisticated 

mechanisms that enable them to move adaptively from one geographic location to another. 

Contributions of these Southward Migrants

The Panel considered the question of whether these southward migratory movements have 

evolved as adaptive mechanisms and whether they always wind up bringing moths to areas 

where they can overwinter. For some moths, the answer seems to be “yes” (McNeil 1987;

Chapman et al. 2010). However, for the CEW, not enough experimental work has been done to 

determine the final fitness of moths that fly south. It is not known if they fly to the Southern 

U.S. or if they fly much further to more tropical areas. 

In some years these migrants may not contribute much to next season’s population and most of 

the next season’s moths may come from insects that remained in the South all summer. In other 

years, however, there is evidence that the local populations produce an extra “suicide generation” 

in areas like North Carolina (Stinner et al. 1978) and would therefore not contribute substantially 

to the next season’s population. In these years it could be that migrants from the North contribute 

the most genes to the future population.

Selection for CEW Resistance

Considering all of the evidence, the Panel could not make any definitive statement about whether 

CEW being selected for Bt resistance in the Midwest will wind up in the next season’s Bt cotton 

in the South and then be back in Midwest corn later in the season should growers plant seed 

mixtures (en masse) with only a small percentage of non-Bt corn germplasm. Some Panel 

members suggested that high adoption of seed mixes in the Corn Belt might have an effect on 

selection for CEW resistance on Bt cotton later in the season. Currently non-Bt corn is 

considered to serve as a refuge for CEW that are being selected by Bt cotton. There appear to be 

plans for intensifying the use of Bt corn in the South. If this is done based on the use of seed 

mixtures, then there might be very little in the way of a refuge for CEW in the South at some 

times in the year. The Panel considered CEW resistance associated with corn to be a serious 
43

resistance risk to both corn and cotton. The dose of insecticidal toxin in Bt cotton is a major 

factor determining the level of resistance risk (see Appendix 1“Empirical Methods for 

Estimating Dose and Efficacy” and the Panel’s response to Charge Question 8).

Other crops and wild plants in the Midwest might be good hosts for CEW, but most of these 

crops are sprayed with insecticide and would not be good sources for production of susceptible 

insects. Some Panel members suggested that a detailed survey of alternate crops is needed. The 

Panel discussed other situations in which selection for CEW resistance would be increased. 

Some Panel members suggested that selection on the CEW for Bt resistance would occur 

whether the refuge is planted as part of seed mixture or is maintained as a structured refuge. 

SmartStax corn is the result of hybrid crosses involving four different events, MON 89034

(cry1A.105 and cry2Ab2 genes) x TC1507 (cry1Fa gene) x MON 88017 (cry3Bb1 gene) x DAS59122-7 (cry34/35 genes). As these genes are not on the same construct they will segregate 

independently during breeding and selection.

Some Panel members suggested that CEW might be selected for resistance faster in a field 

planted with the SmartStax Refuge-in-the-Bag seed mixture consisting of 5% non-Bt seed and 

95% SmartStax seed because research by Chilcutt and Tabashnik (2004) indicated that kernels of 

non-Bt plants that are fertilized by Bt corn pollen can produce Bt toxin. These researchers found 

that if the percent non-Bt corn is ≤10% almost all of the non-Bt corn would be fertilized by Bt

corn pollen. Therefore, the selection on CEW for Bt resistance might be even stronger as the 

SmartStax RIB is a 5% seed mixture. CEW are likely to feed on kernels that are fertilized by Bt

corn pollen as the small percentage of non-Bt corn plants will likely be pollinated by Bt corn 

pollen coming from only a short distance. The Panel indicated that more information regarding 

Bt pollen exposure scenarios would be useful.

Corn kernels are complex tissues genetically and will express Bt toxins in a complex manner. 

The pericarp is a normal maternal diploid, and will express the maternal phenotype. This means 

that all (or none) of the toxins will be expressed in the pericarp of all of the kernels on a plant in 

a seed mixture. The germ (embryo and cotyledon) is a diploid combination of a haploid 

maternal and haploid paternal genome, and the endosperm, which makes up the bulk of the 

kernel, is a triploid combination of two haploid maternal genomes and one paternal genome. 

The haploid genomes in the endosperm are identical to the haploid genomes in the germ, and the 

endosperm is homozygous for the maternal set and hemizygous for the paternal set. Assuming 

that cry1Fa and cry1A.105/cry2Ab2 are hemizygous and on different chromosomes, a pyramid 

plant will produce the following germ/endosperm maternal haplotypes: 1⁄4 no cry genes, 1⁄4 

cry1Fa only, 1⁄4 cry1A.105/cry2Ab2 only, and 1⁄4 with both cry1Fa and cry1A.105/cry2Ab2. The 

pollen produced by this plant would have the same haplotypes in the same ratios. The non-Bt

plants in a seed mixture will produce all non-cry haplotypes. Self-pollinated pyramided plants 

will have kernels with the following phenotypes in the germ and endosperm: 1/16 no Cry toxin, 

3/16 Cry1Fa only, 3/16 Cry1A.105/Cry2Ab2 only, and 9/16 with both Cry1Fa and 

Cry1A.105/Cry2Ab2. Similar frequencies can be worked out for the other crosses in a seed 

mixture. Assuming random mixing of pollen and a 5% seed mixture, the entire field will have a 

mixture of kernel phenotypes in the germ and endosperm, with 8.3% no Cry toxin, 19.3% 

Cry1Fa only, 19.3% Cry1A.105/Cry2Ab2 only and 53.1% both Cry1Fa and 

Cry1A.105/Cry2Ab2. If cry1Fa and cry1A.105/cry2Ab2 are homozygous and on different 
44

chromosomes, the entire field would have 0.2% no Cry toxin and 99.8% both Cry1Fa and 

Cry1A.105/Cry2Ab2.

The CEW will be differentially selected for resistance when feeding on SmartStax corn kernels. 

This is expected to result in stronger selection for CEW resistance. If the cry genes are 

hemizygous, CEW will feed on kernels containing only one or two toxins instead of all three, 

which will select for single toxin adaptations. If the cry genes are homozygous, there will be 

virtually no refuge for CEW in the seed blend. The Panel noted that future experiments may 

show that the first instar CEW could avoid the toxic kernels, but this is unlikely for larger larvae. 

Larger larvae are likely to feed on multiple kernels and may experience an average of all of the 

kernels in the ear, which would be less than the toxin concentration in the pyramided plant. For 

hemizygous cry genes, the average kernel in a pyramided plant may have ~74%, and the average 

kernel in a non-Bt plant may have ~47% of the toxin in the intact pyramided plant. These lower 

concentrations may result in more rapid resistance evolution. These factors would rarely occur 

in a structured block refuge.

Charge Question 5

To assess dose expression for corn rootworm (CRW) Bt toxins, the level of survival (adult 

emergence) is typically compared between artificially infested Bt and non-Bt corn plots. 

However, density-dependent mortality in non-Bt plots can potentially confound the comparison 

by reducing overall survival and adult emergence. (Density-dependent mortality is not expected 

in Bt plots due to effects of the toxin on young larvae.) To account for this effect, the dose 

calculation can be adjusted by removing density-dependent mortality from the control plots. This 

effectively increases the dose mortality estimate by raising the number of larvae present in nonBt plots relative to the surviving larvae in Bt plots.

For the SmartStax toxins, Dow/Monsanto made a density-dependent adjustment to their dose 

estimates based on density/survival relationships developed by Onstad et al. (2006). The 

resulting dose mortality profile was: Cry34/35Ab1 (99.75%), Cry3Bb1 (99.75%), and 

Cry34/35Ab1/Cry3Bb1 pyramid (99.95%). On the other hand, BPPD has also considered 

separate work by Hibbard et al. (2010), which suggests that density-dependent mortality occurs 

at higher egg density levels than those assumed by Dow/Monsanto. In light of this research, 

BPPD recommended in its 2009 risk assessment of SmartStax that dose should also be evaluated 

without a density-dependent adjustment. The non-adjusted dose profile for the SmartStax toxin 

is: Cry34/35Ab1 (94.2%), Cry3Bb1 (97.5%), and Cry34/35Ab1/Cry3Bb1 pyramid (98.2%).

Please comment on dose estimates for the SmartStax toxins (Cry34/35Ab1 and Cry3Bb1) 

targeting corn rootworm given the different interpretations of density-dependent mortality.

Panel Response

The Panel agreed that a density-dependent mortality adjustment is merited theoretically; but 

questioned the accuracy and precision of the compensatory adjustment values used in the Dow 

model. The Panel agreed with BPPD’s recommendation in the 2009 risk assessment of 

SmartStax to evaluate dose without a density-dependent adjustment and use the non-adjusted 
45

dose profile for each Bt toxin: Cry34/35Ab1 (94.2%), Cry3Bb1 (97.5%), and 

Cry34/35Ab1/Cry3Bb1 pyramid (98.2%). 

Density-dependent mortality estimates were calculated from field studies conducted under 

optimal field conditions that the Panel considered unrealistic and therefore the densitydependence mortality was overestimated. The Panel concluded there is no statistically credible 

method to distinguish between 94% and 99% dose mortality because the dose mortality profiles 

are poorly estimated. Both the unadjusted and the density-dependent adjusted dose mortality 

profiles have very high variances. The Panel recommended that the CRW models be rerun using 

the non-adjusted dose profiles (and lower values) and that the re-analysis consider the variance 

of dose mortality profiles using first order Monte Carlo simulations.

The Panel considered three aspects of the density-dependent mortality dose profile in their 

response to this charge question: 1) larval mortality factors, 2) density-dependent mortality 

estimates, and 3) statistical significance of the density-dependent mortality estimates. 

Factors that Influence Corn Rootworm Survival Rates

The Panel identified a number of biological and ecological factors that cause the dose mortality 

profile to be highly variable, and therefore the expected dose-mortality in SmartStax corn fields 

is probably significantly less than the Dow values reported to the Panel. Some of these factors 

are not fully known or understood, so they cannot be controlled or manipulated. Hybrid 

characteristics influence root structure, size and regeneration. These characteristics, combined 

with environmental conditions such as soil type and excessive rains or drought conditions and 

timing of these events, can have significant impacts on CRW larval mortality regardless of 

density. Larval survival rates were calculated from fields studies conducted under optimal field 

conditions, e.g., consistent environmental conditions, uniform distribution of eggs when they are 

typically found in clumps across the field and no CRW behavioral variants. Given that typical 

field conditions are not optimal, the Panel concluded that the larval survival rates used to 

calculate dose mortality are overestimates of what is expected under natural field conditions. 

One panel member commented that his research on the performance of single-gene Bt CRW corn 

varieties, YieldGard, Cry 3Bb1 and Herculex, Cry 34/35Ab, with a near isoline (control) showed

that efficacy of the Bt CRW corn depended on the pest density (Tollefson, unpublished). As the 

density of larvae attacking the Bt CRW corn roots increased, the loss of larvae due to densitydependent mortality on the near isoline increased. This reduced the relative number of adults 

that emerged from the near isoline and it appeared that the Bt CRW corn provided greater 

protection. In some instances, there were more adults that emerged from the Bt CRW corn than 

from the near isoline, due to density-dependent mortality on the unprotected roots.

The protection provided by a Bt CRW corn variety that expresses a moderate dose may be 

reduced if the pest pressure is high. One panel member commented that this information has 

been provided by some manufacturers to explain why their Bt CRW corn did not provide the 

level of root protection that the customer expected. The Panel discussed how refuge damage can 

be used to assess pest density by evaluating root injury where soil insecticides have been used for 

control. Soil insecticides will not provide complete control of the CRW larvae attacking the corn 
46

roots. The level of control is usually in the 40-60 % range. If there were a structured refuge with 

an insecticide applied to the soil for CRW protection and there was obvious root injury in the 

refuge then it is possible to conclude that CRW pressure was high and the Bt CRW corn could 

not protect itself against a high number of larvae. If an insecticide were used on the refuge and 

there was not a noticeable amount of injury on plants in the refuge, but lodging and root injury 

was present in Bt CRW corn fields then it is possible to conclude that the Bt corn PIP was not 

expressing lethal dosages in that field. This implies that the dose mortality profile is highly 

variable, and therefore the expected dose-mortality rate in SmartStax corn fields is probably 

significantly less than the Dow values reported to the Panel. The Panel agreed with BPPD’s 

interpretation of the data and the recommendation that the effective SmartStax dose mortality 

should be estimated without the inclusion of a density-dependent mortality adjustment. Using 

the unadjusted dose mortality in the model may provide a more accurate prediction of the 

durability of the CRW protection provided by SmartStax corn. The Panel identified a number of 

biological and ecological factors that cause the dose mortality profile to be highly variable, and 

therefore the expected dose-mortality in SmartStax corn fields is probably significantly less than 

the Dow values reported to the Panel. 

Density Dependent Mortality Estimates

Density dependent mortality does occur in natural populations of CRW. In support of 

Dow/Monsanto, the Panel suggested that this density dependence may occur at quite low egg 

densities, as illustrated by the following. 

Density-dependent larval survival was modeled using Hassell’s (1975) formulation for density 

dependence for insect populations with discrete generations, x the insect population density at 

generation t and t+1, . The parameters, α and β, were estimated from field 

experiments on the survival of naturally laid eggs (Gray and Tollefson 1988) or survival of 

inoculated eggs (Branson and Sutter 1985; Elliot et al. 1989) using nonlinear least squares (SAS 

1999, Proc NLIN). There were clear differences in the fitted equations for natural and inoculated 

eggs (Fig. 5-1, Table 5-1). The field data suggested that α = 10.6 and β = 0.52 were reasonable 

values for density-dependent egg to adult survival of Diabrotica virgifera in non-transgenic corn 

when insecticides were not applied. These values indicated that density dependence did not 

intensify until egg densities exceeded about 100,000/ha and that density dependence was 

moderately strong, allowing the density to return towards a carrying capacity smoothly without 

overshooting. The Hibbard et al. (2010) conclusion that density-dependence may only occur at 

very high densities may be due in part to their efforts to estimate per capita mortality. This 

would normally be the appropriate analysis, but in this case, per capita mortality has high 

statistical error because it compounds the error in the density estimates. The Hassell method 

described above is more sensitive for detecting density dependence.

[ ( ) ] β α −

t+ = t + t x x 1 x 1
47

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80

Adults (Million/ha)

Eggs (Million/ha)

Field Observations

Egg Inoculations

Field Predicted

Egg Predicted

Table 5-1. Estimated density-dependent larval survival parameters for western corn rootworm, 

based on nonlinear least squares regression. Parameter values were α and β, as developed in 

Hassell, with statistics from the analysis, p<<0.05 for both egg inoculations and field 

observations. Measurement error is not taken into account with this analysis.

α β F (d.f.) r

2

Egg inoculations

(Branson and Sutter 1985; 

Elliot et al. 1989)

5.3 0.68 37.09 (2, 2) 0.974

Field observations

(Gray and Tollefson 1988)

10.6 0.52 156.81 (2,10) 0.969

Fig. 5-1. Density-dependent egg to adult survival of western corn rootworm on corn. Egg 

inoculation data were from Branson and Sutter (1985) and Elliot et al. (1989), showing weighted 

means from 5 years for the two studies. Field observations were from Gray and Tollefson (1988) 

and were from 3 years and 4 tillage systems. Lines were fitted by non-linear least squares.

Statistical Significance of Density Dependent Mortality Estimates

In theory, the Panel agreed that parameters should be estimated as accurately as possible, and 

that adjusting dose mortality with a density-dependent adjustment would be helpful, especially as 

density dependence is strong and occurs over a wide range of CRW densities. In this case, the 
48

Panel had serious reservations over manipulating field collected mortality estimates to adjust for 

density-dependent mortality or any other factor that may be considered. There were two reasons. 

First, the Panel is concerned that the values stated in the charge question may not be accurate 

because they were collected under ideal conditions. Many factors reduce mortality in typical 

field conditions, so the typical mortality value may be less than those in the charge question. A 

density-dependent adjustment may provide a false sense of accuracy. Second, the adjustment 

may have little statistical meaning. The original estimates of mortality are not precise or 

repeatable enough to support the adjustment. While parameters should be estimated as 

accurately as possible, any improvements should provide statistically significant improvements. 

If the parameter estimates are not very precise, adjusting them gains little statistical 

improvement. This point is elaborated below. 

The Panel concluded there is no statistically credible method to distinguish between 94% and 

99% dose mortality because the parameters necessary to make the density-dependent adjustment 

to dose were poorly estimated. Both the unadjusted and the density-dependent adjusted dose 

mortality profiles have very high variances. The general formula given in Hibbard et al. (2010, 

p. 83) as reported in Storer et al. (2006) is 

, [Eq. 1]

where D is dose mortality (in percent), A is adult emergence from the Bt CRW-protected variety, 

A0 is adult emergence from the non-Bt variety, and Y(E) is the predicted survival of viable eggs 

at the infestation level used. 

A claim was made that mortality due to Cry34/35Ab1 toxin dose in the isoline corn estimated at 

96.71% was different from the estimate of 99.88% from the toxic dose of transgenic corn 

targeting rootworms, but there was no discussion as to the statistical significance of this 

difference. The uncertainty of estimates using this equation could be computed using the 

variability measured for the individual components by using an approximate estimate of the 

variance of D given below that is derived using the method of statistical differentials (see for 

example Elandt-Johnson and Johnson (1980). 

[Eq. 2]

where V(A), V(A0), V(Y(E)) are estimates of the variance of the equation components and 

Cov(A,A0), Cov(A,Y(E)) and Cov(A0,Y(E)) are the estimated co-variances for pairs of the equation 

components. A simpler estimate for the variance of D could be obtained if it is possible to 

assume that the co-variances are close to zero. Equation 2 could be used in a simple t-test to 

assess the statistical significance of the difference in estimated mortality and/or to establish 

confidence intervals. 

D = (1− (AY(E)/ A0 )×100

































+ 















+ 















+

















+ 















+ 













 ≈

( , ( )) ( ) ( ( )) ( ) ( , ) ( ) 2

( ) ( ) ( ) ( ( )) ( )

(100%)

( )

3 0

0

2

2

0

3 0

0

2

0

2

2

0

2

0

2

0

2

Cov A Y E

A

A Y E Cov AY E

A

AY E Cov A A

A

AY E

V A

A

AY E V Y E

A

A V A

A

V D Y E
49

A, A0, and Y(E) typically have a high variance, so D will have a high variance. This can be 

illustrated with the data provided by the registrants (Table 5-2). In no case is the estimated 

SE(D) less than 10%. Two facts are used in this calculation. First V(x) = SE2

(x), where x is 

either D, A, A0 or Y(E). Second, Cov(A,Y(E)) = Cov(A0,Y(E)) = 0, because Y(E) is estimated 

independently of A and A0. In addition, we assume Cov(A,A0) = 0. In reality, A and A0 are likely 

to be positively correlated, and if they are positively correlated, the estimated SE(D) reported in 

Table 5.2 is underestimated. Finally, it should be noted that the estimated SE(D) for each of the 

four Bt varieties in a year are not independent, because the same A0 and SE(A0) (and if Cov(A,A0) 

≠ 0, then the Cov(A,A0) are also correlated among the four Bt varieties) enter into equation [2] 

for all four varieties. This non-independence will tend to overestimate or underestimate the 

SE(D) for the four varieties in a quantitatively similar manner, and it will also reduce the degrees 

of freedom associated with the estimated SE(D). In conclusion, it is not possible to statistically 

distinguish a D = 90 from a D = 99 for corn rootworm. Given the uncertainty in the estimation of 

density-dependent mortality, the Panel decided that the unadjusted (no density-dependent 

mortality factor) dose should be used in the models.

Table 5-2. Estimation of the standard error on the density-dependent adjusted dose 

mortality.1

D SE(D) n

A or 

A0

2

SE(A) 

or 

SE(A0)

2 Y(E)3 SE(Y(E))3

2006

A Mon 89034 TC1507 Mon 

88017 DAS 59122-7 99.60 23.52 16 6.6 2.5 5 2

A Mon 88017 DAS 59122-7 99.55 24.13 16 7.4 2.2 5 2

A DAS 59122-7 98.92 56.93 16 17.6 5.1 5 2

A Mon 88107 98.57 77.48 16 23.4 7.3 5 2

A0 Control 16 81.6 15.2

2007

A Mon 89034 TC1507 Mon 

88017 DAS 59122-7 99.33 39.72 8 11.9 3.8 5 2

A Mon 88017 DAS 59122-7 99.61 23.56 8 7 2.3 5 2

A DAS 59122-7 97.72 139.14 8 40.5 14.2 5 2

A Mon 88107 98.71 74.88 8 23 6.7 5 2

A0 Control 8 88.9 26.7

1. D, A, Y(E) and A0 are as in equation [1], and SE(D) is the square root of V(D) from equation [2]. 

All values in percent.

2. 2006 data are from Page 166, MRID 474449-11 (Vaughn et al. 2008); Page 27, RPN 07-262

Vaughn et al. (2008), and 2007 data are from Page 168, MRID 474449-11 (Vaughn et al. 2008); 

Page 29, RPN 07-262 (Vaughn et al. 2008)

3. According to Hibbard et al. (2009), Y(E) varies between 5-25%. SE(Y(E)) was estimated from 

Onstad et al. (2006); SE(Y(E)) is positively correlated with Y(E), so SE(D) is positively correlated 

with Y(E), and the tabulated SE(D) are lower bounds.

Charge Question 6 

Northern and western corn rootworm studies have shown that male emergence in 5% seed 

blends can be variable and may be up to 60 times lower compared to emergence in non-Bt plots 
50

(Data submitted by Monsanto). This information was not included in any of the models used in 

the SmartStax seed blend analysis. The SAP (2009) concluded that a reduction in the number of 

males from Bt seed blends could have a negative impact on the effective refuge. BPPD is 

concerned with the potentially negative effects a reduction in male emergence might have on 

product durability.

Please comment on the potential effects of lowered male emergence of Northern and Western 

corn rootworm on the durability of the seed blend and whether this information should be 

incorporated into the risk assessment.

Panel Response

The Panel expressed concern for the potential effects of lowered male emergence in SmartStax 

RIB 5% seed mixture fields. The Panel recommended further studies to gather additional data on 

male andfemale emergence ratios. The impacts of beetle fitness and emergence timing on mate 

selection should also be considered in potential mating scenarios as described by Kang and 

Krupe (2009) since the fitness of female and male adults may influence mate selection. 

Further, the Panel advised EPA to evaluate male and female emergence ratios in all species, 

behavioral traits (WCR variant and NCR diapause-resistant) and WCR pesticide resistant 

populations in field trials with naturally-occurring populations and in field trials that were 

artificially-infested with corn rootworm.

Adult male emergence counts, beetle weight and emergence timing data provided to the Panel

were limited and highly variable. Based on these data, the Panel made the following 

observations: males emerging from SmartStax RIB 5% seed mixture may be less fit and not as 

proficient in mating with females as males that emerged in non-seed mix fields. The loss of 

males and reduced fitness in the 5% seed mixture fields could mean that there would be fewer 

than expected males present when the receptive females are emerging. The Panel also examined 

whether adults emerging from 100% SmartStax plots experienced any development delays. 

Emergence timing data reflects temporal separations in adult emergence in the SmartStax RIB 

5% seed mix fields versus non-Bt corn fields, although some adults emerging from 100% 

SmartStax plots did not experience any developmental delays. The later data does not support 

the hypothesis that larvae that survive on Bt plants experience emergence delays resulting in a 

bimodal distribution from the seed mix plots. Genetic mechanisms may be involved in the 

survivability of Bt feeding larvae, but supporting data are not currently available to determine 

survival mechanism(s) with little or no apparent loss of fitness. Based on the data provided it is 

not clear if the temporal separations in adult emergence in the SmartStax RIB 5% structured 

refuge fields are a result of feeding on Bt plants or other factors.

The mixing of emerging adults from refuge plants and SmartStax plants and mate selection may 

be more important than male/female ratio. A Panel member reported previous studies have 

shown that CRW move about 17 meters per day (Coats et al. 1986). Adult males typically do not 

engage in long-range dispersal (Tollefson and Coats, unpublished), but female adults have been 

shown to travel as far as 25 miles in long distance flights (Coats et al. 1986). In addition, 

juvenile hormone plays a role in female adult long distance movement behaviors (Coats et al. 
51

1987). Panel members indicated timing of emergence of males and females from Bt and non-Bt 

is also important to mating of any resistant and susceptible adults.

Part B: Modeling of Resistance Evolution

Charge Question 7

The durability of the proposed 5% SmartStax seed blend strategy was compared to the durability 

of a 5% structured refuge for lepidopteran and corn rootworm target pests. Monsanto developed 

a deterministic three locus model for ECB/SWCB and Dow created a stochastic two locus model 

for CRW. Separate analyses were conducted using EPA/ORD’s two locus and three locus 

deterministic, probabilistic model to estimate the risk of resistance evolution with a 5% seed 

blend and structured refuge. The applicants and EPA/ORD each made conservative 

assumptions, though of differing degrees, for parameters determined to be sensitive in the 

models. For example, more conservative initial resistance allele frequencies and fitness 

assumptions significantly lowered the time to resistance in EPA/ORD’s model for ECB and 

SWCB. In Monsanto’s modeling of ECB and SWCB, a greater degree of dispersal between 

compliant and non-compliant fields significantly affected the estimated time to resistance.

Please comment on the appropriateness of the assumptions and inputs used for the following 

parameters in the Monsanto, Dow, and EPA/ORD models:

• Initial resistance allele frequency for single traits Cry1A.105, Cry2Ab2, Cry1F, 

Cry34/35Ab1, and Cry3Bb1 for all modeled pests;

• Survival/fitness for all modeled pests; and

• Dispersal for ECB and SWCB as modeled by Monsanto and EPA/ORD. 

Panel Response

The Panel pointed out the importance of being mindful of the basic differences in model 

structure among the Monsanto, Dow, and EPA/ORD models because these differences influence 

whether certain parameters are present in the model and the range of potential durabilities that 

can be model outcomes. Assumptions about model structure are in many respects more critical 

than the parameter values themselves. Model structure involves the equations that characterize 

dynamic processes, embody assumptions about causality, and specify the parameters that could 

be quantified. The assumptions and data used to estimate parameter values are subsequent to 

choices about model structure. In relation to the discussion of the parameter values specified in 

the Charge Question, the Panel addressed structural issues related to the parameters as well as the 

estimation of the parameter values themselves. An additional discussion about model structure is 

also available in the response to Charge Question 9. 

1) Initial Resistance Allele Frequency for the Single Traits (Cry1A.105, Cry2Ab2, 

Cry1Fa, Cry34/35Ab1 and Cry3Bb1) for all Modeled Pests (all models). The Panel 

concluded that the initial resistance allele frequencies chosen for Cry1A.105, Cry2Ab2, 

and Cry1Fa were appropriate for both ECB and SWCB as “best guesses” in the face of 
52

limited information. Therefore, some are possibly overestimates and hence may

underestimate durability. For WCR and NCR, the initial resistance allele frequencies for 

Cry34/35Ab1 and Cry3Bb1 were probably underestimates of the actual values and 

probably overestimate durability. For WCR and NCR, the Panel suggested that a 

different model structure may be more appropriate for assessing resistance development

because resistance will likely be determined by multiple loci acting with small effects, 

not a single major locus. In this situation a quantitative genetics model might be a better 

model structure. The Panel noted that there is likely to be substantial geographical 

variation in allele frequencies, and therefore initial resistance allele frequency should not 

be treated as a single value for a species. Because resistance could develop locally from 

initially high local allele frequencies and then spread geographically, worst-case 

scenarios must be used that assume high initial allele frequencies.

2) Cross-Resistance Potential (Monsanto and EPA/ORD Models). Cross-resistance is 

a special case of the more general problem of estimating survival/ fitness values, and 

the Panel focused on the Lepidopteran-active toxins because cross-resistance may 

greatly reduce durability of SmartStax for targeted lepidopterans. In general, there is 

considerable evidence to support hypotheses of cross-resistance, especially between 

Cry1A.105 and Cry1Fa in many insect species. The Panel concluded that there is some 

evidence of partial cross-resistance of Cry1A.105 and Cry2Ab2 with a Cry1Fa 

resistance allele in ECB. The Monsanto and EPA/ORD models assumed that there 

would be no cross-resistance and consistently treated Cry1A.105 as a unique protein 

involving a novel mode of action with no cross-resistance between Cry1A.105 and each 

of its component toxins, Cry1Fa and Cry1Ac. The assumption of no cross-resistance 

would overestimate the durability of SmartStax. The Panel agreed that the potential for 

cross-resistance should be considered in any model. 

3) Survival/Fitness for all Modeled Pests. The Panel concluded that the fitness values 

used in the ECB, SWCB, WCR, and NCR models were underestimated for the 

heterozygotes. For ECB and SWCB potential epistasis among resistance loci were 

insufficiently examined. The Monsanto and EPA/ORD models ignored densitydependent mortality and complex selection associated with corn kernels. In addition, 

the Monsanto model did not model between-plant movement of larvae in a way that 

addresses the risks to resistance evolution. The Panel noted that none of the parameters 

for a quantitative genetics model for WCR or NCR have been estimated. All of these 

factors will overestimate the durability of SmartStax.

4) Dispersal for ECB and SWCB as Modeled by Monsanto and EPA/ORD. The 

Panel concluded that adult dispersal for ECB and SWCB would likely randomly mix 

the adults within fields of SmartStax seed mixtures. Nonetheless, this does not rule out 

the possibility of non-random mating; for example, timing of adult emergence could 

lead to resistant individuals having a greater chance of mating with each other. 

Furthermore, there could be non-random oviposition in seed mixtures; for example, 

damage to non-Bt plants could increase relative oviposition rates on Bt plants. In these 

examples, non-random mating and oviposition would speed resistance evolution. 
53

Therefore, seed mixtures do not rule out the possibility that non-random mating and 

non-random oviposition decrease durability.

The Broader Philosophical Context of Modeling

Before addressing the specific issues related to the model parameters, the Panel made 

philosophical comments about mathematical models. All models are idealizations and 

abstractions of reality. They pull from the world those processes we already think are important 

and provide conceptual landscapes to structure our thinking, let dynamic complexity play out, 

and allow us to make judgments and predictions about what we will find in the world. It is 

sometimes naively assumed that if we get the parameters right, then what we want to know about 

the world will just fall out—that parameter values alone are what drives uncertainty about the 

processes the models are designed to capture. This belief is a fallacy that leads to 

overconfidence in the output of models.

The Panel pointed out that it is important to be mindful of the idealistic nature of models. That is 

why the Monsanto, Dow, and EPA/ORD models differ in fundamental ways. There is a growing 

body of literature from philosophers of science examining how simulation is being used to make 

scientific inference. These considerations are important in understanding how to interpret the 

Monsanto, Dow, and EPA/ORD models, and the context for their proper use. To illustrate the 

philosophical issues surrounding modeling, consider the following questions:

1) Why do the frequency-only models provided by Monsanto, Dow, and EPA differ? The 

models are ostensibly targeting the same processes. 

2) Why are different results obtained? It is more than the choices of parameter values 

alone. Differences arise from how the models picked out the abstractions and 

idealizations that they wanted to capture. This is fine, and pluralistic approaches are 

being touted as the way to study complex systems. William Wimstatt has developed 

this into the formal conceptual framework he calls ‘robustness,’ the idea that truth 

emerges from agreement between independent false models (Wimstatt 2007). Richard 

Levins made a similar point over four decades ago (Levins 1966). Nonetheless, from a 

practical point of view of designing IRM strategies, there needs to be a formal way of 

adjudicating among models when they give different results. 

3) Why does each model assume that there is a single locus with alleles coding for 

resistance to single toxins? A strong case can be made for such model structure when 

dealing with cultivars that have a high concentration of toxin relative to the pest’s 

susceptibility. This may be true for ECB, but it is clearly not the case for the CRW. 

For CRW, these models may be less appropriate than quantitative genetic models.

4) How can we assess the models in detail, as opposed to having to simply trust the 

modelers? The Panel made suggestions to make the models more transparent, and 

more easily interpreted. First, using peer-reviewed models would have helped provide

assurance that the models had been properly evaluated by independent modelers. 

Second, testing the model using bridge principles is important to assure evaluators that 

the models were capturing relevant processes in the right way. For example, a model 
54

should be able to be run under the assumptions of standard population-genetics theory 

to obtain known theoretical results. Testing these results would have been very helpful 

and provided a bridge to the more complex cases the model was targeting. Peck (2008) 

has examined the philosophical underpinnings of simulation models and concluded that 

simulations can be a vital tool for understanding complex ecological questions, but they 

require more work than other types of models. The Monsanto, Dow and EPA/ORD 

models were hard to compare because the Panel was not given access to their testing 

and algorithms. 

Initial Resistance Allele Frequency for Single Traits Cry1A.105, Cry2Ab2, Cry1Fa, 

Cry34/35Ab1, and Cry3Bb1 for all Modeled Pests

ECB and SWCB

The Panel concluded that the estimated initial allele frequency values chosen for Cry1A.105, 

Cry2Ab2 and Cry1Fa may be overestimates (conservative values) of the actual ECB and SWCB 

values. However, the data provided to the Agency and in the published literature to estimate 

initial allele frequencies for each toxin are sparse except for Cry1A toxins. Therefore, the Panel 

acknowledges that this conclusion is necessarily tentative, and the values used for initial allele 

frequencies should be regarded by EPA as guesses rather than science-based estimates.

Virtually no data exist to support the estimated values used for the initial frequency for resistance 

to Cry2Ab2. If we assume that resistance to Cry2Ab2 is similar to that for Cry1Ab in ECB and 

SWCB, then we would also assume that initial resistance allele frequencies were similar. If we 

assume that resistance to Cry2Ab2 is rare in ECB and SWCB, then we should also allow that 

there could be significant cross-resistance between the Cry1A.105 and Cry 2Ab2 toxins. 

There are few data available to support the estimated values used for the initial frequency for 

resistance to Cry1Fa. Some unpublished information indicates that ECB may have resistance 

alleles to Cry1Fa at frequencies considerably above 0.001. Thus, the values used in the models 

may be conservative, or they may be near or below the actual resistance frequency (for Cry1Fa). 

There are no data for SWCB, so we must rely on the assumption that resistance allele 

frequencies are similar in both species. 

The Panel pointed out that there could be substantial geographical variation in initial resistance 

allele frequencies for all toxins and all pests. This raises the concern that resistance could evolve 

locally and then spread to other geographical areas. Therefore, for IRM a worst-case scenario 

(i.e., the highest frequency of resistance alleles) should be adopted, because if this worst-case 

occurs in one geographical location, it may determine the overall durability of SmartStax (see 

also Charge Question 9). The Panel cautioned against assuming that there is one value for the 

frequency of resistance to a given toxin in a given target pest.

WCR and NCR

The Panel stated that the estimated resistance allele frequencies for Cry34/35Ab1 and Cry3Bb1

used in the models were not overestimates of the expected values. This is because the Bt toxin 
55

concentrations in the corn cultivars being examined are low relative to susceptibility of WCR 

and NCR. In this situation resistance will likely be determined by multiple loci acting with small 

effects, not a single major locus (as assumed), and a quantitative genetics model might be more 

appropriate for assessing resistance development. For lepidopteran species examined for 

resistance to high dose events, high frequencies or multiple loci of low-level resistance that could 

be effective against low-dose events have been located. NCR and WCR have not been examined 

in this way, but the rapid development of a resistant colony of WCR to mass-selection by 

Cry3Bb1 on a rather small effective population size implies that resistance is common in this 

species. 

The Panel suggested that a starting point for a quantitative genetics model might be to assume 

that there are many alleles with small effects that exhibit additive gene action for resistance to 

either Cry34/35Ab1 or Cry3Bb1. Under these conditions, assumptions about the initial additive 

genetic variance for each trait, the initial mean value for the trait and any genetic interactions 

between the genes associated with Cry34/35Ab1 and Cry3Bb1 would have a significant effect on 

the rate of resistance evolution. In general, the initial allele frequencies for quantitative loci with 

small effects will be substantially higher than that assumed by the models where there is a 

resistance allele at a single locus for each toxin, and evolutionary rates may be considerably 

faster.

Uncertainties Regarding Cross-Resistance Potential between Cry1A.105 and Cry1Fa

Based on the provided materials, the Panel concluded that the overall cross-resistance potential 

of the chimeric CryA.105 protein remains unclear with regard to the Cry1Fa toxin. The Panel 

agreed that the potential for Cry1Fa and Cry1A.105 cross-resistance should be considered in any 

model. Data presented in the review materials indicate there is some level of partial crossresistance in the major allele for Cry1Fa resistance in ECB to both Cry1A.105 and Cry2Ab2 (see 

discussion below). In the MON 89034 BRAD, EPA indicated that additional information was 

needed concerning the cross-resistance potential of Cry1A.105, Cry1Fa and Cry1Ac (US EPA 

2008). When SmartStax was registered in 2009, further analysis of the possibility of crossresistance was required as a term and condition of the registration (US EPA 2009a). The Panel 

received no subsequent materials regarding this issue and could not discern how Monsanto and 

Dow were addressing this registration requirement. A response to BPPD requests for additional 

information was compiled and reviewed for CRW modeling in the July 1, 2009 IRM review 

document, but did not include new data regarding cross-resistance potential of the Cry1A.105 

and Cry1Fa toxins (BPPD 2009b). The Panel agreed that the potential for Cry1Fa, Cry2Ab2

and Cry1A.105 cross-resistance should be considered in any model. The IRM models provided

to the Panel assumed that there would be no cross-resistance and consistently treated Cry1A.105

as a unique protein involving a novel mode of action with no cross-resistance between 

Cry1A.105 and each of its component toxins, Cry1Fa and Cry1Ac. Additional discussion on 

cross-resistance is found in the Panel’s response to Charge Question 8. 

Indications from Plutella xylostella

There are many species that have evolved resistance to Bt, but the most well-studied is. Plutella 

xylostella L. (diamondback moth, DBM), As much as known about DBM resistance, there is 
56

much still unknown about the mechanisms and molecular genetics of insect resistance to the 

various Cry toxins. DBM was the very first species to evolve resistance to Bt in the field, and 

the species that causes by far the most geographically widespread control failures. DBM is a pest 

in crucifers which is subject to intensive selection in the field as chemical pesticides and Bt 

sprays are applied frequently. DBM evolved resistance rapidly to chemical pesticides and it was 

no surprise that DBM would be the first species to evolve resistance to Bt in the field. Tabashnik 

et al. (1997) discovered that a single resistance gene in DBM conferred high levels of resistance 

to four Bt toxins, Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F. Despite considerable effort, the 

mechanism of DBM resistance to Bt still remains unresolved (e.g., Baxter et al. 2008). The 

Panel considered the possibility that a single resistance gene may confer resistance to multiple Bt 

toxins in other insect species, but has not yet been selected strongly enough to be found.

On the other hand, various studies suggesting that insensitivity to Cry toxins may not evolve via 

a single genetic mechanism (Baxter et al. 2005; Heckel et al. 2007). Consistent with the findings 

of Tabashnik et al. (1997), Baxter et al. (2008) reported that a single quantitative trait locus 

(QTL) confers resistance to Cry1A toxins in the DBM, but that it segregated independently of 

genome positions that encode the known glycoprotein receptors of Bt toxins. Similarly, single 

nucleotide polymorphism (SNP) markers for bre5, aminopeptidase N, or cadherin were shown 

not to co-segregate with Cry1Ab resistance in the ECB (Coates et al. 2008). 

Although the DBM is the most persuasive example, there is also evidence for cross-resistance 

between Cry1A and Cry1Ftoxins in other species, such as Heliothis virescens (tobacco 

budworm, TBW) (Gould et al. 1995) that may be due to sharing at least one common receptor

(Jurat-Fuentes and Adang 2001) and H. armigera, Spodoptera exigua (Hernández and Ferré 

2005). Hernandez and Ferré (2005) reasoned that their binding data along with previous binding 

data and observed cases of cross-resistance suggested that this pattern of cross-resistance is

widespread among lepidopteran species.

Three Cross-Resistance Studies

During the meeting, the Panel requested that EPA provide them with additional information

concerning the assessment of cross-resistance between Cry1A and Cry1Fa toxins. 

BPPD IRM Review of the Potential for Cross-Resistance between Cry1Ab and Cry1Fa (US 

EPA 2010a)

One of these documents was the USEPA-BPPD IRM review of the potential for cross-resistance

between Cry1Ab and Cry1Fa toxins based on data provided by Pioneer Hi-Bred International 

(US EPA 2010a, found in SAP docket for this meeting, EPA-HQ-OPP-2010-0772 and also in the 

EPA regulatory docket: EPA-HQ-OPP-2010-0183-0028). Pioneer used a number of approaches 

to investigate Cry1Fa-Cry1Ab cross-resistance including structural comparisons, midgut 

(receptor) binding assays, pore formation analysis, and tests with Cry1Fa-resistant colonies. 

Based on these findings, EPA concluded that cross-resistance is unlikely between Cry1Fa and 

Cry1A.105 in the three species of concern here, the ECB, SWCB, and CEW. 
57

The Panel highlighted several points from the EPA review (US EPA 2010a, quoted below) that 

raised concerns about cross-resistance. 

1) These data largely show that Cry1Ab and Cry1F elicit toxic responses through separate 

pathways. Midgut binding studies identified several shared binding sites in each of the 

targets which could indicate some degree of cross resistance. On the other hand, 

experiments with Cry1F-resistant ECB showed no survival on Cry1Ab. The potential for 

low-level cross-resistance between Cry1F and Cry1Ab was addressed by exploring a 

range of values using a sensitivity analysis with the simulation model. (page 2)

2) Results from the Cry1F competition for Cry1Ab binding were similar for ECB, SWCB, 

and CEW (described in Appendix 1 of MRID# 480056-01). In all cases, the presence of 

Cry1F did not inhibit Cry1Ab binding with BBMV [brush border membrane vesicles] 

proteins except at the highest concentrations of competitor. About 20% (ECB and CEW) 

to 40% (SWCB) competition (reduction in Cry1Ab binding) was observed at the higher 

concentrations of Cry1F. Conversely, homologous competition (Cry1Ab/Cry1Ab) 

showed approximately 90% less 125I-Cry1Ab binding at the maximum concentrations. A 

different pattern of results was observed with the reciprocal Cry1Ab competition for 

Cry1F (Alexa-Fluor marked) binding. Heterologous competition with Cry1Ab reduced 

Cry1F binding by approximately 70% (ECB) and 50% (SWCB). (The CEW binding tests 

were deemed unreliable because of high non-specific binding in the Cry1F/Cry1F 

homologous competition.) The high level of competition shown in the ECB and SWCB 

tests suggested that at least some Cry1F receptors are shared with Cry1Ab. (page 5) 

3) However, for Cry1F-resistant ECB, the presence of Cry1F did not significantly inhibit 

Cry1Ab binding indicating that the shared binding site was lost (modified to no longer 

recognize Cry1F). Since homologous competition (Cry1Ab-Cry1Ab) still existed with the 

resistant colony, it is likely that an unshared Cry1Ab binding site remained viable. (page 

5-6)

4) Cry1F competition assays for Cry1Ab binding sites showed that two classes of receptors 

are found in ECB, SWCB, and CEW: one that is recognized by Cry1F and a second that 

is not recognized by Cry1F. (page 7)

5) Separate research conducted by Pereira et al. (2010) found that Cry1F recognizes 

binding sites of 120, 200, and 250 kDa in ECB and that BBMV binding was not reduced 

with ECB selected for Cry1F resistance. The authors suggested that the mechanism for 

Cry1F resistance in ECB is specific and unrelated to Bt toxin resistance mechanisms 

observed in other insects (e.g., reduced BBMV binding, increased activity of gut 

proteases). BPPD also notes that competitive binding studies conducted with a separate 

insect, diamondback moth (Plutella xylostella), showed some competition between Cry1F 

and Cry1Ab for a common binding site, but also that Cry1Ab binds to separate sites 

independent of Cry1F (Granero et al. 1996). (page 11)

Ultimately, the Panel was not as certain as EPA that there is a low likelihood of cross-resistance 

between Cry1A.105 and Cry1Fa for ECB and SWCB and concluded that predictions of crossresistance remain uncertain. The Panel cited other studies that also indicated cross-resistance 
58

between these toxins. For example, the DBM does show cross-resistance between Cry1Ac and 

Cry1Fa (Tabashnik et al. 1997) and showed some competition between Cry1Fa and Cry1Ab for 

a common binding site (Granero et al. 1996). In fact, there have been several binding studies for 

different insects, including DBM, ECB and TBW, that showed Cry1Fa and Cry1Ab/Cry1Ac 

share a common binding receptor as well as unique binding receptors (Denolf et al. 1993; Hua et 

al. 2001; Jurat-Fuentes and Adang 2001; Hernández and Ferré 2005). Members of the Panel

would not exclude the possibility that a mechanism of resistance as found in DBM (i.e., one

binding receptor modification conferring resistance to multiple Bt toxins) exists in one or more

species including ECB, SWCB, and CEW. The Panel remained unconvinced that there was a 

low likelihood of cross-resistance based on the studies conducted with the Cry1Fa-resistant 

colony. Finding that Cry1Fa resistance did not confer resistance to Cry1Ab might be due to 

multiple pathways to resistance (Baxter et al. 2005; Heckel et al. 2007).

MON 89034 BRAD (US EPA 2008)

The Panel considered a second EPA document, the MON 89034 Biopesticides Registration 

Action Document (BRAD) (US EPA 2008). The BRAD contained a detailed review of the 

likelihood of cross-resistance between all three toxins (US EPA 2008). The Panel observed that 

BPPD had concerns about the potential for cross-resistance between Cry1A.105 and Cry1Fa that 

were not resolved. Monsanto developed a corn event, MON 89034, which produces the 

Cry1A.105 and Cry2Ab2 insecticidal toxins to protect against corn stalk-boring pests such as 

ECB, SWCB, and CEW. MON 89034 is one of four events, MON 89034 x TC 1507 x MON 

88017 x DAS 59122-7, used to create SmartStax. As detailed in the BRAD (US EPA 2008), 

Cry1A.105 is a chimeric protein consisting of domains I and II and the C-terminus of Cry1Ac 

and domain III of Cry1Fa with overall amino acid sequence identity to the Cry1Ac, Cry1Ab and 

Cry1Fa proteins of 93.6%, 90.0% and 76.7%, respectively. The X-Ray crystallography studies 

demonstrated that there was high main chain structural similarity between the modeled crystal 

structures of Cry1Aa, Cry1Ab, Cry1Ac, and Cry1A.105 proteins (US EPA 2008). Such close 

similarity causes a heightened interest in examining the potential for cross-resistance between 

Cry1Ac and Cry1Fa and Cry1A.105 toxins. In their review, BPPD concluded that crossresistance was a real possibility for these toxins. The Panel agreed with this conclusion and 

highlighted two major points of interest from the BRAD to support this conclusion. 

1) Competition binding experiments indicate a common receptor. BPPD presented 

several lines of evidence that indicated that Cry1Fa and Cry1Ab/Cry1Ac share a common 

binding receptor although each of these proteins has unique binding receptors as well 

(Denolf et al. 1993; Hua et al. 2001; Jurat-Fuentes and Adang 2001; Hernández and 

Ferré, 2005). (US EPA 2008, discussion on pgs. 79-80). Specific competition binding 

experiments using brush border membranes from ECB demonstrated that Cry1F shares a 

binding site with Cry1Ab/Cry1Ac, though the level of cross-resistance between Cry1F 

and Cry1Ab/Cry1Ac is not as strong as Cry1Ab vs. Cry1Ac.” Hernández and Ferré’s 

conclusion (as cited in the BRAD) was

...in the case of corn, primary pests susceptible to Cry1Ab and Cry1Fa, such as ECB 

(and SWCB and CEW), would necessitate the importance of establishing the binding site 
59

model for this species in order to develop an appropriate resistance management 

strategy” (Pages 76-77). 

2) ECB-resistant colonies. Cross-resistance studies using ECB resistant colonies indicated

that Cry1Ab-resistant ECB were partially resistant to Cry1F although Cry1F-resistant 

ECB were not cross-resistant to Cry1Ab and only slightly resistant to Cry1Ac. EPA 

noted that similar trends have also been shown with TBW (see Head and Storer 2008).

BPPD’s conclusions stated in the 2008 BRAD were:

“In the case of corn, primary pests susceptible to Cry1Ab and Cry1Fa, such as ECB (and 

SWCB and CEW), would necessitate the importance of establishing the binding site 

model for this species in order to develop an appropriate resistance management 

strategy” (Pages 76-77).”

And

“Cry1F can be considered partially cross-resistant to Cry1Ab and Cry1Ac. The 

availability of binding sites may explain the partial cross-resistance: Cry1Ab and Cry1Ac 

could have more different sites to bind with than Cry1F so that resistance to Cry1F still 

allows for some binding of Cry1Ab or Cry1Ac.” (Page 91). 

The Panel concluded that the issue of cross-resistance between Cry1A.105 and Cry1Fa was left 

unresolved in the BRAD. 

SmartStax IRM Plan (MRID 474449-11, Appendix 1)

The Panel considered a third document, the SmartStax IRM plan submitted to BPPD by 

Monsanto and Dow (MRID 474449-11, entitled “Insect resistance management plan for MON 

89034 x TC1507 x MON 88017 x DAS-59122-7 (Head and Storer 2008). Appendix 1 (Schlenz 

et al. 2008) contained a summary of a study that examined the response of Cry1Fa-resistant (and 

otherwise susceptible to Cry1A.105 and Cry2Ab) and Cry1Fa-susceptible (a colony susceptible 

to all three toxins) ECB and fall armyworm (FAW) colonies to Cry1A.105 and Cry2Ab2. Slopes 

of the concentration-mortality relationship were estimated from data provided in this study and 

are shown in Table 7-1. The statistical error associated with these slope estimates was not 

reported. However, the variation in mortality among replicate experiments was small for about 

1/3 of these experiments (MRID 474449-11, Appendix 1, Pages 45-47 Schlenz et al. 2008)), so 

perhaps these estimates are not too variable. 

Resistant FAW and ECB had lower slopes than susceptible colonies when exposed to Cry1Fa

(Table 7-1). This may be expected, because the resistant colonies should be able to tolerate 

Cry1Fa better than the susceptible colonies. Unexpectedly, the resistant FAW and ECB colonies 

also had lower slopes than the susceptible colonies when exposed to either Cry1A.105 or 

Cry2Ab2 (Table 7-1). Results of this study indicate partial cross-resistance in the Cry1Fa 

resistance allele in ECB against Cry1A.105, Cry1Fa-resistant ECB had a resistance ratio of 133, 

and against Cry2Ab2, they had a resistance ratio of 12. In addition, growth inhibition was 
60

significantly reduced on Cry1A.105 and slightly, but not significantly reduced on Cry2Ab2. 

These data suggest that partial cross-resistance occurs with a Cry1Fa resistance allele in ECB. 

To evaluate the potential cross-resistance to enable realistic modeling, the concentrationresponse relationship for the Cry1Fa RS heterozygote on Cry1A.105 and Cry2Ab2 is essential. 

In addition, it is critical to evaluate survival of these Cry1Fa genotypes on plants expressing 

Cry1A.105 and Cry2Ab2, together and separately. However, the absence of an estimated 

statistical error makes any inference about cross-resistance uncertain. The Panel recommended a

full statistical analysis of the concentration-mortality data.

Table 7-1. Partial cross-resistance in Cry1F resistance allele in ECB against Cry1A.105 and 

Cry2Ab2. Data from MRID 474449-11, Appendix 1, (Schlenz et al. 2008, p. 44). Slopes and 

GI50 and LC50 were estimated with a probit model.

Insect 

Strain1

Protein 

Toxin

 Growth Inhibition2 Mortality2

Slope GI50 (95%CI) ng/cm2 Slope LC50 (95% CI) ng/cm2

s-ECB Cry1A.105 0.00 <1.5 1.10 6.1 (3.9-8.9)

r-ECB Cry1A.105 0.89 5.9 (2.2-15.8) 0.64 813 (488-1478)

s-ECB Cry2Ab2 0.99 27.1 (14.6-50.4) 1.56 595 (451-784)

r-ECB Cry2Ab2 1.09 39.3 (22.3-69.4) 0.62 7206 (3476-20108)

1. s-ECB is a colony susceptible to Cry1Fa, r-ECB is a colony resistant to Cry1Fa.

2. s-ECB and r-ECB are significantly different when the 95% CIs do not overlap.

Survival/Fitness for all Modeled Pests

ECB and SWCB

The fitness values used for the ECB/SWCB models include assumptions about survival that 

likely lead to overestimates of durability. These models assume low heterozygote survival and 

independent fitness of each Cry toxin (multiplicative fitness), and ignore density-dependent 

larval mortality, general knowledge about ECB larval movement, and complex selection in the 

ear of non-Bt plants in seed mixes. In a few cases, non-multiplicative fitness (epistasis) was 

considered, but this was not done in a realistic way or in a way that would lead to underestimates 

(conservative estimates) of durability. In addition, the Monsanto model did not model betweenplant movement of larvae in a way that addresses the risks associated with that movement.

Theoretical Considerations

Assuming that the initial frequencies of Bt resistance alleles are below 0.001, the single most 

influential parameters in the ECB model are those associated with the fitness values of 

heterozygous individuals. This can be seen by examining a single-locus model for a single Bt

toxin. Although SmartStax has multiple stacked toxins, the same principles of resistance 

evolution derived from single-locus models are still relevant.
61

The evolutionary dynamics of a single-locus, high-dose system can be approximated by 

for complete adult dispersal, where p is the frequency of the resistance allele, Q is the proportion 

of refuge, L is the survival of RR in the Bt field (typically set to 1), F is the fecundity per capita 

in the Bt field, and h is the degree of dominance (Ives and Andow 2002). This is a second-order 

approximation, and assumes that survival of SS homozygotes on Bt plants is very low. The lefthand side of the equation is the rate of change of resistance allele frequency. The proportion of 

Bt is represented by (1-Q) and is always between 0 and 1. The reproduction of RR homozygotes 

in Bt fields is represented by LF. Both (1-Q) and LF are always much greater than p. 

Dominance h is the main factor that determines the rate of resistance evolution, because except 

when resistance failure is approached (p > 0.01), the magnitude of the right-hand side of the 

equation is dominated by the value of h. Specifically, h will be greater than p during most of the 

time of resistance evolution, and therefore dominance will be the most significant factor driving 

the rate of resistance evolution.

These concepts generalize to the multi-locus case when all genes are high-dose. The term (1-Q) 

will continue to exert its role. LF must be generalized to encompass a multi-dimensional fitness 

differential, and h must be generalized to encompass multi-dimensional heterozygote fitness. 

This generalized h will determine the rate of the evolutionary process if it is much larger that the 

corresponding R allele frequencies.

The Monsanto, Dow, and EPA/ORD analyses did not include detailed explorations of epistasis in 

the multi-locus fitness interactions that affect dominance h. A few cases were modeled with 

epistasis with the EPA/ORD model. However, the types and magnitudes of epistasis considered 

were not worst-case scenarios, and therefore they may overestimate durability compared to the 

predictions that would be made if realistic epistasis were considered (see also Charge Question 

8).

Empirical Considerations

The fitness of heterozygotes for ECB and SWCB was assumed to be only 2-5 fold (2x and 5x) 

that of the susceptible genotypes, in comparison to the assumption in Dow’s CRW model that 

heterozygotes are 25-fold more tolerant of the Cry proteins than susceptible homozygotes. In the 

absence of RS genotypes, one could look to the only widespread field resistance to Bt, in the 

DBM, where heterozygotes seem to be at least 5X more tolerant than susceptible larvae (Tang et 

al 1997). If the survival of heterozygous ECB and SWCB were higher than 5-fold the survival 

of susceptible homozygotes for all toxins, then resistance of ECB and SWCB would evolve at a

much higher rate than that predicted by the Monsanto and EPA/ORD models and durability 

would be overestimated.

In addition, as explained in more depth in the Panel’s response to Charge Questions 8 and 9, the 

fitness values of larvae that move from plant-to-plant are determined from a restricted set of 

assumptions. The values used in the models do not seem to reflect the empirical data from Davis 

(1 Q)LF( p h)

p

p = − +∆
62

and Onstad (2000), and general knowledge about ECB larval movement is not considered (see 

response to Charge Question 2).

Because some ECB feed within corn ears, they, like CEW larvae (see the response to Charge 

Question 4) may be feeding on kernels that have zero, one, two, or three toxins, or if kernel 

tissue from several kernels is mixed during feeding (such as for later instars), some average of 

these. This could allow survival of heterozygous larvae. The toxicity of these kernels to larvae 

is not known, nor is the fraction of ECB larvae that feed on kernels. This is an empirical 

question in need of further experimental data.

Because the SmartStax RIB 5% seed mixture approach under consideration will use the same 

corn plants as refuges for corn rootworm and lepidopteran pests, it is important to recognize that 

if there is damage to the refuge plants from rootworm, these plants may grow more slowly than 

the plants that are protected from rootworm. Similarly, in years when ECB first generation 

damage to whorl stage corn is high, the damage to refuge plants could make them less robust 

than the Bt plants. By the time of the second generation of ECB moths and CEW moths are 

flying and choosing host plants, the refuge plants may have been crowded out by the faster 

growing Bt plants and would not be attractive to ovipositing moths. This could significantly 

decrease the effectiveness of non-Bt plants to serve as a refuge.

WCR and NCR

The fitness values used for WCR and NCR will likely lead to overestimates in durability. 

Theoretical Considerations

The EPA/ORD model appears to assume that the default value for h should be 0.05, with the 

minimum at 0.01 and the maximum at 0.2. These assumptions would be theoretically reasonable 

if the corn plants provided a classically defined high dose for the NCR and WCR, but this is 

clearly not the case. Tabashnik et al. (2004) among others have pointed out that there is no 

reason to assume effective recessiveness of any Bt resistance allele unless the toxin is at a high 

dose. Some Bt resistance alleles have been found to have a recessive phenotype in 

concentration/mortality or concentration/growth assays (Gould et al. 1995), while others are 

additive or dominant (Gould et al. 1992, 1995). Given the dose in the current corn cultivars, the 

effective recessiveness of a resistance allele in the field is likely to reflect 

concentration/mortality and concentration/growth assay results. This leads to a default h of 

about 0.5. Fig. 7.1 A-D demonstrates how sensitive outcomes of the model are to changing the 

value of h when using the Monsanto and Dow data for mortality of susceptible larvae on the one 

and two toxin plants. This figure is based on output of a simple two-locus model from Jongsma 

et al. (2010) that gives reasonably parallel results to the EPA/ORD model, which in fact likely 

leads to overestimates of durability (see Charge Question 9).

The Panel noted that the EPA/ORD modelers conducted a sensitivity analysis in which a Beta 

distribution was used to describe the values of h. Although the extremes for h used (0.01 and 

0.2) could be included in the analysis, using a Beta distribution means that many values of h will 

be close to the mode value for h which is 0.05 (see Fig. 7.1A), and therefore the analysis is 
63

weighted towards giving overestimates of durability (see also responses to Charge Questions 2 

and 9). From the EPA document (US EPA 2009b) the Panel thought that Dow used an h value of 

0.32 as their worst-case scenario in the model. Given the moderate toxin dose in the corn 

cultivars for rootworms, an appropriate worst-case scenario would be to use an h of 1.0 and as 

mentioned earlier, a mode of 0.50.

Beyond the issue of h for single alleles, is the consideration that when the dose is moderate any 

allele that confers higher fitness to individuals than that of the population mean will increase in 

frequency. In the long term, field resistance to Bt corn could involve dozens of alleles, each with 

small effects. Quantitative genetic models are best for dealing with such situations.

Empirical Considerations

The Panel examined EPA’s SmartStax IRM review (US EPA 2010b) and determined that the 

Dow model used fitness values for WCR derived from field experiments conducted on test plots 

of the corn cultivars that were planted under ideal conditions of soil fertility and moisture. 

Under these conditions, the mortality values of WCR from larvae to adult were very high on 

each of the single-toxin cultivars and on the dual-toxin cultivar when measured relative to 

mortality on a non-Bt cultivar (Cry3Bb1 = 99.75%, Cry34/35Ab1 = 99.75%, Pyramid = 

99.95%). The mortality was slightly lower when there was an assumption of no densityC.

 Fig. 7.1
64

dependent mortality (Cry3Bb1 = 94.2%, Cry34/35Ab1 = 97.5%, Pyramid = 98.2%). The Panel 

pointed out that the data were given as mean mortality values without the breakdown of the 

variance in mortality values. Some locations in some years might have mortality values that 

were 5-10% lower or higher than the mean. In the response to Charge Question 5, the Panel 

noted the high variance associated with estimated WCR survival and the potential overestimation 

of mortality.

The Dow model depended heavily on these data. Based on these mortality values, the model 

would predict that two toxins inhibit resistance evolution (redundant killing) if there is no crossresistance to the two toxins. The Panel, however, identified situations in which under suboptimal field conditions, the mortality due to single toxins would be slightly lower than the 

values used by Dow in the model and thus resistance would evolve more rapidly, especially if h

is larger than assumed. The Panel surmised that under stressful conditions, the mortality levels 

would be low compared to ideal conditions (toxin expression is affected by the plant’s health), 

and when rootworm populations are high or very low; the mortality caused by the toxins would 

also be affected (see response to Charge Question 5). 

The Panel provided a simple example of how important minor variation in these mortality values 

could be using the Jongsma et al. (2010) model. The question asked was what would happen if 

under field conditions, instead of causing an average of 95.8% mortality, each of the single toxin 

plants caused 90.0% mortality. By comparing Fig. 7.1A and B, a small decrease in mortality could 

cause resistance to evolve in a 40% shorter period of time. The sensitivity analysis run by the 

EPA/ORD modelers would not pick up this effect because of their focus on parameter 

uncertainty using a first-order Monte Carlo simulation with a Beta distribution and their choices 

of minimum and maximum values. This will make the sensitivity analysis of EPA/ORD more 

likely to overestimate durability. The Panel appreciated that EPA/ORD modelers conducted a 

sensitivity analysis and suggested that they conduct a more advanced type of uncertainty analysis 

to differentiate the impacts of each variable in the model on the uncertainty of the overall model 

predictions (e.g., Xu et al. 2008; Xu and Gertner 2010a, 2010b; see responses to Charge 

Questions 2 and 9).

Dispersal for ECB and SWCB as Modeled by Monsanto and EPA/ORD

The Panel concluded that the dispersal values used for adult dispersal for ECB and SWCB were 

probably appropriate. This conclusion is based on the following three factors: (i) the seed 

mixture will result in fine-scale intermixing of Bt and refuge plants on the order of meters, (ii) 

adults move from their natal plants prior to mating or oviposition and (iii) adult stages move 

considerably farther than a few meters. Therefore, it is reasonable to assume that adults will 

disperse far enough and frequently enough that the adult population can be considered randomly 

mixed at the spatial scale of fields.

As noted in the Panel’s response to Charge Question 1, it is not clear what would be conservative 

assumptions, and it is not clear how processes that are not modeled could affect resistance 

evolution. For example, because dispersal is extensive, mating is globally random, and the 

genotypes of offspring are well-mixed on the modeled landscape. However, it is not clear that 

mating is random, either globally or locally. Sexual selection theory posits that females may be 
65

selective about mates (epigamic selection) and males may compete for mates (intrasexual 

selection). ECB females do not distinguish between young virgin males and old virgin males, 

but they tend to mate more frequently with experienced males than virgin males. The mating 

choices by males and females may have significant implications for resistance evolution because 

they imply non-random local mating, but these have not been incorporated into resistance 

evolution models. Such mate-selection models tend to lead to complex evolutionary trajectories 

and outcomes, in contrast to the population genetics abstractions of assortative and disassortative 

mating. 

A second process that is not modeled is oviposition choice. ECB females will not discriminate 

between Bt and non-Bt plants when they are grown in separate fields, but they will discriminate 

between damaged and undamaged plants when they are grown near each other. In seed 

mixtures, Bt and non-Bt plants will be growing side by side. If the non-Bt plants are damaged 

during the first generation, second generation ECB females will discriminate against them, 

resulting in lower than expected oviposition on non-Bt plants (refuge plants), reducing the 

effectiveness of the seed mixture to delay resistance evolution.

Thus, although the parameter values for adult dispersal are probably appropriate, the model may 

be missing important processes such as sexual selection and oviposition choice. As a 

consequence, the parameter values that were used for adult dispersal may have minor influence 

on the rate of evolution compared to the processes that were totally ignored in the model 

structure. Hence, the parameter values used in the models may overestimate considerably the 

durability of the pyramided product, not because the values are wrong, but because the wrong 

model was used. 

Charge Question 8

EPA/ORD encountered challenges in the lepidopteran modeling with partitioning nonmultiplicative interactions that occurred between more than two resistance genes since the 

mortality caused by each locus was not independent. With two gene pyramids this non-additivity 

can be assigned to the single two locus interaction, but in a three gene pyramid there are three 

possible two locus interactions. In the absence of data, this non-additivity was partitioned 

equally among the three two locus interactions. As more than two Bt genes are pyramided, this 

problem will have to be addressed so that resistance evolution in the target pests to these 

products can be more accurately simulated.

Does the Panel have any recommendations for distributing non-multiplicative interactions in 

models to evaluate multi-gene pyramided products?

Panel Response

The Panel expressed concern with the way in which survival to different single toxins is 

combined to calculate genotype-specific survival on the pyramided plants in the models of 

resistance evolution. The Panel emphasized that the way in which survival rates of different 

genotypess are combined has a large impact on the predictions that all models make about the 

durability of pyramided crops. It appeared to the Panel that the Monsanto and EPA/ORD models 
66

combined survival rates in ways that generated low heterozygote survival; thereby,

overestimating durability of SmartStax in all simulations. Information on the survival of the 

genotypes conferring resistance to one or multiple Bt toxins for the target pests of SmartStax is 

not available, causing large uncertainties in the predictions of any model of resistance evolution. 

While theory can suggest ways in which survival to single toxins might be combined to generate 

survival to multiple toxins, empirical information is sparse (Appendix 2: High- and Low-Dose

Scenarios and Larval Movement in Seed Mixtures). 

When there is larval movement among plants in seed mixtures, it is necessary to know the

survival rates of all insect genotypes both before and after movement. Thus, more information is 

needed than just the survival of different genotypes on Bt plants, as emphasized in the previous 

paragraph. The concern from the previous paragraph must be addressed for the two or more 

larval stages before the larvae move and after they move (stage-specific survival rates). In 

addition to generating genotypic survival rates to multiple toxins for each larval stage, the stagespecific survival rates must be combined across stages. These stage-specific survival rates may 

be combined multiplicatively if these survival events associated with each stage are independent. 

All of the models the Panel examined combine the stage-specific survival rates multiplicatively. 

However, while this might be a reasonable assumption in the absence of empirical evidence, 

other possibilities should be explored, and to the knowledge of the Panel, this topic has not been 

investigated theoretically.

Non-multiplicative ways in which the resistance of different loci can combine to determine the 

stage-specific or total immature survival of all genotypes to multiple toxins are forms of 

epistasis. Specific mechanisms by which epistasis can occur include, but are not limited to: 1) 

constitutive, low-level expression of Cry-proteases, 2) developmentally restricted expression of 

low levels of Cry-protease, 3) genes regulating expression of receptor genes, and 4) crossresistance. These and other forms of epistasis should be investigated as mechanisms that cause 

non-multiplicative survival rates of the multiple locus genotypes. The consequences of different 

forms of epistasis on resistance evolution should be evaluated.

Below is a discussion of some non-multiplicative ways in which resistance conferred by multiple 

resistance loci could be combined to determine the survival of heterozygotes. This will be 

discussed mainly in the context of two-locus interactions, but the results should also be 

applicable to three-locus interactions.

Multiplicative Interactions among Resistance Loci 

Multiplicative interactions among resistance loci refer to the situation in which the survivals of 

multilocus heterozygotes are equal to the product of the survival of single-locus heterozygotes. 

To illustrate this, suppose the survival of S1S1 and S1R1 individuals were known for plants 

expressing only Bt toxin 1, and denoted L1 and H1, respectively, with the assumption that R1R1

homozygotes have survival 1. Similarly, suppose the survival of S2S2 and S2R2 individuals were 

known for plants expressing only Bt toxin 2, and denoted L2 and H2, respectively. If interactions 

between locus 1 and 2 were multiplicative, then for plants expressing both toxins the survival of 

S1S1S2S2 individuals would be L1L2, the survival of S1R1S2S2 individuals would be H1L2, the 

survival of R1R1S2S2 individuals would be L2, etc. This multiplicative manner of combining 
67

survival to different toxins assumes that the toxins 

act independently, in the sense that insects must 

survive one and then the other of the toxins, with 

the survival probabilities being independent.

Even when survival rates of heterozygotes to 

different toxins combine independently 

(multiplicatively), the impacts of multiple toxins 

on the rate of resistance evolution do not combine 

in a simple way. In other words, the rates of 

resistance to both toxins for insects on singletoxin Bt plants do not lead to simple predictions of 

the rate of resistance evolution to multi-toxin Bt 

plants. When there are pyramid toxins and 

resistance occurs in the form of diallelic loci, the 

rate of resistance evolution at all loci is likely 

determined most strongly by the most-rapidly 

evolving locus; the most-rapidly evolving locus 

drags resistance at the other loci along with it (see 

Appendix 2: Survival of Heterozygotes). This result must be interpreted carefully, however. 

Evolution of multiple resistance loci to pyramid varieties will always be slower, and generally 

much slower, than the resistance evolution of any one locus separately (i.e., if all other loci are

fixed for resistance).

Non-Multiplicative Interactions among Resistance Loci

All non-multiplicative interactions are a form of epistasis. For the two-locus case, there are five

survival values involving heterozygotes that could influence the rate of resistance evolution (Fig.

8-1, shown in purple bold lettering with an asterix). For the three-locus case, there are 19 such 

values. It is likely that changes to the five values in the two-locus case will have differing effects 

on resistance evolution. Changes to the fitness of the S1S1S2R2 and S1R1S2S2 genotypes (Fig. 8-

1, background is shaded in blue) will likely exert the largest effect on resistance evolution, 

followed by the double heterozygote (S1R1S2R2, Fig. 8-1, underlined). This is because the 

S1S1S2R2 and S1R1S2S2 genotypes occur significantly more frequently than the others when the 

resistance alleles are rare, followed by the double heterozygote. This is not fully borne out by 

the Panel simulations (Appendix 2: Survival of Heterozygotes), in that a 5x increase in the 

survival of the double heterozygote has a similar effect on resistance evolution as a 5x increase 

in either or both of the survival of the S1S1S2R2 and S1R1S2S2 genotypes. This may depend on 

how increases in survival are scaled. 

One possible scaling is the following. First, designate the genotype-specific survival rates on a 

single-toxin plant in the margins of the matrix of survival rates on the two-toxin plant (Fig. 8-1). 

For locus 1 on toxin 1, these are L1, H1 and 1, and for locus 2 on toxin 2, these are L2, H2, and 1. 

These will be called single-toxin survival rates. With multiplicative survival rates, the nine 2-

locus genotypic survival rates on a two-toxin plant are generated by multiplying the respective 

single toxin survival rates. These are displayed in the central 3x3 matrix of Fig. 8-1. For each 

set of single-toxin survival values, Hi can be reformulated as hi + (1-hi) Li, where hi is the 

Fig. 8-1. Fitness of 2-locus 

genotypes, showing multiplicative 

fitness based on the marginal 1-

locus fitness values.

L1 H1 K1

S1S1 S1R1 R1R1

L2 S2S2 L1L2 H1L2* K1L2

H2 S2R2 L1H2* H1H2* K1H2*

K2 R2R2 L1K2 H1K2* K1K2
68

dominance of locus i (the relative survival of the heterozygote). The multiplicative fitness of the 

S1S1S2R2 genotype would be ~ ( (1 ) ) i i i Li L h + − h , and the double heterozygote would be 

( ~ (1 ~ ) ~ )( (1 ) ) i i i i i Li h + − h L h + − h where L is the marginal fitness of one of the SS genotypes, i

denotes one of the loci and i

~ denotes the other. As an aside, a general rule of thumb is when h

<< 0.05, the event is high dose, and when h > 0.1 the event is low dose. Several factors enter 

into the specific determination of dose, which complicate this rule of thumb, so in the discussion 

below we use numerical examples which leave little doubt about whether an R allele is to be 

considered high dose or low dose. 

Epistasis occurs when, unlike above, survival is not multiplicative. One way to add epistasis is 

to multiply h by a scaling constant, e, in the two-toxin survival values, but not in the single toxin 

values. When e ≠ 1, there is epistasis, and when e >1, expression of resistance in the 

heterozygote is increased in the two locus genotypes relative to the single-locus case. The 

S1S1S2R2 genotype survival would be ~ ( (1 ) ) i i i i i Li L h e + − h e and the comparable double 

homozygote survival would be ( ~ (1 ~ ) ~ )( (1 ) ) i i i i i i i Li h + − h L h e + − h e .

Mechanisms Leading to Epistasis 

There are numerous mechanisms that lead to epistasis. Although the form and strength of 

epistasis is ultimately an empirical problem, the degree of epistasis is largely unknown for 

pyramid Bt cultivars. Therefore, considering different potential mechanisms that could drive 

epistasis may shed light on how epistasis will affect resistance evolution. The Panel focused on 

one illustrative mechanism, but others are possible (e.g., resistance based on loci regulating 

cadherin or amino peptidase expression).

1) Constitutive, Low-Level Expression of Cry-Proteases. Several species of Lepidoptera 

are known to have multiple kinds of proteases that are expressed at different times during 

development. For example, Sesamia nonagrioides expresses certain proteases only 

during later instars (Ortega et al. 1996). These proteases are the likely reason for why 

this species is able to tolerate and grow on diets containing Cry toxins. It is possible that 

the higher survivals of later instars of ECB and SWCB are related to the expression of 

late-larval proteases (designated Cry-proteases) that can partially or fully degrade and 

detoxify Cry toxins.

Assume that there is a recessive resistance allele to a Cry1A toxin and another recessive 

resistance allele to a Cry2A toxin at a different locus. Assume the Cry-protease is not 

expressed during the 1st and 2nd instars, is expressed at low levels in the 3rd instar, at 

intermediate levels in the 4th instar, and at high levels in the 5th instar. It would have no 

effect on survival of the 1st and 2nd instars, increase survival of the 5th instar substantially, 

and have intermediate effects on 3rd and 4th instars. 

At low and intermediate levels of expression, the Cry-protease might degrade some, but 

not all Cry toxin, thereby abating some of the effect of the Cry toxin. In doing this, it 

would have little effect on the survival of R1R1R2R2 homozygotes, which can tolerate 

even an unabated high dose of Cry toxin, and would have little effect on the survival of 
69

S1S1S2S2 homozygotes, which cannot tolerate slightly abated Cry toxin levels. However, 

it could cause a significant rise in the survival of heterozygotes. Indeed, survival of the 

double S1R1S2R2 heterozygote could be improved nearly to that of the R1R1R2R2

homozygote.

A hypothesized effect of low expression of the protease is to increase the single-toxin 

survival of the heterozygotes. In the example given in Fig. 8-2B, the single-toxin fitness 

of the heterozygote was increased to 0.2, for which h ~ 0.2. This is clearly not high dose, 

but even so, the survival rate of the single heterozygote on a two-toxin plant (S1S1S2R2 or 

S1R1S2S2) is still very low (2 x 10-5). The fitnesses of the single heterozygote and the 

single resistant homozygotes (S1S1R2R2 or R1R1S2S2) are assumed to be multiplicative, as 

the SS genotype does not confer any level of resistance. The conditional dominance for 

allelic variation in resistance at one locus, given that the other locus is fixed as an SS

homozygote is h ~ 0.2 (Fig. 8-2B). 

Similarly, the fitness of the single heterozygote with a resistant homozygote (S1R1R2R2 or 

R1R1S2R2) is expected to be multiplicative, because the RR genotype is hypothesized to 

contribute maximally to fitness. The conditional dominance for allelic variation in 

resistance at one locus, given that the other locus is fixed as an RR homozygote, is also h

~ 0.2 (Fig. 8-2B). 

Fig. 8-2. Hypothesized epistasis for low to intermediate expression of a late-expressing 

protease that degrades Cry toxins. A. Multiplicative fitness with the marginal fitness of 

each locus high dose (h = 0.029). B. Hypothetical epistasis for 3rd and 4th instar larvae 

with low to intermediate expression of the protease, showing non-high dose marginal 

fitness (shaded, light blue boxes with 0.2 in bold) and epistasis (light red box in the center 

with value 0.08). 

 A. 0.0001 0.003 1

 

B. 0.0001 0.2 1

S1S1 S1R1 R1R1 S1S1 S1R1 R1R1

0.0001S2S2 1 x 10-8 3 x 10-7 0.0001 0.0001S2S2 1 x 10-8 2 x 10-5 0.0001

0.003S2R2 3 x 10-7 9 x 10-6 0.003 0.2S2R2 2 x 10-5 0.08 0.2

1R2R2 0.0001 0.003 1 R2R2 0.0001 0.2 1
70

The double heterozygote (S1R1S2R2) is expected to show epistasis in the presence of low 

to intermediate levels of a Cry-protease. This is because the protease may reduce the 

toxin concentration sufficiently that a single resistance allele allows survival. For 

example, if the resistance allele results in an alteration of a cadherin-like receptor in the 

insect midgut, then a heterozygote may have half the population of receptors that can 

bind Cry toxin. Hence, there are likely to be some concentrations of Cry toxin that are 

sufficiently low that they do not bind in sufficient quantities or for a sufficient period of 

time to induce rapid death in the larva. If the protease can produce such concentrations 

when it is at low to intermediate levels of expression, then the double heterozygote may 

survive as well as the double resistant homozygote. In the example in Fig. 8-2B, there is 

a more modest level of epistasis, with S1R1S2R2 survival of 0.08. These considerations 

result in a conditional dominance with the other locus fixed as an SR heterozygote of h ~ 

0.4, which is approximately additive gene action. One could assume that S1R1S2R2

survival ranges from 0.04 (no epistasis) to 0.2.

The survival matrix in Fig. 8-2B could be incorporated into a resistance evolution model 

by assuming that the Cry-protease is expressed constitutively at low levels throughout 

larval development. Although there is no biological or toxicological evidence either way 

regarding this assumption for ECB or SWCB, it may be relevant for other species, such 

as Helicoverpa sp. 

2) Developmentally Restricted Expression of Low Levels of Cry-Protease. Compared to 

constitutive expression, it is more likely that Cry-proteases are expressed in later instars 

of ECB and SWCB. Thus, they may affect fitness in a seed mixture via plant-to-plant 

movement. Glaum et al. (2011) provided a method for calculating the “effective 

dominance” for resistance for larvae moving in a seed mixture for the single-locus case. 

It is possible to generalize this method for the multi-locus case. “Effective dominance” 

could provide an intuitive understanding for how larval movement coupled with stagespecific survival rates affect resistance evolution. 

3) Cross-Resistance. Cross-resistance is not necessarily modeled using a two-locus model 

with epistasis. However, it can be done, as indicated in Fig. 8-3. Here survival on a 

single toxin (the second toxin) is not affected by genetic variation at the second locus, 

and all survival rates are the same, L2. The survival rate of the first locus on toxin 1 is the 

same as in Fig. 8-1. However, survival on a two-toxin plant depends only on the first 

locus. These are the survival rates for complete cross-resistance.

It is also possible that the second locus influences survival rates on the two-toxin plant 

even though it has no effect on survival on a single toxin plant (the second toxin). This is 

illustrated in Fig. 8-4 in which cross-resistance increases the survival of S1R1S2R2 by α

and S1R1R2R2 by β. (Note: this and the following examples add survival associated with 

epistasis. It may be more reasonable to assume a proportional increase in survival related 

to epistasis, in which case epistasis could be modeled using multipliers).

Partial cross-resistance (see earlier discussion in response to Charge Question 7) has 

several possible meanings; here it is defined as resistance that provides some increased 
71

survival on all of the toxins, but does not recover full survival on any of the other toxins 

in the RR homozygote. One possible parameterization of partial cross-resistance is 

shown in Fig. 8-5A. In this case, the maximum possible survival is L2. Epistasis could 

occur for the same reason as detailed for Fig. 8-4. The effect of locus 2, however, should 

extend to both the S1R1 heterozygote and the R1R1 homozygote. The effect of the 

modifier would be to increase the conditional selective differential and in most cases to 

increase conditional dominance. Finally partial cross-resistance associated with locus 1 

could affect survival of heterozygotes at resistance locus 2, which gives major resistance 

to a different toxin as given in Fig. 8-6. This may be relevant for ECB Cry1Fa resistance. 

Constitutive expression of a Cry-protease at sufficiently high levels could cause partial or 

complete cross-resistance. The Cry-protease could degrade multiple Cry toxins, reducing 

or eliminating their toxicity to the insect. Similar to other enzymatic mechanisms, 

constitutive expression of a Cry-protease is likely to be inherited as a dominant allele, or 

at least having additive gene action. Additive or dominant resistance alleles would lead 

to rapid resistance to multiple-toxin plants. In the examples provided above, epistasis 

would further increase the rate of resistance evolution, but it is also possible that epistasis 

could delay the evolution of additive resistance.

Considerations for the two locus case can be readily generalized to three loci. 1) Increase all 

three marginal SR survival rates, analogous to the two locus case above (Fig. 8-2), and multiply 

these through to calculate a new multiplicative survival matrix. This new matrix will be the 

product of three low-dose marginal survival values. 2) Alter the triple heterozygote survival to 

have a conditional dominance much greater than any of the marginal dominances. Additive gene 

action might be one value that could be used. 3) Alter all three double heterozygote survival 

Fig. 8-4. Complete cross resistance of 

locus 1 with a modifier effect of locus 2. 

Epistasis is highlighted in in light red 

boxes, marked in bold with an asterix. .

L1 H1 1

S1S1 S1R1 R1R1

L2 S2S2 L1 H1 1

L2 S2R2 L1 H1+α∗ 1

L2 R2R2 L1 H1+β∗ 1

Fig. 8-3. Complete cross resistance 

of locus 1. Survival rates associated 

with locus 2 have no effect on 

survival on the two toxin plant.

L1 H1 1

S1S1 S1R1 R1R1

L2 S2S2 L1 H1 1

L2 S2R2 L1 H1 1

L2 R2R2 L1 H1 1
72

rates to have slightly larger conditional dominance than the marginal dominance. This assumes 

that two different R alleles will improve survival on the third toxin even when the larva has no R 

allele for the third toxin. A second alternative would be to assume that the conditional 

dominance of the double heterozygotes is the same as the marginal dominance of the third allele. 

This assumes that the R alleles do not interact. This second alternative is less conservative than 

the first, in that evolutionary rates should be slower in the second alternative. However, the first 

alternative may require an assumption about cross-resistance or some other gene-gene 

interaction.

Fig. 8-5. Partial cross resistance of locus 1. A. Multiplicative fitness with locus 1 

conferring partial cross resistance and locus 2 reducing survival rates. B. 

Hypothetical epistasis with a locus for partial resistance associated with locus 2, 

which increases survival of all genotypes with one or more R1 allele. Epistasis is 

highlighted in light red and marked in bold with an asterix.

 A. L1 H1 1 B. L1 H1 1

S1S1 S1R1 R1R1 S1S1 S1R1 R1R1

L2 S2S2 L2L1 L2H1 L2 L2 S2S2 L2L1 L2H1 L2

L2 S2R2 L2L1 L2H1 L2 L2 S2R2 L2L1 L2H1+α∗ L2+α∗

L2 R2R2 L2L1 L2H1 L2 L2 R2R2 L2L1 L2H1+β∗ L2+β∗
73

Empirical Considerations

Case 1. No major resistance alleles for any of the component toxins are available.

Method 1. In this case, it is possible to estimate L1, L2 and their product L1L2 (Fig. 8-1). L1L2 is 

efficacy of the pyramided Bt plant, and can be estimated directly by exposing larvae to the Bt

plant. For high dose events, L1L2 is likely to be very small and difficult to estimate accurately. 

Therefore a standard concentration-mortality experiment can be conducted, incorporating or 

overlaying diluted plant tissue into or on top of diet. In the dose calculation, the idea was to 

convert plant tissue concentrations into bioassay equivalents. Here the idea is the opposite, to 

convert bioassay concentrations into plant equivalents to estimate efficacy on the logit or probit 

scale. Then the mortality power of the intact plant in bioassay equivalents can be calculated, by 

extrapolating the concentration-response profile to 100% of the plant concentration. This 

assumes that the probit or logit model is accurate for extreme mortality values. For example, 

suppose E, efficacy of the intact plant, is very close to 1.00, but difficult to distinguish from 1.00 

statistically. Take a series of dilutions of the plant and measure mortality and estimate the slope 

as S = ΔLogitμ/Δlog[C], where μ is the measured mortality at plant concentration C. C is 

expressed as a fraction of the intact plant, which is C = 1. In addition, the concentration of each 

component toxin should be estimated at each dilution. Then estimate a low variance point on the 

profile, called m, with [Cm] and Logitμm. Extrapolate the profile from [Cm] to [C] = 1 to estimate 

a value for LogitE. E is the inverse transform of LogitE, and this estimated E = L1L2 will always 

be strictly less than 1.00.

If an E < 1.00 can be estimated directly from mortality on the intact pyramided plant, then it is 

possible to calibrate the dilution assay with the mortality that occurs on the intact plant. It is 

important when estimating mortality on the intact plant that if inoculations are used that the 

Fig. 8-6. Partial cross resistance of locus 

1, which provides resistance to toxin 1 and 

partial cross resistance to toxin 2. Locus 2 

provides major gene resistance for toxin 2. 

L1 H1 1

S1S1 S1R1 R1R1

L2 S2S2 L1L2 H1L2 L2

H2 S2R2 L1H2 H1H2 + α H2 + β

1 R2R2 L1 H1 1
74

relatedness of the larvae is considered, because most insect colonies are started with only a few 

thousand individuals at best.

Additional bioassays can be conducted with separate, purified toxin components to estimate L1

and L2 independently. The concentration-mortality profiles for each component toxin 

individually should be estimated, using the same methodologies as previously. The 

concentrations should be similar to the measured concentrations from the plant dilution bioassay. 

A value of L1 and L2 can be estimated from each of the single toxin profiles by interpolating or 

extrapolating to the equivalent toxin concentration in the intact plant. This will use a statistical 

method similar to that described for the dilution assay. It should then be possible to test directly 

the null hypothesis, H0: L1L2 = L1L2 . The null hypothesis is multiplicative fitness (no epistasis) 

for the S1S1S2S2 genotype, and is one direct test for epistasis.

Method 2. Second, epistasis can be tested indirectly for the single heterozygotes. The slopes of 

the concentration-response profiles or the entire profile based on all of the data can be used to 

calculate the expected mortality at toxin concentrations 50x less than that in the intact plant. The 

bioassay and statistical methodology is the same as for estimating dose, which is described in 

Appendix 1. The expected mortality at this concentration should be greater than the LC50 for 

high dose toxins, whether evaluated singly or together. According to EPA-SAP (1998), these 

mortality values may be surrogates for expected heterozygote survival. Designate these 

mortality rates μi,0.02, where μ is the mortality rate for profile i when the concentration is 1/50 the 

full plant concentration (=0.02). When there are two toxins, there are three profiles, and it is 

possible to calculate μ1,0.02, μ2,0.02, and μ12,0.02, where 12 is for both toxins together. Using the 

same reasoning as was used in EPA-SAP (1998), μ1,0.02 is a surrogate for H1, μ2,0.02, is a

surrogate for H2, andμ12,0.02, is a surrogate for the joint fitness, H1L2 and L1H2. It is then possible 

to test the following null hypotheses, H0: L2μ1,0.02 = μ12,0.02 , and H0: L1μ2,0.02 = μ12,0.02 . These null 

hypotheses test for multiplicative fitness of H1L2 and H2L1, respectively (Fig 8-1) forthe single 

heterozygotes, S1R1S2S2 and S1S1S2R2. It is probably possible to rearrange these null hypotheses 

so that they are expressed in terms of the original data instead of the derived values.

Case 2. A major resistance allele exists for one of the toxins, but not for any of the others.

Method 1. Assume that the major resistance allele is at locus 1. In this case, the no epistasis 

hypothesis (multiplicative fitness) can be directly tested using Method 1 described under case 1. 

The three genotypes, S1S1S2S2, S1R1S2S2, and R1R1S2S2 (Fig. 8-1), can be exposed to dilutions of 

plant tissue to estimate the joint products L1L2, H1L2 and K1L2. The marginal values, L1, H1, K1

and L2 can be estimated independently as described above, via four independent bioassays. This 

allows a direct test of the no epistasis hypothesis for three of the nine genotypes in a two locus 

problem (Fig. 8-1). Genotype S1S1S2S2 is used to estimate L1L2, L1, and L2. Genotype S1R1S2S2

is used to estimate H1L2, H1, and L2. Genotype R1R1S2S2 is used to estimate K1L2, K1, and L2. 

Method 2. Second, indirect means similar to those used in Method 2 described under case 1 can 

be used to test for epistasis in the genotypes heterozygous for the other locus, the S1S1S2R2, 

S1R1S2R2, and R1R1S2R2 genotypes (Fig. 8-1). The S1S1S2S2 genotype is used to test for 

epistasis in S1S1S2R2 as described in case 1. The S1R1S2S2 genotype is used to test for epistasis 

in S1R1S2R2 similar to the description in case 1. The R1R1S2S2 genotype is used to test for 

epistasis in R1R1S2R2 similarly. 
75

Method 3. Third, absence of epistasis between resistance loci can be defined as when the 

resistance mechanism associated with each locus operates independently from the mechanism at 

the other resistance loci. This can be tested directly when at least one major resistance allele is 

available at one of the loci. If the mechanisms are independent, then the major resistance allele 

will confer resistance for one toxin, but will not affect survival on any of the other single toxins. 

In other words, the S1R1S2S2 and R1R1S2S2 genotypes may improve survival over that of the 

S1S1S2S2 genotype on toxin 1, but there should be no difference in survival associated with any 

of the three genotypes on toxin 2. 

This can be tested by estimating a concentration-mortality profile for the R1 allele on toxin 2. If 

the profiles for the three genotypes, S1S1S2S2, S1R1S2S2 and R1R1S2S2, are not different, the 

hypothesis of independent resistance mechanisms cannot be rejected and epistasis for toxin 2 is 

small or absent. If they are different, there is likely to be epistasis. If the S1R1S2S2 and R1R1S2S2

genotypes survive better than the S1S1S2S2 genotype, epistasis will increase survival and speed 

up resistance evolution. The epistasis will occur in the S1R1S2S2 and R1R1S2S2 genotypes, and 

may occur in the S1R1S2R2 and R1R1S2R2 genotypes. It may also affect the fitness of the 

S1R1R2R2 and R1R1R2R2 genotypes if the R2 resistance is incomplete.

Based on the data summarized in Table A2.4, the major gene for Cry1Fa resistance in ECB 

shows epistasis for both Cry1A.105 and Cry2Ab2. This epistasis will increase the rate of 

resistance evolution. Estimation of the three locus epistasis problem is considerably more 

complex than two locus epistasis because the fitness of each of the 27 three-locus genotypes is 

determined by the independent action of the three loci plus three kinds of two-locus epistasis and 

one kind of three-locus epistasis. In some cases, it will be possible to test the null hypothesis of 

independent three-locus fitness, such as for the direct tests described above (Method 1), but in 

other cases it may be necessary to evaluate the pairwise null hypothesis that two of the loci have 

independent fitness (e.g., Methods 2 and 3). 

Charge Question 9

Based on a review of the submitted simulation modeling, the preliminary conclusions are:

1) For CRW, a 5% seed mixture and a 5% structured refuge had comparable durability in 

both the EPA and Dow models;

2) For ECB, a 5% seed mixture was less durable than a 5% structured refuge in simulations 

with EPA’s model. However, ECB resistance did not evolve within 158 generations in 

any of the simulations with the 5% seed mixture, similar to the level of durability 

predicted by Monsanto’s model. There was no difference in durability between the 5% 

seed mixture and the 5% structured refuge in Monsanto’s model. Resistance did not 

evolve to either refuge option within 100 generations (the extent to which the model was 

run); 

3) For SWCB, a 5% seed mixture was less durable (78 generations) than a 5% structured 

refuge (118 generations) in EPA’s model simulations. Conversely, with Monsanto’s 

model there was no difference in the prediction for durability between the 5% seed 
76

mixture and the 5% structured refuge. Resistance did not evolve to either refuge option 

within 100 generations (the limit of the model simulations).

Please comment on the reliability of the estimates of resistance evolution by each of the 

three models in light of the biological and parameter uncertainties identified by BPPD. 

Panel Response

Due to the uncertainties associated with the Monsanto, Dow, and EPA/ORD modeling efforts for 

ECB, SWCB, and CRW, the Panel concluded that there was an insufficient scientific basis for 

supporting the SmartStax RIB 5% seed mixture as an effective IRM strategy for all pests of 

concern. As is well-accepted in the scientific literature (e.g., Mallet and Porter 1992; Davis and 

Onstad 2000) and concluded in the 1998 SAP Report on Bacillus thuringiensis (Bt) PlantPesticides and Resistance Management, for high-dose cases when toxicity of the cultivar causes 

low survival of heterozygous pest individuals, seed mixtures will have lower durability than 

structured refuges with the same percentage of Bt plants. This point was illustrated by the 

modeling exercise performed during the meeting (see Appendix 2). Also, there are greater 

uncertainties for seed mixtures than for structured refuges due to the lack of information about 

larval movement for ECB, and SWCB, and how larval movement affects the survival of 

heterozygotes.

In particular, the Panel concluded that for ECB and SWCB, the 5% seed mixtures would lead to 

more rapid resistance evolution than the 5% structured refuges. For CRW, the 5% seed mixture 

will likely have similar durability to the 5% structured refuges due to the low-dose nature of 

resistance. Nonetheless, the overall IRM strategy should be based on the target pest with the 

greatest increase in risk of resistance evolution, which is likely to be SWCB or ECB, rather than 

CRW.

The Panel concluded that the Monsanto, Dow and EPA/ORD models all contain attributes that 

lead to overestimates of the durability of the 5% seed mixture, especially for SWCB. Note that 

the overestimation of the durability of the 5% seed mixture is a separate (though related) issue 

from the underestimation of the difference between the durability of 5% seed mixtures and 5% 

structured refuges; indeed, many of these attributes that lead to overestimation of the durability 

of 5% seed mixtures also lead to overestimate of the durability of 5% structured refuges. The 

Panel concluded that the estimated generations of durability were not scientifically credible and 

that these numbers were likely overestimated for ECB, SWCB, and CRW. The degree of 

overestimation could be considerable, as illustrated by the modeling exercise in Appendix 2.

The Monsanto model overestimated the durability of seed mixtures for several reasons. First, the 

Monsanto model did not account explicitly for the movement of larvae among plants in seed 

mixtures and the resulting potential for large increases in the survival of heterozygotes that will 

greatly speed resistance in ECB and SWCB. Second, they did not report the actual simulated 

durability with either seed mixtures or structured refuges. The EPA should encourage reporting 

of mean, 5% risk and 0% risk durability for all simulation models because the relative model 

results may have meaning even when the estimated number of generations do not. Despite the 

lack of information provided by Monsanto, the Panel concluded that the relative estimates that 
77

could have been reported by the Monsanto model would underestimate the difference in 

durability between seed mixtures and structured refuges. Third, the Monsanto model did not 

consider any fitness values with epistasis. Epistasis can increase survival of single, double and 

triple heterozygotes that will also greatly speed resistance in ECB and SWCB (see Charge 

Question 8).

The EPA/ORD model and analyses, although generally superior to those of Monsanto and Dow, 

also underestimated the difference in durability between seed mixtures and structured refuges, 

and overestimates the durability of seed mixtures, for both ECB and SWCB. The model 

explicitly accounts for movement of larvae among plants and explores fitness values with some 

epistasis. However, it did not explore the realistic larval movement scenarios and hypotheses 

listed in the Response to Charge Question 2, and did not explore epistasis sufficiently as 

explained in the Response to Charge Question 8. Specifically, the Panel stated that there were

three attributes that likely lead to overestimates in the durability of seed mixtures: 1) the model 

contains no density-dependent larval survival, 2) the sensitivity analysis could give artificially 

low estimates of the risk of resistance, and 3) the manner in which survival of heterozygotes is 

computed could lead to overestimates of durability.

The Panel concluded that the Dow and EPA/ORD models for CRW did not underestimate the 

difference in durability between a 5% seed mixture and a 5% structured refuge. Independent 

modeling by the Panel confirmed that there is likely to be no or little difference in the durability 

of a 5% seed mixture and a 5% structured refuge. Nonetheless, the Dow model did overestimate 

the durability of seed mixtures (and also structured refuges). The Dow model, as EPA-BPPD 

noted, uses high-dose values of survival rates of CRW that will overestimate durability. Also, 

these conclusions come with the caveat that none of the models considered multilocus resistance 

mechanisms that could operate in the low-dose case of CRW.

The Panel noted that modeling resistance durability involves high levels of uncertainty, and 

interpreting model results must be done in light of this uncertainty. One source of uncertainty 

(decision model uncertainty) stems from considering only a limited range of models or 

comparisons among models. For example, the charge from EPA-BPPD was to compare a 5% 

seed mixture with a 5% structured refuge, and this constraint on the comparisons limited the 

information that could be obtained from the models. The Panel also indicated that there was 

uncertainty about the structure of the models (model completeness). The models may leave out 

attributes that affect resistance evolution, such as density-dependent larval mortality. A further 

source of uncertainty that surrounds the predictions of the models is parameter uncertainty. 

Although a range of parameter values was investigated in the Monsanto and Dow models, and a 

more thorough sensitivity analysis was done for the EPA/ORD model, the Panel emphasized that 

there was a high degree of uncertainty for pyramided Bt traits and seed mixtures, and 

recommended the use of sophisticated risk assessment techniques, such as scenario analysis and 

information gap analysis, to address these uncertainties.

The Panel was not provided sufficient information to estimate how much the durability of 5% 

SmartStax seed mixture was overestimated or how much the uncertainty mattered. However, the 

magnitude of this uncertainty is potentially large. To give an illustration of this, one element of 

the uncertainty about model completeness, the effect of density-dependence, could by itself have 
78

a large effect. If resistance is 20x faster in a seed mixture than a structured refuge with weak 

density dependence (Appendix 2) and only 7x faster without density-dependence (EPA/ORD 

model) for Leidoptera, then including density dependence in the models could speed up 

resistance evolution 2.8x compared to the EPA/ORD model. Multiplying this against the 

durability predictions reported for the EPA/ORD model, this single source of uncertainty could 

reduce the projected durability of the 5% SmartStax seed mixture to 57 generations for ECB. 

Other factors mentioned above could further reduce durability, so the magnitude of uncertainty is 

clearly large and significant. This example is only for illustrative purposes; the Panel cautions 

that while the models show that the effects of density dependence can be large, quantifying how 

large is impossible given the lack of information Monsanto and Dow know about the target pests 

of SmartStax corn. 

If survival of CRW is modeled with low-dose survival rates using the Panel model, the average 

durability might be 32 generations. This corresponds well with the results reported for the 

EPA/ORD model, but is substantially less than the results reported for the Dow model. None of 

these models address uncertainty related to epistasis.

All of these issues related to uncertainty in the model structure and uncertainty about the 

parameter values are indicated by “-” in Table 9-1. If the durability of the 5% seed mixture is 

overestimated by a feature in the model, this is indicated by “O”. This could occur either 

because the comparison to a 5% structured refuge is overestimated (ratio of durability of 5% 

seed mixture/ 5% structured refuge), or because the durability of the 5% structured refuge is 

overestimated (durability of 5% structured refuge). All of the Lepidoptera models seriously 

overestimate the ratio of durability of a 5% seed mixture compared to a 5% structured refuge. 

The models estimated the ratio at 1/7 and 1/20, but both of these are still seriously overestimated. 

The Lepidoptera models also seriously overestimated the durability of the 5% structured refuge. 

The Panel warns against basing policy on these durability values, and did not evaluate the overall 

durability of the Panel model for this reason. However, the Panel understands the temptation to 

think that durability of 158 generations is a long time, even if it is off by a factor of 2. The Panel 

model was used to indicate to decision-makers that these long times can be quickly reduced by 

model and parameter uncertainties. For example, when density-dependent mortality is included 

(Panel model reported as ‘+A’), it can reduce durability 2.8-fold compared to a model without 

density-dependent mortality. As this is only one of the seven ways that durability was 

overestimated by the Monsanto and EPA/ORD models, at current levels of uncertainty, these 

durabilities may be considerably overestimated. 
Table 9-1. Summary of Panel’s evaluation of the evolution models, indicating how the models have overestimated the durability of 

the 5% seed mix refuge. A = Neither underestimates nor overestimates ratio or durability; U = Underestimates ratio or durability; O = 

Overestimates ratio or durability; other symbols are defined in footnote to this table.

Lepidopteran-active toxins 

(ECB and SWCB)

Coleopteran-active toxins 

(WCR and NCR)

Monsanto 

model

EPA/ORD 

model

Panel 

model

Dow 

model

EPA/ORD 

model

Panel 

model

Ratio of durability of 5% seed mix/ 5% 

structured refuge

O O O A A A

Model Structure

Larval movement - O + + N N N

Selection in ears - O - O - O X X X

Correlations with larval movement 

parameters

- ? - ? - ? N N N

Parameter Values

Larval movement hypotheses - O - O - O N N N

Epistasis - O + O + (O) N N N

Cross resistance - O - O - O N N N

Adult dispersal + (O) + (O) + (O) N N N

Ratio of durability of 10% seed mix / 5% 

structured refuge

Not evaluated (decision model uncertainty)

Durability of 5% structured refuge O O N O O N

Model Structure

Density dependent mortality - O - O + A + A - O + A

Density-dependent selection - (O) - (O) - (O) - O - O - O

Correct locus number + A + A - U + O + O + O

Stochastic - ? -/+ ? - ? + ? -/+ ? - ?

Non-random oviposition - ? - ? - ? - ? - ? - ?

Local non-random mating - O - O - O - O - O - O
80

Cost of resistance - U - U - U - U - U - U

Quantitative genetics X X X - O - O - O

Parameter Values

Initial R allele frequencies + U/A + U/A + U/A + O + O + O

Heterozygote survival - O - O - O - O - O + A

Epistasis - O - O + (O) - O - O - O

Cross resistance - O - O - O - O - O - O

Dose mortality N N N - O - O - O

1st order Monte Carlo simulations - O + O - O - O + O - O

Temporal variation - ? + ? - ? - ? + ? - ?

Spatial variation - O + O - O - O + O - O

Symbols are defined as follows: + = structure included in model or appropriate parameter values explored; - = structure missing from model or appropriate 

parameter values not explored; -/+ = evaluated in some, but not all simulations; N = Not evaluated by Panel; ? = Panel uncertain about direction of effect; () = 

Panel uncertain that magnitude is significant; X = Not needed in model
The Panel recommended several areas that needed further theoretical investigation to assess and 

design IRM strategies for seed mixtures:

1) Modify the Monsanto, Dow, and EPA/ORD models to remove the attributes that likely 

lead to overestimates of durability, or develop new models that more accurately assess 

durability 

2) Investigate the integration of IRM strategies with adaptive IRM management approaches 

that include well-defined triggers for taking remedial actions and clearly identify the most 

appropriate remedial actions.

3) Model IRM at the regional scale to account for risks of resistance evolution that may 

result from long-term, region-wide suppression of pest population densities; and

4) Investigate the incremental introduction of products such as the 5% SmartStax seed 

mixture, which have high levels of risk uncertainty, especially for lepidopteran pests.

This incremental approach would decrease the percent of non-Bt in a seed mixture based 

on coupling monitoring for resistance and population density. 

Monsanto, Dow, and EPA/ORD Models Overestimate Durability

Monsanto Model for ECB (SWCB)

The Monsanto model for ECB and SWCB resistance modeled larval movement among plants in 

seed mixtures implicitly as a decrease in refuge size and appeared to be a frequency-only model 

(although this was not explicitly stated in their documentation). The Panel indicated that these 

attributes would overestimate the time to resistance failure of SmartStax for ECB and SWCB. 

Additional discussion is provided here on the effect of larval movement, while comments on 

frequency-only models are provided in the discussion of the EPA/ORD model.

The Monsanto model assumes that the effect of larval movement in seed mixtures would 

increase the mortality of susceptible larvae. For larvae that are initially on non-Bt plants, if they 

move from the plant, they are likely to move to Bt plants and thus be killed. As Mallet and 

Porter (1992) point out, however, the main effect on resistance evolution of larval movement 

within seed mixtures is to increase the relative survival of heterozygotes. When selection occurs 

both before and after larval movement between plants, the relative survival of heterozygotes goes 

up, and this speeds the rate of resistance evolution. This effect on heterozygotes was not 

included in the Monsanto model. Therefore, this model overestimated the durability of seed 

mixtures because it excluded the main mode of increased resistance evolution caused by 

movement of larvae between plants in seed mixtures. See Appendix 2, Monsanto’s Implicit 

Modeling of Larval Movement.

Dow Model

The Dow model of CRW resistance used high-dose values that were corrected for densitydependent mortality, in contrast to recommendations by BPPD that dose should not be corrected 
82

for density-dependent mortality (lower dose) (see discussion in response to Charge Question 5). 

Because of this, Dow’s model will likely overestimate the durability of SmartStax with regard to

CRW protection.

This notwithstanding, the Panel stated that in low-dose situations, such as for the two Bt CRW 

toxins in SmartStax, a seed mixture is likely similar in durability to a structured refuge 

(Appendix 2: High- and Low-Dose Scenarios and Larval Movement in Seed Mixtures). 

Therefore, a SmartStax RIB 5% seed mixture will likely have similar durability as a 5% 

structured refuge (the current refuge requirement) for CRW.

EPA/ORD Model

The EPA/ORD model contained three attributes that will lead to overestimating the durability of 

the SmartStax RIB 5% seed mixture. 

1) The EPA/ORD model is a frequency-only model that contained no density-dependent 

larval survival for all three target pests (ECB, SWCB, and CRW). As explained below, 

this will lead to longer times to control failure than comparable models that include 

density-dependent larval survival.

2) EPA/ORD performed a sensitivity analysis to assess their model, and this sensitivity 

analysis was performed in such a way that seed mixes will appear to be more durable 

than they actually are. 

3) For ECB and SWCB the model assumes relatively low survival of heterozygotes; as 

discussed under Charge Question 8, relatively low survival of heterozygotes leads to 

overestimates of durability. 

The Panel addressed 1) and 2) in more detail, as described below.

1) Density-Dependent Larval Mortality. For all models with density-dependent larval 

mortality, resistance occurs more rapidly than for the corresponding model with no 

density-dependent mortality, referred to here as “frequency only” model. This is because 

density-dependent mortality acts primarily on non-Bt plants where the density of 

susceptible larvae can be high. The Panel pointed to the importance of distinguishing 

between the relationship between density and mortality (sometimes referred to as the 

strength of density dependence), and the degree of mortality caused by density 

dependence. Even though in some cases density-dependent mortality may increase only 

slowly with density, there can still be high density-dependent mortality if densities are 

sufficiently high. Therefore, the effect of density-dependent mortality on resistance 

evolution is not determined by the value of a parameter in any model, but instead,

depends on both model parameters and the density that larvae achieve in the model.

Density-dependent mortality increases the rate of resistance evolution by decreasing the 

survival of susceptible insects. This has the effect of increasing the survival of 

heterozygous insects (partially resistant) relative to susceptible insects. This, in turn, 
83

increases selection on heterozygotes and speeds resistance evolution. In models, the 

functional form of density-dependent mortality is unimportant provided the insect 

population is not approaching extinction (i.e., decreasing exponentially). By ignoring 

density-dependent mortality, the EPA/ORD model will overestimate the durability of 

SmartStax (see Appendix 2: Density-Dependent vs. Frequency-Only Models).

In general, density-dependent models will be appropriate for all pest species. It is not 

necessary to demonstrate density-dependent mortality in the field in order to use a 

density-dependent model; instead, density-dependent models should be the starting point 

for analysis of resistance evolution. This is because there are only a few special cases in 

which a frequency-only model is appropriate. These include: (a) when the pest 

population is dropping exponentially to extinction, (b) when density-dependent mortality 

occurs for the adults (but not for the larvae), and (c) when density-dependent mortality 

acts equally on all genotypes. The last situation could arise if, for example, a natural 

enemy attacked pests and did not respond to variation in pest density among Bt and nonBt plants within seed mixtures, or between refuge and Bt fields when structured refuges 

are used (Ives et al. 2011). Nonetheless, based on existing evidence density-dependent 

larval mortality occurs at the scale of plants for CRW, ECB, and SWCB, making densitydependent models necessary.

2) Sensitivity Analysis. EPA/ORD conducted a sensitivity analysis by randomly sampling 

from a distribution of parameter values. The Panel noted that this procedure, while 

designed to address parameter uncertainty, does not address other forms of uncertainty, 

such as uncertainty in the structure of the model (e.g., the contrast between frequencyonly vs. density-dependent models, or the procedure EPA/ORD used to assign survival 

rates to genotypes) and uncertainty in the scope of investigation (e.g., whether CEW 

should be considered in the assessment of IRM for SmartStax).

The Panel suggested that care should be taken in distinguishing sources of variation in 

parameters when interpreting the parameter sensitivity analyses presented by EPA/ORD. 

For some parameters (e.g., the probability that larvae move between plants) the greatest 

source of uncertainty might be lack of information about larval movement. In contrast, 

for other parameters (e.g., the initial resistance allele frequencies) uncertainty might be 

due to real variation across the spatial extent of the target pest population. One way to 

conceptualize this difference is to consider the spatial scale of variation. If all of the 

variation in the probability that larvae move between plants, for example, is due to lack 

of information, then this variability will be the same across North America. If a 

sensitivity analysis based on this variation is conducted and shows, for example, that 

there is a 0.99 probability that resistance does not occur in 25 years, then rapid resistance 

evolution is unlikely across entire North America. In contrast, real variation in initial 

allele frequency might occur at the scale of counties, with some counties having high and 

some counties having low initial allele frequencies. If there is a 0.99 probability that 

resistance does not occur in 25 years, and if there are 1000 counties in which the pest 

occurs, then assuming that populations in different counties are evolutionarily

independent, resistance will likely occur in 10 counties within 25 years. Because 
84

resistance to high-dose Bt products will likely spread geographically, continent-wide 

resistance could easily occur within 25 years.

The Panel suggested alternative procedures for risk analysis that should be conducted 

along with the sensitivity analysis conducted by EPA/ORD. First, the models could be 

studied in detail to understand what factors mathematically are expected to cause rapid 

resistance evolution. This might be particularly helpful in comparing predictions from 

different models or for different pests. Second, EPA/ORD could select parameters from 

broad, uniform distributions and determine which sets of parameters give rapid 

resistance. This procedure is conceptually similar to the regression analyses used by 

EPA/ORD, although instead with a focus on extremes and parameter interactions. This 

approach would also avoid the difficulties of regression analyses that make linear 

assumptions about the response of the time to resistance failure on parameter values. 

Using this process might be helpful in determining interactions among parameters that 

lead to resistance.

Panel Model

The Panel model also contained several attributes that will lead to overestimating the durability 

of the SmartStax RIB 5% seed mixture for Lepidoptera pests. These included not evaluating 

selection in the corn ears, and not evaluating cross resistance and larval movement sufficiently. 

The Panel model only evaluated two-locus evolution, which will underestimate the durability of 

both the 5% seed mixture and the 5% structured refuge. However, the ratio between the seed 

mixture and structured refuge should not be greatly affected by this factor. Consequently, the 

Panel did not use its model to consider durability in terms of numbers of generations. Instead, 

the model was used to characterize uncertainty in relative terms, i.e., how many times shorter or 

longer was durability when the uncertain factor was allowed to vary. 

Model Uncertainties 

The Panel stated that model uncertainties should be considered when assessing model predictions 

of the evolution of resistance using different IRM strategies. A general discussion of the types of 

uncertainties that pervaded all models used by Monsanto, Dow and EPA/ORD and suggestions 

on how to address these model uncertainties is provided below.

The models used here target what is expected to be the principal process important to the 

evolution of insect resistances, but they, like all models, represent an abstraction of a much more 

complex ecological reality. Local processes, scale issues, and spatial substructuring can create 

conditions that can cause unpredictable events and influence the rate and emergence of 

resistance). The ecological context of resistance development is often local, scale dependent, 

and hard to predict (Durrett and Levin 1994). Other authors note that the potential for surprises 

in the evolution of resistance are found ubiquitously in these complex systems. For example, 

population substructuring, such as will be found in field-level refuges, is known to convert 

epitasis into additive genetic variance, which can allow resistance to develop (Takahasi 2007;

Jarvis and Cheverud 2009). This is why the Panel stated that it is important to be more cautious 

about the likelihood of resistance vis-à-vis these models.
85

The fact that models are abstractions of reality leads to many kinds of uncertainty that should be 

considered when evaluating risk. Epistemic uncertainty is uncertainty that can be reduced by 

increased knowledge. There are several kinds of epistemic uncertainty (Regan et al. 2002): 

ignorance, decision model uncertainty, model uncertainty and uncertainty about causality, 

interpretive uncertainty, and parameter uncertainty. Each one of these areas is discussed in turn. 

1) Ignorance. There are many sources of ignorance (Regan et al. 2002), but these cannot be 

addressed in a single risk assessment, such as being considered in this Panel.

2) Decision Model Uncertainty. In the case of examining the durability of 5% seed 

mixture, this pertains to the IRM management options given to the Panel for

consideration. The decision models used by Monsanto, Dow, and EPA/ORD appear to 

be constrained to a comparison of a 5% block refuge versus a 5% seed mixture refuge. 

While this is a critical comparison, there is uncertainty that this is the only critical

comparison. For example, the Panel could have been asked to compare a 5% block 

refuge with a 10% seed mixture refuge, or to compare a 5% seed mixture refuge with a 

10% seed mixture refuge. Indeed, 20% seed mixture refuges could have been another 

basis for comparison. Because the modeling results for the 5% seed mixture refuge are 

likely to be overestimates of durability, the narrow decision model creates decision model 

uncertainty.

3) Model Uncertainty and Uncertainty about Causality. Model uncertainty and 

uncertainty about causality is a more pervasive source of uncertainty (Regan et al. 2002). 

The main kinds that are relevant to the present case are associated with model structure 

and predictions/extrapolations. Some of the main sources of uncertainty about model 

structure include: completeness of states, relationships among states, and nonlinearities 

among the relationships (Regan et al. 2002). The model results were not robust to 

uncertainty about completeness of states. Several of the models did not include state 

variables for population density and include variables only for allele frequency. These 

have been referred to as “frequency-only” models. Frequency-only models overestimate 

durability, sometimes by considerable amounts. The Panel noted that the allele frequency 

models for CRW may have an inappropriate structure; quantitative genetics models may 

be less prone to overestimation and would require a different set of state variables. The 

models were also not robust to uncertainty about the relationships among states. For 

example, all of the models assumed random oviposition and random mating. Reasonable

biological processes presented in the response to Charge Question 7 suggested alternative 

relationships that would accelerate resistance evolution for ECB and SWCB. Analogous 

process could be provided for CRW; however, these alternative relationships will be 

nonlinear. Standard approaches to addressing uncertainty about model structure include 

qualitative modeling and comparison of alternative models (Regan et al. 2002). Although 

at least two models were used to evaluate CRW and ECB/SWCB, these models were not 

distinct enough to be considered alternative models. Thus, the Panel concluded that the 

uncertainty about model structure was not effectively evaluated.
86

Several approaches have been used to address uncertainty associated with predictions and 

extrapolations. These include uncertainty factors, scenario analysis, and other more 

novel approaches, such as information gap theory (Regan et al. 2005). Uncertainty 

factors have been widely used in toxicological studies, but they are probably not 

appropriate for the evolutionary models considered here. Some scenario analysis was 

done, but the models were not exercised sufficiently to characterize the robustness and 

conservatism of the predictions. Information-gap theory might be used to address these 

uncertainties. One of the major sources of uncertainty associated with the extrapolations 

of the models is the assumed independence of the potential variation in the parameter 

values. One approach to scenario development is to identify potentially important 

covariance among the parameter values and specify some biological scenarios that could 

give rise to this important covariance. This can be done by running the model with 

systematically varying parameter values that cover the range of possible values of each 

parameter in combination with variation in all of the other parameters. The output can be 

screened to identify the worst cases and the parameter values associated with these worst 

cases can be identified. Inspection of these values will generate hypotheses about 

important covariance, and the model can be run again to test these covariance hypotheses. 

Once potentially important covariance is identified, biological scenarios can be generated 

that show the identified covariance, and these scenarios can be used to guide 

interpretation of the model output.

4) Interpretive Uncertainty. Interpretive uncertainty arises from the necessary subjective 

judgments in the interpretation of the model output. The major interpretive uncertainty 

that runs throughout the discussion of the model results is that the model output is 

believed to be meaningful only in a comparative context, but the model output is often 

interpreted in an absolute context. Specifically, the credible interpretation of the number 

of generations to failure is to relate it to some standard for comparison, for example, a 5% 

block refuge. The results should be presented as the ratio of generations to failure of the 

5% mixed seed refuge to the standard for comparison. However, the model results were 

often interpreted absolutely. For example, if the time to resistance failure was more than 

100 generations, then it was considered sufficiently conservative. This results in 

significant interpretive uncertainty for some of the models because the models were 

terminated before it was possible to determine the relative rate of evolution for the 5% 

seed mixture refuge compared to a standard. 

5) Parameter uncertainty. Parameter uncertainty is associated with measurement error, 

biased estimation, variation in space and time, and dependencies among parameters. 

There are many standard approaches for the first three of these, and we have already 

outlined a robust approach to dependencies among parameters in the broader context of 

model uncertainty. The models were designed to address some of these sources of 

parameter uncertainty. Several deficiencies remained. First the stochastic models and 

the Monte Carlo simulations were not run a sufficient number of times. The number of 

runs should have been proportional to the x

n

, where x is the estimated number of different 

runs for any one parameter, holding all others constant, and n is the number of 

parameters. If x = 3 and there are 12 parameters, this requires more than 500,000 runs. 

Most of the models have substantially more than 12 parameters. Second, as noted in our 
87

response to Charge Question 5, measurement errors were not included in the 

parameterization of dose for CRWs. Third, the Monsanto and Dow models did not 

consider variation in space and time; while the EPA/ORD model did, more could be done 

to investigate spatial and temporal variation.

General Modeling Issues for SmartStax IRM

The Panel recommended that emphasis in modeling assessments of stacked cultivars should be 

placed on durability for the pest that shows the greatest potential rate of resistance evolution. 

The Panel suspected that this may be SWCB for SmartStax, although acknowledged that this was 

only a suspicion. Because of the low-dose characteristics shown by CRW to SmartStax toxins, it 

may have the lowest risk of resistance evolution in SmartStax RIB in comparison to deploying 

SmartStax with a structured refuge.

The 2009 SAP for Pioneer’s Optimum AcreMax 1 seed mixture targeting CRW recommended a 

20% seed mixture given that the SAP was comparing this against a 20% structured refuge (the 

mandated requirement) (EPA SAP 2009). At the same time, the 2009 SAP emphasized that this 

should not set precedence for future policy decisions regarding seed mixtures due to the unique 

low-dose characteristics of Optimum AcreMax 1 against CRW. Larval movement for low-dose 

Bt expression is a less-serious threat to IRM than for high-dose Bt expression, as illustrated in 

Appendix 2: High and Low Dose Scenarios and Larval Movement in Seed Mixtures. The 

Panel emphasized that SmartStax RIB did not have the same low-dose characteristics as 

Optimum AcreMax1 for ECB and SWCB; therefore, the same IRM recommendation is not 

appropriate.

The Panel recommended several areas that needed further emphasis and theoretical investigation 

to assess and design IRM strategies for products such as SmartStax RIB:

1) Modify the Monsanto, Dow, and EPA/ORD models to remove the attributes that likely 

lead to overestimates of durability, or develop new models that more accurately assess 

durability; 

2) Investigate the integration of IRM strategies with adaptive IRM management approaches 

that include well-defined triggers for taking remedial actions and clearly identify the most 

appropriate remedial actions;

3) Model IRM at the regional scale to account for risks of resistance evolution that may 

result from long-term, region-wide suppression of pest population densities; and

4) Investigate the incremental introduction of products such as the 5% SmartStax seed 

mixture, which have high levels of risk uncertainty, especially for lepidopteran pests.

This incremental approach would decrease the percent of non-Bt in a seed mixture based 

on coupling monitoring for resistance and population density. 

Each of these recommendations for additional modeling is described in more detail below. 
88

Remove Model Attributes that Overestimate Durability

As described above (“Monsanto, Dow, and EPQ/ORD models overestimate durability”), all three 

models have attributes that will lead them to overestimate durability of seed mixtures. The Panel 

recommended that these attributes be changed in the models, and the models should be reanalyzed. 

The Panel explained that frequency-only models will overestimate durability relative to densitydependent models. When pest larvae experience density-dependent mortality, the rate of 

resistance evolution does not change greatly with the percentage of non-Bt seed in a seed 

mixture. However, if adoption of Bt crops suppresses pests so effectively that the pest 

population approaches extinction and density-dependent mortality no longer occurs, a frequencyonly model is appropriate. Frequency-only models are much more sensitive to the percentage of 

non-Bt seed in a seed mixture. This is important for IRM because the risk of suppressing pests to 

very low regional densities, and the consequent potential for very rapid resistance evolution, 

increases greatly with low percentages of non-Bt seed in seed mixtures. Simply, IRM depends 

on the persistence of susceptible insects, and if broad adoption of seed mixtures threatens the 

regional persistence of the pest, then the risk of resistance evolution increases greatly.

The Panel emphasized some ways in which the Monsanto, Dow, and EPA/ORD models 

overestimate the durability of SmartStax seed mixtures. Nonetheless, other attributes of the 

models will likely lead to underestimates of durability, such as the absence of a cost of 

resistance. Ideally, we would like to be able to compare those factors that lead to overestimates 

with those factors that lead to underestimates. The Panel, however, stated that the levels of 

uncertainty in the models are too high to allow this type of quantitative comparison. The Panel 

emphasized that the level of uncertainty, and possible overestimation of durability, is likely 

greatest for SWCB. Furthermore, as noted in the Panel’s response to Charge Question 3, ECB is 

not an adequate surrogate for SWCB. A key difference between SWCB and ECB that will 

greatly increase the rate of resistance evolution in models is the lesser sensitivity of SWCB 

larvae to Cry toxins than ECB larvae (registrant study, RPN-09-075, Head et al. 2009). 

Therefore, RS heterozygote survival may exceed 5%, for one or more of the toxins. This will 

neutralize the advantages of the high-dose strategy and lead to more-rapid resistance evolution. 

Given the Panel’s concerns about model uncertainty, assessments of IRM with seed mixtures

must be extremely cautious, erring on the side of underestimating durability.

The Panel cautioned against using the argument that consistent quantitative (as opposed to 

qualitative) results among different models provide strong support for predictions about the time 

to resistance failure. The Monsanto, Dow, EPA/ORD, and Panel models have very different 

structures, yet they cannot be viewed as giving independent information about the time to 

resistance (a quantitative result). For example, all are based on the assumption that resistance 

will occur in the form of major resistance alleles, and that cross-resistance will be minimal. 

They also share many of the same uncertainties, such as how survival rates of genotypes to 

different toxins combine (Charge Question 8).

The Panel also cautioned that model predictions concerning the time to resistance are not 

absolute values. For example, if a model (under a given set of parameters ) predicts that the 
89

time to resistance failure is 100 generations, this prediction should not be taken literally as 

meaning that resistance will not occur within 100 generations. The Panel agreed with the points 

made by Dr. Nicholas Storer (Dow AgroSciences) in his public comments that models are most 

valuable in making qualitative comparisons among different strategies, such as between a 5% 

structured refuge and a 5% seed mixture. This comparison can be made in terms of the relative 

time to resistance failure of two strategies, but the absolute time to resistance failure should not 

be used to assess policy decisions.

Bt Resistance Monitoring and Remediation Plans

During the public meeting, EPA provided the Panel (at the Panel’s request) with the resistance 

monitoring and remediation plans for all of the Bt PIPs that are part of SmartStax corn. Each 

registrant was required to develop and implement monitoring and remedial action plan for each 

PIP. EPA indicated that the current monitoring and remedial action plans for each PIP 

component of SmartStax corn should be applicable to all PIPs as expressed in SmartStax corn. 

The Panel noted that there was no unified monitoring and remedial action plan designed 

specifically for all PIPs as expressed in SmartStax corn. The Panel emphasized the point that 

monitoring and remediation plans are integral components of IRM strategies. Designing 

monitoring and remediation plans requires the same scientific information needed to develop 

IRM strategies. The same high levels of scientific uncertainty discussed throughout this report 

for the durability of SmartStax seed mixtures are also applicable to the development and 

implementation of monitoring and remedial action plans.

Based on previous SAP reports for EPA on Bt resistance management, the premise for doing 

monitoring of the frequency and level of resistance in target species is that such monitoring 

could trigger actions to delay or avoid field resistance from evolving. The current documents 

provide no valuable guidance beyond requiring a response when confirmed genetically based 

field resistance causes economic losses in a local area, and the response is only for the local area 

affected. There is vague language that might imply a possible broader geographic response, but 

there is no plan for implementation. 

The EPA requires that the registrant (e.g., Monsanto or Dow in the case of SmartStax) conduct 

annual resistance monitoring and implement certain actions should resistance be “suspected” and 

additional steps should resistance be “confirmed.” The Panel indicated that the steps to confirm 

field resistance are difficult and ill-defined. It could easily take two years to prove that there was 

“confirmed” resistance even if crops were destroyed by the insects in the same year that 

resistance was discovered. This is because the remedial action plan stated that “all of the 

following criteria are met by progeny from the target pest sampled from the area of suspected 

resistance,” before resistance is considered “confirmed.” Once “field resistance” is confirmed 

then the EPA requires that specific remedial action plan must be quickly developed, but the 

Panel did not see in any of the remedial action plans when these specific remedial action plans 

needed to be implemented. 

The Panel affirmed that the timing of remediation is important. The Panel noted that previous 

SAPs which addressed monitoring and remediation stated that for most pests, once “field 
90

resistance” is reached the resistance alleles would no longer be locally confined and therefore 

local action would often be ineffectual. 

The Panel indicated that more effective approaches for monitoring single allele resistance 

(Andow and Alstad 1998; Blanco et al. 2008; Venette et al. 2000; Mahon et al. 2010) and 

quantitative genetic resistance (Gould 1978; Karowe 1990) have been developed and should be 

considered. Furthermore, approaches for adaptive response to incipient resistance have been 

outlined, and these should also be considered (Andow and Ives 2002; Downes et al. 2010).

As currently written, if CRW developed complete genetic resistance to one of the two Bt toxins 

in these cultivars, no action would be taken to change the refuge requirement. This is because it 

would be impossible to meet the criteria laid out in the documents for establishing “confirmed 

resistance” under these conditions. Furthermore, if there was complete genetic resistance of 

CRW to both toxins in the stacked cultivars, it could still take two years or more before testing 

could meet the criteria in the current document for confirming resistance, and it could take three 

seasons or more before effective remedial action was taken at even a local level. 

As witnessed by the recent disputes over what is genetic resistance to Bt in CEW (Tabashnik et 

al. 2008a, b, c; Moar et al. 2008), the Panel recommended that science-based monitoring and 

remediation plans be integrated using models to assess monitoring and remediation tactics much 

more accurately. This will require more precise definitions of field resistance that make it easy 

to integrate information about field resistance into remediation plans and well-defined triggers 

that require changes in the IRM plan. The remedial action triggers should be informed by the 

most current scientific information about IRM and be adaptive to new scientific information. 

Timing could be critical in responding to resistance, and therefore the steps in a remediation plan

should be clearly defined and easily implementable.

Regional-Scale Pest Abundances and Seed Mixtures

Although field trials were performed by Monsanto and Dow, these trials were not on the spatial 

and temporal scales needed to assess the long-term consequences of 5% seed mixture adoption. 

Recent work by Hutchison et al. (2010) demonstrated that the wide-spread use of Bt plants 

reduces the region-wide population abundance of ECB; similar region-wide effects are possible 

for other target pests in high-dose cases of toxicity. Because of the regional spatial scale of this 

effect, multiple years were required before it was manifested. 

The region-wide decrease in pest abundances affects decisions about SmartStax IRM in two 

ways. First, the benefits of a seed mixture >>5% (e.g., a 20% seed mixture) in reducing pest 

abundances and crop damage may be as great or greater than the benefits measured by Monsanto 

and Dow for a 5% seed mixture, because the Monsanto and Dow trials were not performed on 

sufficient spatial and temporal scales to observe the regional pest suppression demonstrated by 

Hutchison et al. (2010). In other words, a 20% seed mixture will likely give large benefits to 

farmers that increase over years as regional pest densities are reduced. 

Second, reducing regional pest abundances may introduce risks of resistance evolution. If 

regional effects are too great, populations could be driven towards extinction, leading to greater 
91

rates of resistance evolution as predicted by frequency-only models. This risk argues against 

introducing 5% seed mixtures.

Strategies for Increasing the Durability of Seed Mixtures

The Panel considered the possibility that the durability of seed mixtures compared to structured

refuges could be counterbalanced by other measures to increase durability. Unfortunately, this 

approach requires quantitative assessment of the loss of durability due to using a seed mixture

that is not possible given the uncertainties in modeling resistance evolution. For example, the 

loss of durability of seed mixtures depends on the magnitude of movement of larvae among 

plants and the selection they undergo before and after movement. There is high uncertainty 

about these key processes (see Appendix 2: Seed Mixtures vs. Structured Refuges). 

The Panel discussed cautious, incremental approaches to decreasing the percent of non-Bt in a 

seed mixture given the novelty of using seed mixtures for IRM in high-dose situations such as 

SmartStax for ECB and SWCB. For example, a 20% seed mixture, rather than a 5% seed 

mixture could be introduced. Modeling could be used to compare different incremental 

introduction strategies, and use sensitivity analyses, information-gap analyses, for example, 

could be used to assess the risk of resistance evolution under these different strategies.

The second approach to the introduction of seed mixtures involves the use of adaptive 

management strategies that integrate modeling and monitoring. Due to model uncertainties, 

introduction of seed mixtures would be implemented along with monitoring not only for 

resistance but also for the abundance of target pests. As discussed previously (at the start of 

“General modeling issues for SmartStax IRM”), monitoring should focus on the target pest that 

poses the greatest risk of resistance evolution. This incremental approach would decrease the 

percent of non-Bt in a seed mixture based on coupling monitoring for resistance and population 

density. For example, if a 20% seed mixture was used as the initial IRM strategy then this 

percentage could be incrementally lowered (perhaps every 5 years) if monitoring had not 

indicated any signs of resistance or severe population declines. However, if resistance is 

detected, then remediation should start immediately. Remedial action should also be taken if 

population densities reach low levels, as this would signal much higher risks of resistance 

evolution due to the loss of density dependence.
92

ADDITIONAL DISCUSSION

Identification of Research Needs Concerning Major Corn Pests and Future Use 

of Bt Corn Products

At the end of the meeting, the Panel held an open discussion regarding research needs 

concerning IRM for the major corn pests and the future use of Bt corn products. Each of these 

research areas were discussed in detail throughout the document. A quick, but perhaps not a 

complete list of these areas is provided below. These areas are not listed in any priority.

1) Adult and larval movement, especially SWCB

2) Adult dispersal patterns, male and female differences, mating choice (in Bt and 

refuge fields)

3) CRW and density-dependent mortality estimations

4) Survival of heterozygotes

5) Effects of CEW migration on selection for resistance in Bt corn and Bt cotton

6) Effect of pollination of non-Bt corn with Bt pollen and selection of CEW 

resistance in kernels

7) Determine the percentage of ECB feeding in ears and percentage feeding in 

axils on pollen

8) Re-examine the 1998 SAP definition of high dose with respect to a seed 

mixture. Selection for resistance to single toxins versus selection for resistance 

to multiple toxins (combination of single toxins in one plant)

9) Measurement of the effect of redundant mortality in multi-toxin products 

(pyramids)

10) Uniformity of the seed mixture and uniformity of distribution of non-Bt plants 

in the field

11) Examine other mechanisms of resistance other than detoxification, e.g., 

behavioral resistance mechanisms

12) Examine cross-resistance potential and its effect on resistance, include crossresistance (partial cross-resistance) in modeling simulations

13) Interpretation of models: model structure, inputs, outputs – sensitivity and 

uncertainty analyses
93

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SmartStax (MON 89034 x TC1507 x MON 88017 x DAS 59122-7) corn. Memorandum 

from A. Reynolds to M. Mendelsohn, dated March 16, 2009. 

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104

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429187, EPA-HQ-OPP-2010-0183-0028). Memorandum from A. Reynolds to M. 

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Appendix 1. Empirical Methods for Estimating Dose and Efficacy

Dose

Dose was estimated using a method suggested by the EPA SAP (1998).

The “dose” of insecticidal toxin in a Bt plant is a major factor determining the level of resistance 

risk. Dose depends on both the concentration of the Cry toxins in the Bt plant and the genetic 

characteristics of the target pest. A “high-dose” is defined as one that kills a high proportion 

(>95%) of heterozygous resistance genotypes similar to homozygous susceptible genotypes 

(Tabashnik 1994; Roush 1997; Gould 1998; Andow 2001). For a high dose, resistance is 

recessive or nearly so. A “low dose” is anything that is not a high dose. All other things being 

equal, there is a greater resistance risk associated with low dose species than high dose species.

Theory for Dose Calculation

Caprio et al. (2000, p. 808) suggested that a high dose might be defined when the LC50, RS for the 

RS heterozygote is >50 times greater than the LC50, SS for the SS susceptible homozygote. The 

EPA SAP (1998) suggested a similar criterion, using 25 times, instead of the 50 times suggested 

by Caprio et al. (2000). Because the Caprio et al. (2000) suggestion is based on a literature 

review, the Panel uses the Caprio et al. (2000) suggestion for calculating dose. 

When the R allele is not available, this ratio of LC50’s cannot be evaluated directly. The EPA 

SAP (1998) suggested several ways to evaluate this when the R allele is not available. The basis 

for these ideas is that if and only if LC50,RS > 50 LC50, SS for the plant, then the plant toxin 

concentration, designated [TP], diluted 50-fold must kill >50% of the SS larvae. In other words, 

the killing power of the plant toxin concentration is at least 50 times greater than that needed to 

kill 50% of the SS larvae. Indeed, for any dilution factor x, such that the toxin concentration 

[TP]/x kills > 50% of the SS larvae, it can be concluded that [TP] has at least x times the killing 

power of the LC50, SS. Moreover, if [TP]/x kills y% of the SS larvae, it can be concluded that 

[TP]/x is an LCy, SS. Thus to meet the high dose standard, it must be shown that when x = 50, 

[TP]/x produces an LCy, SS where y > 50. If for any x < 50, it is demonstrated that [TP]/x kills 

<50% of the SS larvae, then [TP]/50 cannot kill > 50% of the SS larvae, and it can be concluded 

that the plant is low dose. 

A test conducted with x = 1, provides the killing power, or efficacy, of the plant without dilution. 

Thus, efficacy measures the killing power when x = 1. Specifically, if E is the efficacy, the 

undiluted plant has an LCE, SS. Obviously, if E < 50%, the plant cannot be high dose. Using the 

efficacy data in this way, it is possible to avoid most of the potential problems that were outlined 

by Caprio et al. (2000, p. 809). 

To determine expected dose using EPA SAP (1998) and Caprio et al. (2000), it is necessary to 

calculate expected efficacy. 

Efficacy can be calculated as: 

1 ( / ) E = − mt mc , [Equation A1.1]
107

with a variance of: 

( ) ( ) 1 ( )

2

2

2

c

c

t

t

c

V m

m

m V m

m

V E 















+ 













 = , [Equation A1.2]

where E is efficacy, mt is the mean of the treatment, mc is the mean of the control, and V is the 

variance, assuming Cov(mt,mc)=0. 

The means may be any measure of pest attack under field conditions. In general, various 

measures of efficacy are of interest because comparison of the various measures provides 

reinforcing evidence of the accuracy of any one measure. For the estimate of dose, mortality 

(survival) may be the more appropriate measure, but the mortality estimates must be consistent 

with other measures of efficacy. 

Finally, the slope of the concentration-response relationship is needed. Using the slope and one 

known point on the concentration-mortality profile for the plant-expressed toxin(s), it is possible 

to calculate the dilution factor that would produce the LC50, SS. The dilution factor, x, is the 

factor by which the undiluted plant matter must be diluted in order to produce an LC50, SS. It is 

calculated as: 

( )





 



 −

= S

obit E obit

x

Pr ( ) Pr (0.5)

10 , [Equation A1.3]

where E is efficacy, and S is the slope in the concentration-mortality relationship, in this case 

with probit transformed efficacy data. A straight-forward modification of this equation can be 

made if a logit model were used instead of a probit model. 

Note that by using an independently estimated E, the concentration-mortality profile is scaled to 

units of dilution on the x-axis. It is known that laboratory bioassays often do not predict field 

mortality at the same toxin concentration. There are many possible reasons for this that are 

mostly related to the inability to accurately mimic in the laboratory the exposure of an insect 

feeding on a plant. Assuming that the laboratory mortality assay models field mortality to some 

proportionality constant that can be applied to the laboratory toxin concentration, using E in the 

Probit equation implicitly defines the hypothetical proportionality constant. To illustrate this 

point, let p be the hypothesized proportionality constant. Define [Ca] to be the toxin 

concentration in the laboratory bioassay that gives a mortality equal to Probit[E]. Define [Cp] to 

be the toxin concentration in the plant that produced efficacy E. Then p = [Ca]/[Cp]. The 

assumed existence of proportionality constant is not unusual or extreme, because it is also 

assumed in justifying a discriminating dose bioassay for use on field populations.

It might appear that it is important also to assume that the slope of the concentration-mortality 

bioassay is equal to the slope of the diluted intact plant-mortality profile. This additional 

assumption is not essential, because it is entailed in the assumption of proportionality constant. 

To illustrate, suppose there is mortality E on the intact plant ([Cp,intact] = 1), and mortality M at a 

half dilution ([Cp,half] = 0.5). The slope on a Probit scale would be equal to (Probit(E)-

Probit(M))/(log [Cp,intact] – log [Cp,half]) = (Probit(E)-Probit(M))/(log 1 – log 0.5). Assume that 

there is a proportionality constant, and as above, let [Ca] be the toxin concentration in the 
108

laboratory bioassay that gives a mortality equal to Probit[E]. Then from the definition in the 

previous paragraph, [Ca] = p [Cp], or equivalently, [Cp] = [Ca]/p. This means that [Ca,1] = p [Cp, 

intact] and [Ca,2] = p [Cp,half]. The slope of the laboratory bioassay is determined from Probit(E), 

Probit(M), [Ca,1], and [Ca,2], where Probit(E), Probit(M), and [Ca,1]are independently determined 

and [Ca,2]is only constrained by p, the proportionality constant. The slope of the laboratory 

bioassay is equal to (Probit(E)-Probit(M))/(log [Ca,1] – log [Ca,2]). Substituting, this is 

equivalent to (Probit(E)-Probit(M))/(log p [Cp, intact] – log p [Cp,half]) which simplifies to 

(Probit(E)-Probit(M))/(log [Cp,intact] – log [Cp,half]), proving that the assumption of 

proportionality is sufficient.

A straight-forward modification of this method can be made if a logit model were used to 

analyze the raw data instead of a probit model. In some ways a logit model might be superior. 

The dilution factor, x, is likely to have a large standard error. To reduce the compounding of 

error that occurs during the multiplication and division of random variables, it might be better to 

estimate log x. If this is done, the logit transform may enable relatively easier access to the raw 

data in constructing a MLE for log x or for bootstrapping error estimates. 

Estimating Pest Attack in the field

Efficacy data were provided by the registrants in MRID 474449-11 (Head and Storer 2008), 

which are summarized in Table A1-1. The data are from field plots during 2006 and 2007 with 

either natural pest populations or artificial infestations. There are diverse measures of pest attack 

that are useful for checking for consistency in measures of control efficacy. 
Table A1-1. Measures of levels of pest injury and damage on pyramided and single gene products by the major 

Lepidopteran pests of corn1

ECB/SWCB2 SWCB2 CEW2 SCB2 FAW2 FAW PR2

mean (SE) mean (SE)

mean 

(SE)

mean 

(SE)

mean 

(SE)

mean 

(SE)

mean 

(SE)

Leaf Damage Rating (0-

9)

MON 89034 TC1507 

MON 88017 DAS 59122-

7

0.2 (0.2) 0.5 (0.3) 0.7 

(0.4) 2.3 (0.2)

TC1507 0.0 (0.0) 1.1 (0.2) 0.8 

(0.2) 6.5 (0.3)

MON 89034 0.2 (0.1) 1.1 (0.4) 0.6 

(0.2)

2.1 

(0.1)

Control 5.2 (1.8) 8.4 (0.4) 7.0 

(0.7) 7.1 (0.2) 7.4 (0.2)

1 loc, n=4 4 loc, 

n=16

7 loc, 

n=28 2 loc, n=8 2 loc, n=8

% Infested Plants

MON 89034 TC1507 

MON 88017 DAS 59122-

7

24.2 (7.1)

TC1507 50.9 (5.7)

MON 89034 

Control 57.4 (5.2)

15 loc, 

n=35-36

Larvae/ Plant

MON 89034 TC1507 

MON 88017 DAS 59122-

7

0.0 (0.0) 0.1 (0.1) 1.1 (0.3)

TC1507 0.0 (0.0) 0.4 (0.2) 1.9 (0.3)

MON 89034 0.0 (0.0)

Control 0.5 (0.1) 0.9 (0.3) 13.1 (1.9)
110

1 loc, n=4 3 loc, 

n=12

1 loc, 

n=2

Cavities/ Plant

MON 89034 TC1507 

MON 88017 DAS 59122-

7

0.6 (0.3) 0.1 (0.1) 0.1 (0.1)

TC1507 0.4 (0.3) 0.0 (0.0) 0.9 (0.5)

MON 89034 0.0 (0.0)

Control 3.9 (1.3) 4.8 (0.4) 18.5 (3.0)

7 loc, n=14 2 loc, n=8 1 loc, 

n=2

Stalk Damage (cm/plant)

MON 89034 TC1507 

MON 88017 DAS 59122-

7

0.0 (0.0) 0.6 (0.4)

TC1507 0.0 (0.0) 0.0 (0.0)

MON 89034 0.0 (0.0) 0.0 (0.0)

Control 0.6 (0.2) 60.9 (14.7)

1 loc, n=4 5 loc, 

n=20

Ear Damage Rating 

(cm/plant)

MON 89034 TC1507 

MON 88017 DAS 59122-

7

0.1 (0.0) 0.6 

(0.3)

TC1507 3.8 (0.3) 2.0 

(0.4)

MON 89034 0.4 (0.1) 0.8 

(0.3)

Control 5.0 (0.5) 3.3 

(0.2)

4 loc, 

n=15-16

2 loc, 

n=7-8

Number of Damaged 

Kernels

MON 89034 TC1507 6.5 (1.4)
111

MON 88017 DAS 59122-

7

TC1507 28.2 (3.4)

MON 89034 6.1 (0.8)

Control 29.7 (6.8)

2 loc, 

n=8

Larvae/Ear

MON 89034 TC1507 

MON 88017 DAS 59122-

7

0.6 (0.3)

TC1507 2.8 (0.4)

MON 89034 0.7 (0.5)

Control 1.4 (0.2)

3 loc, 

n=9-12

1. Data are from MRID 474449-11. pages 170-180 (Vaughn et al. 2008), and RPN 07-262, pages 31-41 (Vaughn et al. 2008). 

Data were not collected in the gray cells. “loc” is the number of different geographic locations where the varieties were 

evaluated. “n” is the number of replicate plots summed across all locations. Data were pooled across sites and years, 

weighting means and variances by sample size. 

2. ECB = European corn borer, SWCB = Southwestern corn borer, CEW = Corn earworm, SCB = Sugarcane borer, FAW = 

Fall armyworm, mainland USA, FAW PR = Fall army worm in Puerto Rico
 Estimating Efficacy from Field Data

Efficacy was estimated using equations A1-1 and A1-2 and the data in Table A1-1. When 

multiple measures of efficacy were available, they were mutually reinforcing, implying that 

the data were internally consistent. 

The data confirm high efficacy for ECB and SWCB for all three products that were evaluated 

(Table A2-2). For ECB, efficacy was highest for stalk damage compared to leaf damage 

rating or cavities per plant. The relatively poor suppression of cavities is probably indicative 

of the high rates of older larval movement, which is typical for ECB. For SWCB, efficacy 

was higher for larvae/plant than the injury and damage indexes. For both species, 

suppression of leaf damage rating was not as high as some of the other injury and damage 

ratings. This is because the leaf damage rating is very sensitive to small amounts of grazing 

by neonates, and is not strongly related to yield loss. The percent of infested plants is a good 

predictor of yield loss in first generation ECB and SWCB, but this parameter was not 

measured. For these species, the higher reported efficacies are probably more characteristic 

of field control of larvae than the lower values. We conclude the efficacy of the triple-stack 

is probably 0.990-1.000 for ECB and SWCB.

Efficacy for SCB is also fairly high, with all values above 0.85; MON 89034 was not 

evaluated by itself for SCB. The efficacy of the triple-stack was systematically higher than 

for TC1507, and larval suppression by the triple-stack was 0.916. Cavities per plant were 

even more strongly suppressed by the triple-stack at 0.995. The efficacy of the triple-stack 

was probably between 0.916 and 0.995 for SCB.

Efficacy for CEW varied strongly by event. The triple-stack had higher suppression than 

TC1507 alone, and was systematically higher than MON 89034 alone. For the triple-stack, 

efficacy was highest for ear damage rating (0.980). Suppression of larvae per plant was 

0.889 and suppression of larvae per ear was 0.589 for the triple-stack. An additional trial (In 

MRID 474449-11, page 420, Huckaba and Storer (2008)) found considerably better 

suppression of live larvae in the ear, which would raise this value to perhaps 0.690, but 

without the detailed statistics, it is difficult to combine these values accurately. Larval 

suppression is probably a better measure of field control.

Efficacy for FAW varied strongly by event and locality. On the USA mainland, TC1507 had 

lower suppression of ear damage rating than the other events, and MON 89034 was similar to 

the triple-stack. On the mainland, the triple-stack may have an efficacy of 0.811-0.907. 

Puerto Rico has a population of Cry1Fa-resistant FAW (Cry1Fa is the active ingredient in 

TC1507). TC1507 has poor efficacy against the Puerto Rican FAW population; efficacy is 

similar against the triple-stack and MON 89034. Efficacy is systematically lower against 

both the triple-stack and MON 89034 in the Puerto Rican population compared to the 

mainland FAW population. As this might indicate cross-resistance, this should be examined 

with a controlled experiment. 

For estimating dose, the most relevant efficacy measure is neonate to adult survival. None of 

the reported efficacy measures provide this value, but larval density is the closest measure 

(Table A1-2). Most of the estimates of efficacy are mutually reinforcing, increasing the 
113

credibility of the data. In no case for the triple stack is E < 50%, so the efficacy data do not 

by themselves determine if the triple-stack is low or high dose against any of these pests. 

However, the values allow a rough ranking of species based on the likelihood of being high 

dose. Of all of the species, ECB and SWCB are most likely to be high-dose, followed by 

SCB. Of all of the species, CEW and FAW in Puerto Rico are most likely to be low-dose. 

Mainland FAW is intermediate.
Table A1-2. Measures of efficacy (reduction compared to control) of pyramided and single gene 

products against the major Lepidopteran pests of corn1

ECB/SWCB SWCB CEW SCB FAW FAW 

PR

E (SE) E (SE) E (SE) E (SE) E (SE) E (SE)

Leaf Damage Rating (0-9)

MON 89034 TC1507 

MON 88017 DAS 59122-7

0.962 

(1.000)

0.946 

(0.711)

0.907 

(0.677)

0.674 

(0.070)

TC1507 1.000 

(0.000)

0.869 

(0.200)

0.884 

(0.260)

0.085 

(0.053)

MON 89034 0.962 

(0.500)

0.869 

(0.345)

0.919 

(0.351)

0.721 

(0.035)

% Infested Plants

MON 89034 TC1507 

MON 88017 DAS 59122-7

0.578 

(0.296)

TC1507 0.113 

(0.138)

MON 89034 

Larvae/ Plant

MON 89034 TC1507 

MON 88017 DAS 59122-7

1.000 

(0.000)

0.889 

(0.000)

0.916 

(0.273)

TC1507 1.000 

(0.000)

0.556 

(0.000)

0.855 

(0.159)

MON 89034 1.000 

(0.000)

Cavities/ Plant

MON 89034 TC1507 

MON 88017 DAS 59122-7

0.845 

(0.503)

0.990 

(1.400)

0.995 

(1.000)

TC1507 0.897 

(0.751)

1.000 

(0.000)

0.951 

(0.556)

MON 89034 1.000 

(0.000)

Stalk Damage (cm/plant)
115

MON 89034 TC1507 

MON 88017 DAS 59122-7

1.000 

(0.000)

0.990 

(0.639)

TC1507 1.000 

(0.000)

1.000 

(0.000)

MON 89034 1.000 

(0.000)

1.000 

(0.000)

Ear Damage Rating 

(cm/plant)

MON 89034 TC1507 

MON 88017 DAS 59122-7

0.980 

(0.002)

0.811 

(0.413)

TC1507 0.240 

(0.111)

0.393 

(0.188)

MON 89034 0.920 

(0.250)

0.766 

(0.398)

Number of Damaged 

Kernels

MON 89034 TC1507 

MON 88017 DAS 59122-7

0.781 

(0.221)

TC1507 0.051 

(0.249)

MON 89034 0.795 

(0.139)

Larvae/Ear

MON 89034 TC1507 

MON 88017 DAS 59122-7

0.589 

(0.519)

TC1507 0.000 

(0.255)

MON 89034 0.504 

(0.702)

1. Data were not collected in the gray cells. ECB = European corn borer, SWCB = Southwestern corn borer, CEW 

= Corn earworm, SCB = Sugarcane borer, FAW = Fall armyworm, mainland USA, FAW PR = Fall army worm in 

Puerto Rico
Concentration-Mortality Assay: Slopes and LC50

The registrants provided data that can be used to estimate the slope of the concentrationmortality profile for Cry1Fa, Cry1A.105, or Cry2Ab separately, for some of the species, i.e., 

only FAW and ECB (MRID 474449-11, Appendix 1, Schlenz et al. (2008)). Slopes of the 

concentration-mortality profiles for ECB and FAW are shown in Table A1.3. For each species, a 

colony resistant to Cry1Fa (and otherwise susceptible to Cry1A.105 and Cry2Ab) and a colony 

susceptible to all three toxins were evaluated. The statistical error associated with these slope 

estimates was not reported. However, the variation in mortality among replicate experiments 

was small for about 1/3 of these experiments (Pages 45-47, Schlenz et al. (2008)), so perhaps 

these estimates are not too variable. 

Resistant FAW and ECB had lower slopes than susceptible colonies when exposed to Cry1Fa

(Table A1-3). This may be expected, because the resistant colonies should be able to tolerate 

Cry1Fa better than the susceptible colonies. 

Unexpectedly, the resistant FAW and ECB colonies also had lower slopes than the susceptible 

colonies when exposed to either Cry1A.105 or Cry2Ab (Table A1-3). This suggests that there 

was some level of epistasis and partial cross-resistance. However, the absence of an estimated 

statistical error makes any inference about epistasis or cross-resistance uncertain, and the 

registrant should be encouraged to provide full statistical analysis of the concentration-mortality 

data.

Table A1-3. Estimated slopes of the concentration-mortality relationships for FAW and ECB 

colonies against Cry1F, Cry1A.105, and Cry2Ab.1

Toxin Species Colony2 Estimated Slope

(∆probit mortality/ ∆Log10[ ])3

Cry1Fa FAW Susceptible 1.45

Resistant 0.36

ECB Susceptible 1.46

Resistant 0

Cry1A.105 FAW Susceptible 1.60

Resistant 0.63

ECB Susceptible 1.10

Resistant 0.64

Cry2Ab FAW Susceptible 0.47

Resistant 0.46

ECB Susceptible 1.56

Resistant 0.62

Average4 FAW Susceptible 1.17

Resistant 0.55

ECB Susceptible 1.37

Resistant 0.63

1. Toxins exposed singly in a diet overlay assay. MRID 474449-11 (Schlenz et al. 2008).

2. Susceptible colony is susceptible to all three toxins. Resistant colony is resistant to Cry1Fa, but susceptible to 

the other toxins.
117

3. Slopes were taken from Table 1, page 44, MRID 474449-11 (Schlenz et al. 2008). Units are change in probit 

transformed mortality rate divided by change in log10 toxin concentration.

4. Average does not include Cry1Fa for resistant colonies.

Lethal concentrations, 50% (LC50’s), for the concentration-mortality profiles were reported in

Table 1, page 44, MRID 474449-11 (Schlenz et al. 2008). The 95% confidence intervals were 

also reported, when they could be estimated. As expected, the resistant colonies had a much 

higher LC50 on Cry1Fa than the susceptible colonies. Unexpectedly, the resistant colonies also 

had a higher LC50 for both Cry1A.105 and Cry2Ab (Table A1-4). This suggests some level of 

epistasis and partial cross-resistance between Cry1Fa and the other two toxins. 

Table A1-4. Estimated LC50’s and 95% confidence intervals (in parentheses) associated 

with the concentration-mortality relationships for FAW and ECB colonies against 

Cry1F, Cry1A.105, and Cry2Ab.1

Toxin Species Colony LC50 (ng/cm2)

Cry1Fa FAW Susceptible 377 (279-510)

Resistant >10000

ECB Susceptible 179 (133-243)

Resistant >10000

Cry1A.105 FAW Susceptible 181.2 (138-240)

Resistant >10000

ECB Susceptible 6.1 (3.9-8.9)

Resistant 813* (488-1478)

Cry2Ab FAW Susceptible >10000

Resistant >10000

ECB Susceptible 595 (451-784)

Resistant 7206* (3476-20108)

1. See footnotes for Table A1-3.

* LC50 for resistant colony is significantly different from the LC50 for the susceptible colony.

Estimating Dilution Factors and Determining Dose

As noted above, the registrants provided data that can be used to estimate the slope of the 

concentration-mortality profile for Cry1Fa, Cry1A.105, or Cry2Ab separately for some of the 

species (Appendix 1, Schlenz et al. 2008). Although it would have been preferable to 

characterize concentration-mortality with a mixture of the toxins, the mixture would likely 

generate a steeper slope and lower estimates of dilution factors. Consequently, use of the 

average of the separate slopes likely biases the result toward a determination of high-dose. In 
118

addition, it would have preferable to have independent estimates for all of the significant target 

pests.

Calculated values for x are given in Table A1-5 using equation A1-3. To be a high-dose, x > 50. 

ECB/SWCB has a dilution factor greater than 50 and the triple-stack is generally considered to 

be high-dose against these pests. The dilution factor for SWCB alone may range from 32 to well 

above 50. The triple-stack is likely to be high dose for SWCB alone. The dilution factors are 

lower for SCB, but the triple-stack may be high dose. The dilution factors for CEW and FAW 

are much lower than 50 and the triple stack are considered to be low dose for these species. 

Three points should be emphasized about this analysis. First, the logit transform of mortality 

may provide better parameter estimates than the probit transformation when estimated mortality 

in the concentration-mortality assay is near 0 or 1. Second, the slopes should be estimated using 

realistic exposure methods so that the assumption of proportionality of concentrations is met. 

The diet overlay method may overestimate or underestimate the slope compared to incorporating 

whole plant tissue. This would, respectively, underestimate or overestimate the dilution factor. 

Third, the propagation of statistical error in E and S needs to be determined. 

In any event, this analysis shows that readily available data can be used to determine dose, and 

additional standard and/or standardizable experiments can be conducted to obtain improved 

determinations of dose. Moreover, it shows that dose cannot be inferred from efficacy alone. 

The slope of the concentration-mortality relationship is also needed. For example, if E = 0.907 

and S = 0.55, the dilution factor would be 254, a high dose even without extremely high efficacy. 

Indeed, with S = 0.55, E can be as low as 0.825 and still be high dose. Alternatively, a very high 

efficacy, E = 0.999, and a steep slope, S = 2, would give a dilution factor x = 35, which is not 

high dose. Any slope greater than 1.81 would also give a low dose, despite the extremely high 

efficacy. 

Table A1-5. Estimated dilution, x, of triple-stack plant toxin concentration to reach an LC50

1

Lepidopteran 

Target Pest1

Estimated 

Efficacy (E)

Slope2

(∆probit 

mortality/ 

∆Log10[ ])

Estimated 

Dilution 

Factor (x)

Panel Determination 

of Dose

ECB/SWCB 1.000 1.37 5203 High dose

SWCB 0.980-1.000 32-5203 Probably high dose

CEW 0.589-0.889 1.6-11.1 Low dose

SCB 0.916-0.995 15.1-159 Possibly high dose

FAW 0.811-0.907 1.17 5.7-13.5 Low dose

FAW PR 0.674 0.55 6.6 Low dose

1. ECB = European corn borer, SWCB = Southwestern corn borer, CEW = Corn earworm, SCB = 

Sugarcane borer, FAW = Fall armyworm, mainland USA, FAW PR = Fall army worm in Puerto Rico

2. Average slope for susceptible ECB and FAW or resistant FAW (for FAW PR) exposed to Cry1F, 

Cry1A.105, or Cry2Ab separately. Slope for ECB was used for SWCB. Slope for FAW was used for 

CEW and SCB. From Table A2.3.

3. Dilution factor calculated assuming the efficacy was 0.9999 instead of 1.000. If efficacy was only 

0.99, the dilution factor would still be at least 50.
Appendix 2: Two-Locus Model of Resistance Evolution

This Appendix provides the Ives et al. model of resistance evolution (Ives et al. 2011) and how it 

was used by the Panel to address Charge Questions 8 and 9. As a two-locus model, it is most 

appropriate for investigating the evolution of resistance to SmartStax by CRW. Nonetheless, it is 

also used here to investigate the high-dose case appropriate for ECB and SWCB. Although there 

are three toxins targeting ECB and SWCB, and hence a three-locus model of resistance might be 

called for, the two-locus model can nonetheless be used to investigate the conceptual issues 

involved in resistance evolution. The factors affecting resistance evolution in the two-loci model 

are the same as those in a single-locus model (Ives et al. 2011), and there is no reason to suspect 

that a three-loci model differs.

The description of the model is largely a repeat of what is found in Ives et al. (2011). The base 

model keeps track of both allele frequencies and insect densities, and is similar to the singlelocus model analyzed by Ives and Andow (2002). The base model is described first followed by

modifications for seed mixtures and larval movement among plants, for explicit spatial structure 

and limited adult movement.

Base Model

The base model assumes that adults have high movement rates among fields and therefore are 

effectively uniformly distributed among refuge and Bt fields in proportion to their areal extent; 

each generation a proportion Q of the adult population occurs in refuge and 1 – Q in Bt fields. 

Mating within fields is random, with females producing F offspring. Resistance to each of two 

Bt toxins is governed by diallelic, independently segregating loci, with R1 and S1 denoting 

resistant and susceptible alleles to Bt toxin 1, and R2 and S2 denoting the resistant and susceptible 

alleles to Bt toxin 2. Thus, there are nine genotypes of offspring whose frequencies within fields 

are at Hardy-Weinberg equilibrium. The survival of offspring with genotypes R1R1, R1S1, and 

S1S1 from Bt toxin 1 are given by s1RR, s1RS, and s1SS, and similarly s2RR, s2RS, and s2SS give the 

survival associated with Bt toxin 2. Survival on plants containing both of the Bt toxins are 

assumed to be multiplicative, as is expected if toxins have independent modes of action 

(Raymond et al. 1989). For example, the survival of an S1S1S2S2 individual on Bt plants is s1SS

× s2SS, and the survival of an R1S1R2S2 individual is s1RS × s2RS. For baseline parameter values, 

we assume s1RR = s2RR = 1, s1RS = s2RS = 0.0595, and s1SS = s2SS = 0.01; for these values, the 

dominance of both resistance alleles is h = 0.05. Although the model of Ives et al. (2011) can 

incorporate a cost of resistance, here survival of all genotypes on non-Bt plants is 1.

Following any mortality caused by Bt, there is density-dependent survival given by (1 + x)

–1

where x is the density (all genotypes) of surviving larvae within a field. The specific form of this 

survival function makes little difference for any of our qualitative or quantitative conclusions. 

Because the model explicitly keeps track of the number of individuals of different genotypes, 

rather than just genotype frequencies, density-dependent survival changes the rate of resistance 

evolution. To investigate this effect, the model can be run as a frequency-only model that does 

not keep track of population densities, only gene frequencies. The frequency-only model is the 

same as the model including density dependence, except the term (1 + x)

–1 is removed and 
120

genotype densities are converted to frequencies each generation. The frequency-only model is 

essentially identical to two-toxin models analyzed previously (Mani 1985; Gould 1986; Roush 

1998; Gould 2006).

To run simulations, we assume initial resistance allele frequencies are 0.005 for both resistance 

genes. Failure of Bt crops (i.e., when the insect population is resistant) is assumed to have 

occurred when both resistance alleles exceed a frequency of 0.5. In some scenarios, in particular 

when the proportion of refuges (or non-Bt plants in seed mixtures) is very small, insect densities 

can be very low when this criterion for Bt failure is reached. Nonetheless, once resistance allele 

frequencies reach 0.5, the resistant population recovers from low density very rapidly, so using 

this criterion to assess resistance failure gives similar results to those obtained by using a 

threshold density of insects.

For the case of seed mixtures, all fields are the same and contain a fraction q of non-Bt plants and 

(1 – q) of Bt plants, with females depositing eggs such that larvae initiate on non-Bt and Bt plants 

in proportion to their prevalence. We follow Mallet and Porter (1992) in assuming that larvae 

have two stages (a = young and b = old) and move between plants with probability μ between 

stages. When they move, they move to non-Bt or Bt plants with probabilities q and (1 – q). For 

comparison among cases with different larval movement probabilities μ, we assume that survival 

on Bt plants of susceptible genotypes in different larval stages, s

a and sb

, combine 

multiplicatively. Thus, the total survival of a susceptible larva to Bt toxin 1 that remains on a Bt

plant is s1SS = s

a

1SS × s

b

1SS, and the total survival of a heterozygous larva to Bt toxin 2 that 

remains on a Bt plant is s2RS = s

a

2RS × s

b

2RS. Similarly, the total survival of a heterozygous larva 

to Bt toxin 2 that moves from a Bt to a non-Bt plant is s2RS = s

a

2RS × w

b

2RS, where w is used to 

denote the survival on non-Bt plants. The survival rates to the two toxins are then combined as 

before to give the total survival from both toxins; thus, the total survival of a S1S1R2S2 larva that 

moves from a Bt to a non-Bt plant is s

a

1SS × w

b

1SS × s

a

2RS × w

b

2RS. Finally, density-dependent 

mortality occurs at the scale of individual plants after mortality has been caused by Bt; thus, the 

survival of insects on a plant is given by (1 + xp)

–1 where xp is the density of second-stage larvae 

per non-Bt or Bt plant.

The spatially structured model is built on a 50 by 50 grid of same-sized fields, with a proportion 

Q being refuge fields and a proportion (1 – Q) being Bt fields. Assuming that the entire 

landscape is made up of refuge or Bt fields ignores the possibility of fields of other crops or noncrop habitat. Biologically, this is equivalent to assuming that, even though different types of 

habitats may be available on a real landscape, these habitats are permeable to dispersing adults 

who move through them as if they were not there. Refuges are distributed randomly on the grid, 

and crop rotation is included by randomly rearranging refuges on the grid. Cases in which fields 

are either rotated every three insect generations or never rotated are investigated.

When males disperse from their natal fields, they do so before mating, whereas females disperse 

from natal fields following mating. The probability of dispersing a linear (Euclidean) distance x

from natal fields is proportional to dx

, so dispersal drops off exponentially with distance; the 

fraction of adults remaining in their natal field is proportional to d0 = 1. At d = 0.9, the results 

from the spatially explicit model are almost identical to the base model that assumes complete 

spatial mixing of adults. Finally, the grid of fields has “wrap-around” boundaries (i.e., the grid is 
121

on a torus), so that insects dispersing off one side of the grid appear on the opposite side; this 

assumption makes the dynamics on the 50 by 50 grid better approximate the dynamics expected 

for a much larger grid while remaining computationally manageable. Comparison of simulations 

on the 50 by 50 grid to those on a 100 by 100 grid showed no effect of grid size on the 

conclusions. Within fields, we make the same assumptions as in the base model. There is 

random mating and no movement of larvae among plants. Following mortality from Bt toxins, 

survival depends on the density within each field according to the equation (1 + xij)

–1

, where xij is 

the density of larvae in the ij-th field.

Limited Dispersal and Spatial Refuges

Although there are justifiable concerns about low adult dispersal rates causing rapid resistance 

evolution, generally this requires very low dispersal rates for both sexes (Fig. A2-1a). Also, the 

risks posed by low dispersal rates depend on the rotation of refuge fields, with lower rotation 

rates reducing the risk of resistance evolution (Fig. A2-1b). The Panel stated that for the pests 

targeted by SmartStax, it is unlikely that adult dispersal rates are so low that the risk of resistance 

evolution are greatly increased, at least under current refuge implementation requirements set by 

the EPA.

Survival of Heterozygotes

The survival of genotypes, especially of heterozygotes, in comparison to susceptible 

homozygotes, plays a central role in determining the rate of resistance evolution. Thus, it is 

necessary to know how the survival rates combine for two or three resistance loci. When larvae 

move between Bt and non-Bt plants, it is also necessary to know the survival of all genotypes 

before and after movement. There is high uncertainty about the survival of all genotypes and 

how these survival rates are affected by larval movement. The analyses performed by the Panel

illustrate the sensitivity of resistance evolution to these uncertainties. Five cases of the relative 

survival of pre- vs. post-movement larvae were considered.

1) In Case 1, survival rates are equal between pre- and most-movement stages, with survival 

rates for each stage multiplicatively combining to give the overall survival of larvae. 

2) In Case 2, all Bt mortality occurs before movement.

3) In Case 3, all Bt mortality occurs after movement (Fig. A2-2). 

Comparing these three cases, Case 1 leads to the fastest resistance evolution rate, while 

Case 2 has slowest resistance evolution rate (Fig. A2-3a). Furthermore, for Case 2 the 

rate of resistance evolution decreases with increasing larval movement. This happens 

because all Bt mortality occurs before movement, and allowing larvae to move spreads 

larvae among plants and decreases density-dependent mortality. This decrease in 

density-dependent mortality favors susceptible larvae that would otherwise be confined to 

non-Bt plants, thereby decreasing the relative survival of heterozygotes and slowing 

resistance. This explanation can be tested using the frequency-only model (Fig. A2-3b); 
122

when density-dependent mortality is removed, Cases 2 and 3 give the same rates of 

resistance evolution that are independent of larval movement. 

4) In Case 4, most but not all mortality occurs before movement.

5) In Case 5, most of the mortality occurs after movement. 

In both of these cases, resistance evolves more rapidly than in Case 1 (Fig. A2-4). Note: 

Case 4 could be viewed as being intermediate between Cases 1 and 2, while Case 5 could 

be viewed as being intermediate between Cases 1 and 3. Nonetheless, in both cases the 

rate of evolution is not intermediate.

To illustrate the sensitivity of resistance evolution to the survival of heterozygotes, consider the 

case of increasing the survival rates of S1S1R2S2, S1R1S2R2, and S1S1R2R2 individuals by a factor 

of five (Fig. A2-5a). In each case, the rate of resistance evolution increases, although this 

increase is least for S1S1R2R2 individuals (Fig. A2-6a). This cannot be solely due to the relative 

rarity of R2R2 homozygotes, because when the initial frequencies of both R1 and R2 are the same, 

the frequencies of S1R1S2R2 and S1S1R2R2 genotypes are the same. When the survival rates of

heterozygotes, S1S1R2S2 and S1R1S2S2, and S1S1R2R2 and R1R1S2S2,are increased symmetrically 

(Fig. A2-5b), the results are similar to those in Fig. A2-6b. This suggests that asymmetrical 

changes in the survival of heterozygotes can have impacts on resistance evolution almost as large 

as symmetrical changes.

This last result can be interpreted as meaning that when there are asymmetrical effects on 

resistance loci, the rate of resistance evolution will be determined by the most rapidly evolving 

locus. To illustrate this, we considered three cases; (i) s1SS = s2SS = 0.01, (ii) s1SS = 0.01 and s2SS

= 0.001, and (iii) s1SS = s2SS = 0.001. The rate of evolution for case (ii) is most similar to the rate 

of evolution to case (i), which gives the most rapid case of resistance (Fig. A2-7). 

Monsanto’s Implicit Modeling of Larval Movement

In Monsanto’s analysis, they model larval movement by decreasing the size of the refuge 

(proportion of plants that are non-Bt) to account for mortality of larvae that move from non-Bt

plants to Bt plants. This implicit method of accounting for the movement of larvae does not 

include any effects on the selection of heterozygotes. As first demonstrated by Mallet and Porter 

(1992), this is the main effect by which planting seed mixtures speeds resistance evolution, and 

therefore the Monsanto model will overestimate the time to resistance failure.

To illustrate this with the SAP model, the model was run under the baseline conditions, and 

Monsanto’s procedure was then used to model the case of reducing the proportion of non-Bt

plants in the seed mixture in a way that matches the survival of susceptible insects in the SAP 

model (Fig. A2-8). This demonstrates that the procedure Monsanto used to implicitly model 

larval movement shows very little effect of larval movement on the time to resistance failure up 

to the point that the insect population decreases towards extinction.
123

High and Low Dose Scenarios and Larval Movement in Seed Mixtures

The effect of larval movement to increase the rate of resistance evolution applies mainly to the 

high-dose case, rather than the low-dose case. This is illustrated in Fig. A2-9 using the low-dose 

survival rates given in the EPA/ORD model for CRW.

Density-Dependent vs. Frequency-Only Models

The EPA/ORD model included only allele frequencies and not insect densities. This will give 

higher estimates of the time to resistance failure than a corresponding density-dependent model 

for the situation in which populations are persistent and density dependence operates at the scale 

of individual plants (Fig. A2-10).

Seed Mixtures vs. Structured Refuges

In this situation, the rate of resistance evolution between seed mixtures and structured refuges is 

compared for the case in which insects have long-range adult dispersal and larvae move among 

plants. For the case of long-range adult dispersal, the case of structured refuges will be the same 

as the case of seed mixtures when there is no larval movement. Therefore, the decrease in time 

to resistance failure caused by larval movement represents the lower durability of seed mixtures

relative to a structured refuge.

For the baseline parameter values and the density-dependent model, when 0.3 of the larvae move 

between plants, no seed mixture with the proportion non-Bt seeds less than 40% performed as 

well as a 5% structured refuge (Fig. A2-11 a-c). This is because, when the proportion of non-Bt

plants is sufficient to maintain the insect population (to the right of the arrows), the time to 

resistance failure actually decreases with the proportion of non-Bt seeds in the seed mixture until 

the seed mixture contains a majority of non-Bt seeds. This unintuitive result is caused by the 

combined effects of larval movement and density dependence.

When the effect of density-dependent survival is removed by using the frequency-only model, 

the time to resistance failure increases monotonically with the proportion of non-Bt seed in the 

seed mixture (Fig. A2-11 d-f). In this case, a 5% structured refuge is roughly equivalent to a 

20% seed mixture when 0.3 of the larvae move among plants; however, if 0.6 of the larvae 

move, then a seed mixture of >60% is required for equivalent durability to a 5% structured 

refuge. 

These results show the very large loss of durability caused by a seed mixture under high-dose 

conditions. For the more-realistic case of the density-dependent model, when there is a modest 

rate of larval movement (0.3 of the larvae move), there is effectively no seed mixture that 

provides the durability of a 5% structured refuge. Even under the unrealistically optimistic case 

of a frequency-only model, a 20% seed mixture is needed.

In conclusion, it is important to remember that comparisons between seed mixtures and 

structured refuges are only intended to show the very strong effects that larval movement can 
124

have for seed mixtures. The Panel has repeatedly noted that there are sufficient uncertainties in 

all models that quantitative predictions should be used with extreme caution. That includes the 

comparisons between seed mixtures and structured refuges.

Fig. A2-1. For structured refuges, the effect of adult dispersal distance on the rate of resistance 

evolution (generations to resistance failure, defined when the frequency of both resistance alleles 

exceeded 0.5). In the top panel, refuge fields are rotated every three generations, while in the 

bottom panel refuge fields never rotate. For lines labeled “male”, males have mean dispersal 

distances given by the x-axis, while females have infinite dispersal distances; the opposite 

applies for the lines labeled “female”. For lines labeled “both”, the mean dispersal distances of 

both males and females are given on the x-axis. Gray lines give the case of "infinite" dispersal 

computed for the spatially implicit model. Data are simulated on a 50x50 grid of fields with 

“wrap-around” boundaries. Baseline parameter values are: proportion of refuge fields Q = 0.05; 

female fecundity F = 100; s1SS = s2SS = 0.01; h1 = h2 = 0.05; and initial allele frequencies 0.005.
125

Fig. A2-2. Survival of genotypes before (upper tables) and after (lower tables) for five cases that 

differently distributed selection on larvae before and after movement between plants. In all 

cases, the total survival of each genotype on Bt plants if there is no larval movement is the same.

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR

RR 1 1 1

SR 1 1 1

SS 1 1 1

SS SR RR

RR 0.1 0.24 1

SR 0.024 0.06 0.24

SS 0.01 0.024 0.1

SS SR RR

RR 1 1 1

SR 1 1 1

SS 1 1 1

SS SR RR

Stage 1 Stage 2

Case 1 Case 2 Case 3

RR 0.31 0.5 1

SR 0.16 0.24 0.5

SS 0.1 0.16 0.31

SS SR RR

RR 0.31 0.5 1

SR 0.16 0.24 0.5

SS 0.1 0.16 0.31

SS SR RR

RR 0.032 0.12 1

SR 0.004 0.015 0.12

SS 0.001 0.004 0.032

SS SR RR

RR 0.032 0.12 1

SR 0.004 0.015 0.12

SS 0.001 0.004 0.032

SS SR RR

Case 4 Case 5
126

Fig. A2-3. Generations to control failure as a function of the proportion of larvae moving 

between plants for the density-dependent model (top panel) and frequency-only model (bottom 

panel). Survival rates for cases 1-3 are given in Fig. A2-2. Baseline parameter values are: 

proportion of non-Bt plants in seed mixture q = 0.05; F = 100; and initial allele frequencies 

0.005.
127

Fig. A2-4. Generations to control failure as a function of the proportion of larvae moving 

between plants for the density-dependent model. Survival rates for cases 1, 4 and 5 are given in 

Fig. A2-2. Baseline parameter values are: proportion of non-Bt plants in seed mixture q = 0.05; 

F = 100; and initial allele frequencies 0.005.
128

A. Asymmetric Changes 

B. Symmetric Changes 

Fig. A2-5. Baseline values for total survival of each genotype highlighting those survival rates 

that were increased by 5-fold in the simulation experiments to investigate the effects of 

heterozygote survival on the rates of resistance evolution (Fig. A-6). The top table (A) gives the 

case of asymmetric changes and the bottom table (B) gives the case of symmetric changes. Blue, 

red, and green shading denote S1S1R2S2, S1R1S2R2, and S1S1R2R2 genotypes (asymmetrical case) 

and S1S1R2S2 and S1R1S2S2, S1R1S2R2, and S1S1R2R2 and R1R1S2S2 genotypes (symmetric case).

RR 0.01 0.06 1

SR 0.0006 0.0035 0.06

SS 0.0001 0.0006 0.01

SS SR RR
129

Fig. A2-6. Generations to control failure as a function of larval movement (proportion of larvae 

moving) for the experiment in which survival of different genotypes are increased by a factor of 

5 (see Fig. A2-5). Baseline parameter values are: proportion of non-Bt plants in seed mixture q = 

0.05; F = 100; s1SS = s2SS = 0.01; h1 = h2 = 0.05; and initial allele frequencies 0.005.

Asymmetric

Symmetric
130

Fig. A2-7. Resistance evolution for two loci under different selection pressures. (a) Example 

trajectories of allele frequencies when s1SS = 0.01 and s2SS = 0.001, where the black line 

corresponds to the frequency of the resistance allele in the fastest-evolving locus, R1. (b) 

Generations to resistance failure as a function of larval movement (proportion of larvae moving 

between plants). Baseline parameter values are: proportion of non-Bt plants in seed mixture q = 

0.05; F = 100; h1 = h2 = 0.05; and initial allele frequencies 0.005.
131

Fig. A2-8. (a) Generations to resistance failure (frequency of both alleles > 0.5), (b) survival of 

S1S1S2S2 homozygotes, and (c) relative survival of heterozygotes for the cases in which larval 

movement is explicitly or implicitly modeled. For implicit modeling of larval movement, the 

survival of S1S1S2S2 susceptibles is decreased (by decreasing the proportion of non-Bt plants in 

the seed mixture, q) (panel b), while there is no change in the relative survival of heterozygotes 

(panel c). In (a), the break in the slope of the line for implicit larval movement (gray line) 

corresponds to the point above which the insect population starts to decline towards extinction. 

Baseline parameter values are: proportion of non-Bt plants in seed mixture q = 0.05; F = 100; 

s1SS = s2SS = 0.01; h1 = h2 = 0.05; survival is divided equally between pre- and post-larval 

movement; and initial allele frequencies 0.005.
132

Fig. A2-9. (a) Generations to resistance failure (frequency of both alleles > 0.5), (b) survival of 

S1S1S2S2 homozygotes, and (c) relative survival of heterozygotes for high- and low-dose cases. 

In the high-dose case, s1SS = s2SS = 0.01 and h1 = h2 = 0.05, and relative survivals are combined 

multiplicatively. In the low-dose case, values provided by EPA/ORD for CRW are used: sSSSS = 

0.018, sSSSR = 0.019, sSSRR = 0.038, sSRSS = 0.02, sSRSR = 0.0233, sSRRR = 0.0861, sRRSS = 0.058, 

sRRSR = 0.1051, and sRRRR = 1. Baseline parameter values are: proportion of non-Bt plants in seed 

mixture q = 0.05; F = 100; and initial allele frequencies 0.005.
133

Fig. A2-10. (a) Generations to resistance failure (frequency of both alleles > 0.5), (b) survival 

of S1S1S2S2 homozygotes both before and after density dependence occurs in the densitydependent model, and (c) relative survival of heterozygotes for the density-dependent and 

frequency-only models. Baseline parameter values are: proportion of non-Bt plants in seed

mixture q = 0.05; F = 100; s1SS = s2SS = 0.01; h1 = h2 = 0.05; survival is divided equally between 

pre- and post-larval movement; and initial allele frequencies 0.005.
134

Fig. A2-11. Generations to resistance failure (frequency of both alleles > 0.5) for densitydependent and frequency-only models for F = 100, 50, and 20 (labeled) when larval movement is 

0, 0.3, and 0.6. In the density-dependent model, arrows mark the proportion of seed mixtures

below which the population decreases towards extinction. Baseline parameter values are: s1SS = 

s2SS = 0.01; h1 = h2 = 0.05; survival is divided equally between pre- and post-larval movement; 

and initial allele frequencies 0.005.