OFFICE OF 

PREVENTION, PESTICIDES, AND 

TOXIC SUBSTANCES 

MEMORANDUM February 4, 2010 

DP barcodes: 370457 

 Decisions: 390180 

 

SUBJECT: Science Assessment of Pioneer’s modeling addenda (MRIDs 478710-01, 

478879-01, and 479108-01) in support of their application of registration 

for Optimum AcreMax1 Insect Protection 

TO: Mike Mendelsohn, Senior Regulatory Specialist 

 Microbial Pesticide Branch 

 Biopesticides and Pollution Prevention Division (7511P) 

FROM: Jeannette Martinez, Ecologist 

 Microbial Pesticide Branch 

 Biopesticides and Pollution Prevention Division (7511P) 

SECONDARY Alan Reynolds, Entomologist 

REVIEW: Microbial Pesticide Branch 

 Biopesticides and Pollution Prevention Division (7511P) 

ACTION REQUESTED: 

The BPPD1

 has been asked to review three addenda by Pioneer (MRIDs 478710-01, 478879-01, 

and 479108-01) to complement their original modeling chapter (MRID 473567-08) submitted to 

support the Sec(3) registration request for Optimum® AcreMaxTM 1 Insect Protection (OAM1). 

OAM1 is a Bt corn seed blend consisting of 1) seeds that express a stack of Cry1F to target 

Lepidoptera corn pests and Cry34/35 to target corn rootworm (CRW) and 2) seeds that express 

Cry1F only (refuge seed for CRW). The registrant requests a 5% seed blend registration with a 

separate 20% block refuge (containing 100% Cry34/35) for Lepidoptera planted immediately 

adjacent to the Bt field. 

 

1

 The use of BPPD in this review refers to the BPPD IRM team consisting of Jeannette Martinez and Alan Reynolds. 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 
- 2 -

CONCLUSIONS AND RECOMMENDATIONS 

1) Overall Conclusions

BPPD has reviewed Pioneer’s submissions for Optimum AcreMax1 (OAM1) including revised 

modeling with new movement data. In addition, BPPD conducted an independent modeling 

analysis of Pioneer’s proposal using a model (deterministic and stochastic version) developed by 

EPA/ORD. After careful review of Pioneer’s modeling (including parameter assumptions) and 

based on the analyses conducted by ORD (modeling efforts and probability analysis), BPPD 

concludes that the registrant’s assertion that a 5% seed blend is always superior to the current 

20% block refuge deployment strategies for OAM1 and corn rootworm” is not scientifically 

supported. 

Pioneer’s model made a number of assumptions on CRW movement, refuge location, and 

percent non-compliance. In their model, when these assumptions were included, a 5% seed blend 

was more durable than a 20% block refuge. However, BPPD has concerns that some of Pioneer’s 

assumptions may not be realistic or conservative enough to support a reduced refuge for OAM1. 

The following are Pioneer’s modeling parameters and assumptions which were not supported by 

BPPD’s and ORD’s analyses and were of concern: random mating, neonate dispersal, adult 

movement, planting location of block refuges, 0% non-compliance, Lepidopteran refuge. 

Considering all of the identified uncertainties and concerns discussed throughout this risk 

assessment, BPPD concludes that the modeling conditions/parameters used in Pioneer’s model 

(compliance, CRW adult dispersal, neonate movement rather than late-instar movement, refuge 

placement to maximize distance between refuges) are unfavorable to blocks while being more 

favorable to blends. This had the effect of biasing Pioneer’s modeling analysis towards more 

durable seed blends and less durable block refuges. 

2) ORD and Pioneer models: a side-by-side comparison of modeling output using 

Pioneer’s benchmark parameter values

A side-by-side comparison (using Pioneer’s benchmark parameter values)between the ORD’s 

and Pioneer’s deterministic models showed that the ORD model predicted greater durability for 

blocks than for seed blends (see Table below). Predicted durability estimates were similar 

between the two models except for the 20% block refuges. A likely explanation for the predicted 

greater durability for 20% block refuge strategy is that ORD chose an effective refuge of 15% 

(due to non-random oviposition behavior and choice for refuge placement by growers). In order 

to duplicate Pioneer’s block refuge result (7yrs), the effective refuge size would need to be 

decreased to 1%. This parameter value (1%) was felt to be unrealistically low for a universal 

assumption (for further discussion see Section IV in this review). 
- 3 -

Side-by-side comparison between predicted durability of ORD and Pioneer model with Pioneer’s benchmark 

parameters 

Durability (time to resistance) Refuge Scenario Pioneer EPA/ORD1

No Refuge 5 years 3 years 

20% Block Refuge 7 years 14 years 

5% Seed Blend Refuge 10 years 9 years 

(Table 17 in Section IV) 115% effective refuge was chosen in ORD’s model as most likely value and fixed 30% non-compliance 

3) ORD’s risk profile based on likely more realistic or conservative assumptions

The objective was to use BPPD’s and ORD’s assumptions for ‘likely more realistic and 

conservative’ parameter values and assumptions and to predict durability for 20% block and 5% 

seed blend under these conditions. In addition, a probability distribution was generated using a 

PERT distribution (setting a minimum, most likely, and maximum value for each parameter of 

concern) to associate probabilities with predicted outcomes. 

a) Deterministic Model

Considering other movement data and recalculating Pioneer’s movement rate by adjusting for 

errors, ORD’s deterministic model predicted that the mean time to resistance for the block refuge 

strategy was 11.95 years (56.55% of the simulations exceeded 10 years). For the seed mixture 

simulations, the mean time to resistance was 8.1 years (13.5% of the simulations exceeded 10 

years) (see Table below). 

ORD's risk profile for Cry34/35 deployed in a 20% block and 5% seed blend based on more realistic 

assumptions and probability analysis 

20% Block 5% Blend 

Mean Durability 11.95 yrs 8.1 yrs 

% of simulations 

exceeding 10 years 56.6% 13.1% 

(Table 18 in Section IV) 

See Section C.2.b and Table 11 and Table 12 for assumptions and parameter values 

b) Stochastic Model

The stochastic simulations with more conservative assumptions (i.e. square field size and no 

long-distance dispersal) predicted that durability for Cry34/35 deployed with a 20% block refuge 

would last approximately 13.9 years for fields of typical size (40 ha); for large 144 ha fields 

(which are not average size but outliers), durability is predicted to be around 8.4 years. 
- 4 -

4) Options

i) Additional Data: 

Pioneer’s conclusions that a 5% seed blend is superior to the 20% block refuge were the result of 

a number of uncertain (possibly unrealistic) assumptions that would have to hold true for the 

entire Corn Belt. These include: 

• Refuges would have to be planted consistently along the shorter side of the Bt fields. 

This is against grower practice. 

• Growers would have to move their refuge locations yearly. This is believed to be 

unlikely. 

• Growers would have to plant refuges consistently over areas of lowest egg densities. 

This is probably not a realistic assumption. 

• Adult male movement would have to be limited to Pioneer’s estimate of 15 m/d. 

ORD’s analysis demonstrated that Pioneer underestimated dispersal; after having 

corrected for errors, adult movement was similar to what was reported by Spencer 

(2009). 

EPA/ORD’s interpretation of the data suggest that non-random oviposition and refuge 

placement are most important rather than non-random mating being the threat to a block 

refuge system. This suggests a potentially plausible method that Pioneer might use to 

bolster their claims that failure of the block refuge system is imminent. If the relative 

density of eggs could be estimated in transects across fields of different sizes, and if more 

accurate estimates of how growers plant and move their refuges over years were 

available, then major differences between the ORD and Pioneer models could be 

resolved. Such a study might also provide an estimate of the contribution of beetles 

dispersing long distance. 

ii) Different blend percentages:

ORD explored the durability for different percentages of seed blends (2.5%, 5%, 7.5%, 

and 10%) and reported that predicted durability for the 10% seed blend was 11.3 yrs 

(with 51.6% of the simulations exceeding 11 years). While this is still lower than the 

predicted durability for a 20% block refuge, the 10% seed blend had greatest durability 

among the percentages modeled and came closest to the predicted 20% block durability. 

iii) Temporal and/or geographically limited registration:

BPPD has considered temporally/geographically-limited registrations in the past because 

they may reduce the risk for resistance evolution. These registrations are typically limited 

by total U.S. acreage (usually <250,000) and have county-level acreage caps (usually 

<20,000). However, the modeling analyses conducted by Pioneer and ORD were not 

designed to address the risk of a time limited registration for OAM1. Therefore, BPPD 

cannot specifically quantify the risk of resistance evolution for such an option. 

iv) Seed blend with a pyramid PIP:

BPPD notes that models predict that pyramids of two PIP toxins can increase the 

durability of the traits and allow for a reduction of the percentage of refuge. ORD’s 
- 5 -

modeling showed that if two traits with the characteristics of Cry34/35 were pyramided 

and deployed as a 5% seed blend that durability would increase by more than three fold 

(52 years) over the sequential use of the two single toxins. Pioneer could consider the 

addition of a second CRW-active toxin to OAM to improve the durability and potentially 

justify a 5% seed blend refuge. 
- 6 -

BACKGROUND 

In February, 2008, Pioneer Hi-Bred International, Inc. submitted the original application to 

register Optimum® AcreMaxTM 1 Insect Protection, a new corn seed blend product that contains 

two different kinds of seeds from the already registered products Herculex XTRA and Herculex 

I. The new product would incorporate the CRW refuge into the seed bag. In addition to the seed 

mix request, Pioneer also requested a reduction in refuge for CRW from the standard 20% to 

3.5% (±1.5). 

A seed blend strategy and reduction in refuge from 20% to >2% represented a paradigm shift for 

BPPD’s Insect Resistance Management (IRM) program, and based on the recommendations of a 

prior SAP (2000), the Agency was discouraged from permitting seed mixtures as a refuge option 

for Lepidoptera due to potentially high movement of larvae between Bt and non-Bt plants. BPPD 

conducted a risk assessment (BPPD 2008a and 2008b) and presented preliminary conclusions to 

the Scientific Advisory Panel for peer review on February 23-24, 2009. 

The SAP (2009) expressed concern about the proposal to greatly reduce refuge for a single toxin, 

non-high dose PIP. The Panel concluded that there was no compelling evidence to support a 

reduction in refuge from 20% to >2% from the data provided by Pioneer or found in the public 

literature. However, the Panel agreed that a seed mix strategy would lend itself to corn rootworm 

biology. The final recommendation by the Panel was the use of a 20% seed blend for OAM1 

due to the non-high dose characteristics of Herculex Cry34/35 corn. Among other things, the 

Panel commented on the study by Lefko et al. (2008), upon which Pioneer based its assumptions 

for the value of the major resistance gene frequency (0.00035) in the simulation model. The 

Panel concluded that there was no rigorous evidence in Pioneer’s reported data that the 

frequency of major resistance genes was very low. Using the formulae by Andow and Alstad 

(1998), the Panel calculated the initial frequency of the resistance allele to be 0.0015 but stated 

that Lefko et al. (2008) could not be used reliably to establish an initial frequency or a reasonable 

upper bound estimate for the frequency of the major resistance allele due to a number of 

problems with the study. The SAP further stated that considering the limitations of the study, 

even this estimated value (0.0015) could not be considered a generous upper bound estimate. 

With respect to Pioneer’s dominance value chosen for the major resistance allele (0.05), the 

Panel stated that such a value was a reasonable upper bound estimate for genes that confer more 

than 300 fold resistance (fitness of RR genotype/fitness of SS genotype). Evidence suggests, 

however, that resistance in CRW may be based on genes of relatively minor effects that confer 

less than 50-fold resistance. Caprio et al. (2000) suggested that for genes of this level of effect, 

no upper bound of less than 1.0 could be assumed. 

In October 2009, Pioneer submitted an amended request for a 5% OAM1 seed blend registration. 

To support this request, the registrant provided three new addenda (MRIDs 478710-01, 478879-

01, and 479108-01) to their original modeling submission, which are subject of this review. 

BPPD received permission from Pioneer to consult with academia and other CRW experts on the 

latest data submission. Their comments and expert opinions are cited in this document. BPPD 

will discuss its independent simulation analyses of the seed mix proposal using “USEPA 

population genetics two locus model – POPGEN” and a variation of Caprio et al.’s stochastic 
- 7 -

model (2006). The results of BPPD’s independent modeling are discussed in Section III of this 

review. 

I. CRW BIOLOGY

a. Additional Data on CRW Biology 

Nowatzki et al. (2003a and 2003b) conducted a field study (mark-recapture) with western corn 

rootworm (Diabrotica virgifera virgifera, WCR) using a rubidium labeling technique to quantify 

short range, in-field movement of adult beetles from the time of their emergence. The experiment 

was conducted in two corn fields near Mead (NE) during 2002 and 2003. 

Corn plants were treated with RbCl at the center of a non-transgenic corn field and then infested 

with 200 WCR eggs/plant. Larvae feeding on the treated plants were labeled with RbCl nondisruptively as were newly emerged adults. Beetle movement was then tracked with traps 

positioned in a circular pattern at fixed distances (50 ft, 100 ft, 200 ft, 300 ft, 400 ft, and 600 ft) 

from the center infested area to the field edge. Beetles were collected throughout the emergence 

time at regular intervals. 

Newly emerged beetles were collected concurrently from the center infested area of the fields in 

2003, fed a non-Rb treated diet of corn in the lab, and sub-sampled daily for five days in order to 

determine the Rb-decay rate after emergence. First order exponential decay model was fit to 

natural log transformed data (Rb concentrations). According to the model, labeled beetles were 

distinguishable from unlabeled beetles for approximately four days (no difference between male 

and female beetles). 

The Rb-decay curve allowed 1) estimating the time from when beetles left the treated area to 

when they were captured in the traps but 2) required an assumption regarding unknown Rb 

concentration of individuals after they had stopped feeding on Rb-treated plants (= t0). Nowatzki 

et al. assumed that a beetle’s t0-Rb concentration was equal to the average Rb concentration of 

beetles captured in traps within the Rb-treated region of the corn field. The rate of movement 

was calculated using 1) the distance traveled from the center of the field to the trap and 2) 

estimated time elapsed since feeding had ceased. 

Estimated beetle movement results separated by timing and sex are listed in Table 2. In 

summary, males moved an average of 13.9 m/day prior to female emergence and 15.2 m/day 

during peak female emergence when the data were pooled across both years. Females moved on 

average 1.9 m/day during peak female emergence and 13.1 m/day during post-female emergence 

(data pooled across years). 

Pioneer concluded from this study that male WCR beetles movement was much less than what 

was previously assumed (45 m/day). The registrant proposes that 15 m/day for males and 13 

m/day for females should be a more realistic estimate of WCR beetle in-field movement. 
- 8 -

Table 1. Time periods capturing beetle movement based on biological significance 

Relevant time periods Female presence Estimated movement rate 

Pre-female emergence > 10% Male 

Peak-female emergence 40-70% Male and female 

Post-female emergence > 90% Female 

Table 2. Male and Female Beetle Movement Results from Rb Study (data from 2002 and 2003) 

Time 

Period

Sex N Mean 

Movement 

(m/day) 

St Dev 

(m/day) 

Min. 

(m/day) 

25th

Percentile

(m/day) 

Median 

(m/day) 

75th

Percentile

(m/day) 

Max. 

(m/day) 

2002

Prefemaleemergence 

M 305 13.2 23.1 0 0 4.9 15.9 220.8 

Peak- F 59 0.06 0.47 0 0 0 0 3.6 

femaleemergence M 125 16.0 26.2 0 0.7 7.0 23 188.5 

Postfemaleemergence 

F 26 12.8 17.7 0 0 0 30.4 41.6 

2003

Prefemaleemergence 

M 47 18.6 30.2 0 0 6.3 25.7 152.7 

Peak- F 16 8.8 13.8 0 0 0 17.9 38.9 

femaleemergence M 13 7.6 15.2 0 0 0 10.2 40.0 

Postfemaleemergence 

F 41 13.3 13.6 0 0 7.9 25.5 41.4 

Combined Across Years

Prefemaleemergence 

M 352 13.9 24.2 0 0 4.9 18.6 220.8 

Peak- F 75 1.9 7.2 0 0 0 0 38.9 

femaleemergence M 138 15.2 25.4 0 0 6.7 22.6 188.5 

Postfemaleemergence 

F 67 13.1 15.2 0 0 6.4 26.0 41.6 

Modified from MRID 478879-01 
- 9 -

II. PIONEER MODELING (MRIDs 478710-01, 478879-01, and 479108-01) 

A. Purpose 

With the support of three further modeling analyses, Pioneer seeks to demonstrate that the 

addition of a refined and better characterized modeling parameter - an explicit in-field dispersal 

component for adult corn rootworm - adds to the durability of a seed mixture such as OAM1 and 

makes the 5% seed blend superior to block refuge strategies for 59122-7 corn. 

B. Simulation Modeling: Assumptions and Scenarios 

a) Assumptions:

Pioneer’s original model (MRID 473567-08) was deterministic and spatially implicit but has 

since been modified to be spatially explicit between and within Bt and refuge fields in order to 

accommodate the incorporation of adult CRW in-field movement. No previously published or 

publicly available CRW model has incorporated and simulated the effects of such a parameter on 

the durability of 59122 maize; rather, previous models assumed an instantaneous and even 

distribution of refuge beetles in the Bt corn once they reached the edge of the field. Pioneer 

reported that such previously assumed CRW mixing/movement characteristics become 

increasingly unrealistic as Bt field size increases. 

Pioneer modified and varied several input values for main modeling parameters (see value 

ranges in Table 3) based on the feedback received from the SAP 2009 and EPA to more 

conservatively assess the durability of 59122-7 maize for different refuge deployment scenarios. 

In Pioneer’s revised modeling, the effect of a major resistance gene on the trait durability in 

absence of a minor gene was investigated as to avoid uncertainties around the interaction of the 

two genes. In the original model, Pioneer had assumed, based on interpretations of results by 

Lefko et al. (2007) that the presence of a minor gene allowed for sufficient survival of 

susceptible genotypes which helped delay the onset of major resistance evolution. In the latest 

modeling submission, the registrant provided 1) worst-case, 2) benchmark, and 3) best-case 

scenarios for main parameters in the model. Most other assumptions of the original model 

remained in the modified version (e.g., density-dependent mortality, overwintering egg 

mortality) with the exception of, for example, pre-dispersal tasting mortality. 

The model consists of a grid representing 100 m x100 m (1 ha) subunits (cells) within a field or 

set of fields. The program has a limit of 50 x 50 cells for a total evaluation of 2500 ha. For the 

simulations reported in Pioneer’s addenda, a maximum size of 4x10 cells (400 m x 1000 m) or 

40 ha was assumed. Within each cell, the WCR simulation model by Onstad (2006) was 

implemented to capture larval population dynamics. Movement of CRW adults between cells 

was based on the data generated by Nowatzki et al. (2003b). A cell in the model either 

represented a refuge block, a 59122 maize block, or a seed mix block (5% seed blend) depending 

on the scenario modeled. Once male dispersal had occurred, mating was initiated, and 

oviposition was modeled uniformly within each cells. 
- 10 -

Overwintering mortality occurred during the egg stage and was set at 50% (Godfrey et al. 1995). 

Larvae were represented by two life-stages (early and late-stage larva), and larval movement and 

mortality occurred during the early stage only (first instars). Density dependent mortality for 

later-instars was introduced by using Onstad’s density dependent survival algorithm (Onstad et 

al., 2006). Emergence of adults was based upon the emergence curves by Nowatzki et al. (2008). 

Males emerged on average 7 days before females; emergence in Bt fields was further delayed by 

an additional 7 days for both sexes. 

The model code was verified to confirm the expected behavior of the model for extreme 

scenarios (Onstad communication with Pioneer). 

Table 3. Modeling Parameters and Input Values 

Parameter Range values 

Worst-case -

BPPD 

defined* 

Worst-case -

Pioneer 

defined 

Benchmark 

value Best-case value 

Initial adult 

density/ha N/A 80,000 80,000 80,000 80,000 

Proportion of males N/A 0.5 0.5 0.5 0.5 

Plants/ha N/A 79,027 79,027 79,027 79,027 

Initial frequency of 

resistance gene 0.0005-0.01 0.01 0.01 0.001 0.0005 

Dominance 0.025-0.2 0.2 0.1 0.05 0.025 

Early SS-survival 0.0125-0.05 0.05 0.025 0.025 0.0125 

Early RR-survival N/A 1 1 1 1 

% off-type seed N/A 0.75 0.75 0.75 0.75 

Fecundity N/A 356 356 356 356 

SS-oviposition rate N/A 0.32 0.32 0.32 0.32 

RR-oviposition rate 0.32-1 1 1 1 0.32 

Probability of 

leaving Bt N/A 0.625 0.625 0.625 0.625 

Probability of 

leaving non-Bt N/A 0.5 0.5 0.5 0.5 

Early larval survival 

during movement N/A 0.5 0.5 0.5 0.5 

Overwintering 

survival N/A 0.5 0.5 0.5 0.5 

♂ movement pre-♀

emergence (m/d) N/A 15 15 15 15 

♂ movement peak-

♀emergence (m/d) N/A 15 15 15 15 

♀ movement postmating (m/d) N/A 12 12 12 12 

% block compliance 70-100 70-100 70-100 70-100 70-100 

BPPD extracted this information from MRID 478710-01 and MRID 478879-01 * BPPD asked Pioneer to explore more conservative parameter values than in their worst-case situation. These values are 

reflected in the assumptions named ‘worst-case - BPPD defined’. 

Shaded fields indicate parameters whose values were changed for output runs 
- 11 -

b) Refuge Deployment Strategies Modeled:

Several scenarios were modeled with different refuge deployment approaches and assumptions 

about grower compliance with refuge requirements (MRID 479108-01). BPPD lists some of 

these in Table 4. Pioneer then varied input values of uncertain parameters (Table 3) to explore a 

possible range of durability outcomes for each scenario and also to compare durability results 

between different deployment options. 

Scenario #1 involved modeling of 100% 59122 in continuous corn fields without any designated 

refuge portion. 

Scenario #2 involved modeling of 59122 maize in a continuous landscape with a 20% block 

refuge. The refuge was moved to different locations twice during the time modeled. Pioneer 

stated that, based on communications with growers and consultants, moving refuges from year to 

year was a realistic assumption, Grower compliance with refuge was assumed to be 70%. 

Scenario #3 simulated the infield strip refuge option with a 20% refuge and 80% compliance 

with refuge requirements. 

Scenario #4 was a variation of scenario 2) and incorporated the planting of 59122 corn by 

growers for four years followed by a 5% seed blend starting the fifth growing season. 59122 corn 

was modeled with 20% block refuges (location moved once) and a 75% grower compliance with 

refuge requirements. 

Scenario #5 modeled the 20% infield strip refuge with 75% compliance with refuge 

requirements. After four initial years with infield strip refuges, a 5% seed blend was introduced. 

Scenario #6 involved the modeling of a 5% seed blend without a prior deployment of a block 

refuge. 

Table 4. Different Refuge Deployment Scenarios Modeled by Pioneer 

Scenario 

Number 

Refuge Option 

#1 No refuge 

#2 20% Block (moved location twice) with 70% compliance 

#3 20% strip refuge with compliance ranging from 50-83% 

#4 20% Block only (moved location once) for 4 yrs with 75% compliance; introduction of 5% 

seed blend after 4th year 

#5 20% strip refuge only for 4 yrs with 75% compliance; introduction of 5% seed blend after 4th

year 

#6 5% Seed blend only; no inclusion of prior use-history 

C. Sensitivity Analyses 

Pioneer conducted numerous sensitivity analyses for parameters with uncertainty by varying the 

input values in block, strip, and seed blends to assess effects on estimated durability of DAS59122-7. Those parameters that had an effect on the estimated durability are labeled ‘sensitive’ 

in Table 5. However, in their original submission Pioneer used the descriptions ‘relatively 
- 12 -

sensitive’ and ‘relatively insensitive’. Since Pioneer did not define what was meant by ‘relative’ 

as compared to potentially ‘very’ sensitive, BPPD simplified the descriptions by using the terms 

‘sensitive’ and insensitive’ only. The parameters investigated were: 

 

• % seed blend: range of 2.5% - 10% 

• Strip width (number of refuge rows): range of 4 rows – 20 rows 

• % strip compliance: 50% - 100% 

• % block refuge compliance: 50% - 100% 

• Block refuge position: fixed or variable 

• Initial adult density; range of 20,000-160,000 

• Overwintering survival: 0.2, 0.5, and 0.8 

• Male movement (during pre- and peak-female emergence): range of 5m/d- 45m/d 

• Female movement (post-mating): range of 5m/d - 45m/d 

• Dominance of resistance gene: range of 0.05 – 0.2 

• Survival rate of SS-genotype at early life-stage: range of 0.0125 – 0.05 

• Initial resistance allele frequency: range of 0.0005 – 0.01 

Whether parameters were or were not sensitive is displayed in Table 5. In Pioneer’s model, dose, 

initial R-allele frequency, dominance and overwintering survival were sensitive parameters 

under all three deployment strategies. Compliance rate was a sensitive parameter for the 20% 

strip and block refuge. Movements of male and female adults during different time periods were 

sensitive parameters and varying the distance dispersed per day affected the durability of the trait 

when deployed as a 20% block refuge. Varying adult movement in the 5% seed blend and infield 

strip option did not affect the durability of the trait. 

Table 6 shows the durability outcomes (in years) for sensitivity analyses of various seed blends 

implemented after four years of only 1) 20% block refuge (refuge in same location and 100% 

compliance), 2) 20% strip refuge with 75% compliance, and 3) 20% block refuge (refuge moved 

once in four years) with 75% compliance. The last scenario in Table 6 looks at only a seed blend 

option without considering prior use history. Each deployment strategy was modeled with worst 

case, benchmark, and best-case parameter values/assumptions. Among all scenarios and 

combinations of assumptions and seed blends, the 20% strip refuge assuming 100% compliance 

had the greatest durability among all refuge deployment options. Among the remaining refuge 

options, the following order of decreasing durability was observed. 

For Pioneer’s defined best-case parameter values, durability measured in years was second 

highest for the 20% block refuge with 100% compliance with seed blends ranging from 2.5% to 

10% (19 to >20 years). The seed blend deployment without considering prior use history resulted 

in a durability ranging from 17 to > 20 years. There were small differences in years predicted to 

resistance between the blend percentages, and the higher blends had better durability. The 20% 

strip refuge with 75% compliance and various seed blends resulted in a durability ranging from 

16 to >20 years. The 20% block refuge deployment assuming 75% compliance followed by seed 

blends resulted in a durability ranging from 15 to19 years. 
- 13 -

For Pioneer defined worst-case parameter values, a 20% block refuge with 100% compliance as 

well as the 20% strip refuge with 75% compliance both followed by seed blends after four years 

had the greatest durability (5 years for seed blends ranging from 2.5-10%). Seed blends ranging 

from 2.5% to 10% resulted in a durability of 4 years (2.5%-3.5%) or 5 years (5%, 7.5%, and 

10%). There was essentially no difference in predicted durability between seed blends of 

different percentages for the worst-case assumptions. The 20% block refuge with 75% 

compliance followed by seed blends of different percentages resulted in four years of durability. 

For Pioneer’s benchmark parameter values, durability for seed blends ranged from 8 to12 years; 

higher blend percentages predicted higher durability. For 20% block refuge with 100% and 75% 

compliance followed by seed blends after four years, durability ranged from 8 to 10 years and 

from 7 to 9 years, respectively. 

Table 7 shows the sensitivity results for trait durability measured in years for the block refuge 

when compliance was varied for the 20% block refuge. With an assumption of 50% compliance, 

the trait was predicted to last 8 years; with 70% and 80% compliance, the trait was predicted to 

last 10 years; and with 90% compliance, the trait was predicted to last 13 years; with 100% 

compliance, the trait was predicted to last more than 20 years. 

Table 8 shows the sensitivity results for trait durability measured in years for the strip refuge 

when compliance was varied for the 20% strip refuge strategy. When farmers were compliant 

50% of the time with planting refuges, then Pioneer’s model predicted that resistance to the trait 

evolved in 8 years. With 70%, 80%, and 90% compliance, resistance did not evolve until 9 years, 

10 years, and 12 years, respectively. 

Table 9 shows the sensitivity results for three deployment strategies (20% block with 100% 

compliance, 20% infield strip with 100% compliance, and 5% seed blend without prior usehistory) when main parameters, such as dominance and initial allele frequency of the major 

resistance gene, and dose assumptions were varied. 

When dominance for the major resistance allele was increased from 0.025 to 0.2, the durability 

decreased from 17 to 6 years for the 20% block refuge, 20 to 7 years for the 20% strip refuge, 

and 15 to 5 years for the 5% seed blend. The relationship between deployment options and their 

estimated trait durability was generally: 20% Strip > 20% Block > 5% Blend. 

When initial resistance allele frequency was increased from 0.0005-0.01, the durability decreased 

from 11 to 6 years for the 20% block refuge, 16 to 8 years for the 20% strip refuge, and 12 to 5 

years for the 5% seed blend. The relationship between deployment options and their estimated 

trait durability was generally: 20% Strip >5% Blend ≈ 20% Block. 

When survival of the susceptible genotype was increased from 0.0125 to 0.05, the durability 

increased from 9 to 13 years for the 20% block refuge, 14 to 18 years for the 20% strip refuge, 

and10 to 12 years for the 5% seed blend. The relationship between deployment options and their 

estimated trait durability was generally: 20% Strip > 20% Block ≈ 5% Blend. 
- 14 -

Table 5. Sensitivity analyses for three different refuge deployment scenarios 

Parameters 20% strip 

Refuge 20% block 5% seed blend Comments 

Dose Sensitive Sensitive Sensitive 

Mortality for SSgenotypes was 

varied from 95-

98.75% 

R-allele 

frequency Sensitive Sensitive Sensitive 

Initial frequency was 

varied from 0.0005-

0.1 

Dominance Sensitive Sensitive Sensitive Value was varied 

from 0.025-0.2. 

Compliance Sensitive Sensitive N/a 

100% was assumed 

when the parameter 

value was held 

constant. Sensitivity 

values were explored 

ranging from 50-

90%. 

Initial 

density Insensitive Insensitive Insensitive 

Sensitivity values 

were explored 

ranging from 

20,000-160,000. 

Movement1 Insensitive Sensitive Insensitive 

Movement2 Insensitive Sensitive Insensitive 

Movement3 Insensitive Sensitive Insensitive 

Sensitivity values 

were explored 

ranging from 5-45 

m/day. 

Refuge 

location N/a Sensitive N/a 

Unclear from 

submissions what 

was modeled. 

Overwinter. 

Survival Sensitive Sensitive Sensitive Value was varied 

from 0.2-0.8. 

In the sensitivity analyses, 

Pioneer changed the value 

of one parameter at the time 

to determine the effect such 

a change had on the 

durability of the Bt trait 

under two refuge 

deployment scenarios. For 

the remaining parameters 

that were held constant, 

benchmark parameter values 

were assigned for most 

cases. 

Compliance was varied 

from 50%-90%. Hence noncompliance ranged from 

10%-50% and represented 

the percentage of growers 

who planted 0% refuge. 

1

 male movement prior to 

female emergence 

2

 male movement during 

peak female emergence 

3

 post-mating female 

movement 

Table extracted and modified from MRID 478879-01 and MRID 479108-01
- 15 -

Table 6. Sensitivity Analysis for Seed Blend Percentage and Effects on Durability of 59122 Corn 

Worst Case 

–Pioneer defined 

Initial Resistance Allele 

Frequency=0.01 

Dominance Value =0.1 

surv-ss= 0.025 

ovip-ss=0.32 

ovip-rr =1 

Benchmark 

Initial Resistance Allele 

Frequency=0.001 

Dominance Value =0.05 

surv-ss= 0.0125 

ovip-ss=0.32 

ovip-rr =1 

Best-case 

Initial Resistance Allele 

Frequency =0.0005 

Dominance value=0.025, 

surv-ss= 0.0125 

ovip-ss=0.32 

ovip-rr =0.32 

Refuge Various % Seed Blends Following Block Refuge after 4 Years 

Deployment 

Strategy 2.5 3.5 5 7.5 10 2.5 3.5 5 7.5 10 2.5 3.5 5 7.5 10 

20%+Fixed+ 

100% 

compliance

followed by 

various % 

blended refuge 

5 5 5 5 5 8 9 9 10 10 19 >20 >20 >20 >20 

20% strip with 

75% and 

100% 

compliance; 

followed by 

various % 

blended refuge 

5 

6 

5 

6 

5 

6 

5 

6 

5 

6 

9 

10 

9 

10 

10 

11 

10 

12 

11 

12 

16 

18 

18 

19 

19 

>20 

20 

>20 

>20 

>20 

20% block

with variable 

locations 

(moved once in 

four years) 

with 75% 

compliance 

rate; followed 

by various % 

blended refuge 

4 4 4 4 4 7 8 8 9 9 15 17 18 19 19 

Various % 

blended 

refuge1

 

4 4 5 5 5 8 9 10 11 12 17 19 >20 >20 >20 

Table extracted and modified from MRID 478879-01 and MRID 479108-01 

1

 These are seed blend options that do not take into consideration prior use-history of 59122 corn. 
- 16 -

Table 7. Sensitivity Analysis for 20% Block Refuge Compliance and Predicted Durability 

Block Compliance 

Parameter Low Med High 

Refuge Scenarios 20% block fixed 

location 

20% block fixed 

location 

20% block fixed 

location 

Initial Adult Density/Hectare 80000 80000 80000 

Proportion of Males 0.5 0.5 0.5 

Plants/Hectare 79072 79072 79072 

Initial Resistant Allele Frequency 0.001 0.001 0.001 

Dominance Value of Resistant Allele 0.05 0.05 0.05 

Survival Rate of Susceptible at Early 

Stage 0.0125 0.0125 0.0125 

Survival Rate of Resistant at Early 

Stage 1 1 1 

Offtype Seed Proportion 0.0075 0.0075 0.0075 

Fecundity 356 356 356 

Oviposition Rate of Susceptible in 

Toxin 0.32 0.32 0.32 

Oviposition Rate of Resistant in 

Toxin 1 1 1 

Probability of Leaving Bt 0.625 0.625 0.625 

Probability of Leaving Normal 0.5 0.5 0.5 

Early Larval Survival Rate During 

Movement 0.5 0.5 0.5 

Over-winter Survival Rate 0.5 0.5 0.5 

Pre Female Emergence Male 

Movement* 45 45 45 

Peak Female Emergence Male 

Movement 15 15 15 

Post-Mating Female Movement 12 12 12 

Block Refuge Position (Fixed or 

Years Changed) Fix Fix Fix 

Block Compliance (% Compliance) 50% 70%/80%/90% 100% 

Block Durability 

(Year when R allele > 50%) 8 10/10/13 >20 

Table extracted and modified from MRID 478879-01 
- 17 -

Table 8. Sensitivity Analysis for 20% Infield Strip Refuge Compliance and Predicted Durability 

Strip Compliance Parameter

Low Med High 

Refuge Scenarios 20% Strip 20% Strip 20% Strip 

Initial Adult Density/Hectare 80000 80000 80000 

Proportion of Males 0.5 0.5 0.5 

Plants/Hectare 79072 79072 79072 

Initial Resistant Allele Frequency 0.001 0.001 0.001 

Dominance Value of Resistant Allele 0.05 0.05 0.05 

Survival Rate of Susceptible at Early 

Stage 0.0125 0.0125 0.0125 

Survival Rate of Resistant at Early Stage 1 1 1 

Offtype Seed Proportion 0.0075 0.0075 0.0075 

Fecundity 356 356 356 

Oviposition Rate of Susceptible in Toxin 0.32 0.32 0.32 

Oviposition Rate of Resistant in Toxin 1 1 1 

Probability of Leaving Bt 0.625 0.625 0.625 

Probability of Leaving Normal 0.5 0.5 0.5 

Early Larval Survival Rate During 

Movement 0.5 0.5 0.5 

Over-winter Survival Rate 0.5 0.5 0.5 

Pre Female Emergence Male Movement 15 15 15 

Peak Female Emergence Male Movement 15 15 15 

Post-Mating Female Movement 12 12 12 

Strip Refuge Position (Fixed or Years 

Changed) NA1 NA NA 

Strip Width (rows) 4 4 4 

Strip Compliance (% Compliance) 50% 70%/80%/90% 100% 

Strip Durability 

(Year when R allele > 50%) 8 9/10/12 14 

Table extracted and modified from MRID 478879-01 
- 18 -

Table 9. Predicted Durability for DAS-59122-7 When Sensitive Parameters Were Varied for 5% Seed Blend, 

20% Block, and 20% Infield Strip Refuge 

Dominance Initial Resistance 

Allele Frequency Survival Refuge Deployment 

0.025 0.05 0.1 0.2 0.0005 0.001 0.01 0.0125 0.025 0.05 

20% block (fixed) + 

100% compliance 17 9 7 6 11 9 6 9 11 13 

20% strip, 100% 

compliance 20 14 10 7 16 14 8 14 15 18 

5% seed blend 15 10 7 5 12 10 5 10 11 12 

Table created from information extracted from MRID 478879-01 

D. Output Results 

The “output results” section discusses Pioneer’s durability estimates for different 59122 

deployment options for the different parameter scenarios (i.e. worst-case, benchmark, and bestcase), while Pioneer’s sensitivity analysis section explored the ranges of values for relevant 

parameters in order to determine how robust a predicted output result would be. Predicted output 

(durability) and sensitivity analyses need to be considered jointly. 

 

Table 10 lists the results from simulation runs with different assumptions (worst-case, best-case, 

and benchmark assumptions) and various deployment strategies: 

1) no refuge; 

2) 5% seed blend following a 

i) 20% block refuge (location moved twice in four years) with 70% compliance; 

ii) 20% block refuge (location moved once in four years) with 75% compliance; 

iii) 20% strip refuge with 75% compliance; 

iv) 20% strip refuge with 85% compliance; and 

3) 5% seed blend without prior-use history and 100% compliance. 

After a meeting with the BPPD IRM team in October 2009, Pioneer modeled a new worst-case 

scenario, which is referred to in the Table 10 as ‘worst case - BPPD defined’. 

Assuming BPPD’s worst case defined parameters, the no-refuge scenario resulted in the ‘least 

amount of time to product failure’ (3 years), while both the 20% block refuge scenarios and the 

5% seed blend (with no prior use history) scenario predicted four years to product failure. The 

20% strip refuge scenarios with 75% and 80% compliance resulted in the greatest durability (5 

years). 

Assuming Pioneer’s worst-case defined parameters, the 20% strip refuge scenario with 83% 

compliance resulted in the greatest durability (6 years), while a) the 20% strip refuge with 75% 

and 50% compliance and b) the 5% seed blend predicted 5 years to product failure. The 

remaining scenarios predicted the same period to product failure as under BPPD’s worst case 

defined assumptions (3 years for no refuge and 4 years for the 20% block refuge scenarios). 

For benchmark parameters, the trait durability was predicted to be highest for the 20% strip 

refuge with 75% compliance (80% compliance not evaluated) and the 5% seed blend (10 years 
- 19 -

both) followed by the two block refuge deployment strategies (8 years both) and no refuge 

strategy (5 years). 

For best-case parameter values, the trait durability was predicted to be more than 20 years for the 

5% seed blend, 19 years for the 20% strip refuge with 75% compliance (80% compliance not 

evaluated), 18 years for 20% block refuge with 75% compliance rate for four years followed by 

5% seed blend thereafter, 13 years for 20% block refuge with 70% compliance, and 9 years for 

the no-refuge deployment scenario. 

Table 10. Durability for DAS-59122-7 under different refuge deployment strategies as expressed in ‘years to 

product failure’ 

Worst Case -

BPPD defined 

Worst Case –

Pioneer defined 

Benchmark 

Parameters Best-case Scenario 

Refuge 

Deployment 

Strategy 

R-allele freq =0.01

Dominance = 0.2 

SS-survival = 0.05 

Density = 160,000 

Movement= 5m/d1

R-allele freq = 0.01 

Dominance = 0.1 

SS-survival = 0.025 

R-allele freq = 0.001 

Dominance = 0.1 

SS-survival = 0.025 

R-allele freq = 0.0005 

Dominance = 0.025 

SS-survival = 0.0125 

No refuge 3 years 3 years 5 years 9 years 

20% Block (moved 

location twice), 70% 

compliance 

4 years 4 years 8 years 13 years 

20% strip refuge with 

compliance ranging 

from 50-83% 

4 years (50%) 

5 years (80%) 

5 years (60%) 

6 years (83%) Not conducted Not conducted 

20% Block (moved 

location once), 75% 

compliance followed 

by 5% seed blend 

after 4 years 

4 years 4 years 8 years 18 years 

20% strip refuge, 75% 

compliance followed 

by 5% seed blend 

5 years 5 years 10 years 19 years 

5% Seed blend 4 years 5 years 10 years >20 years 

Table extracted and modified from MRID 478879-01 and MRID 479108-01 

SS-oviposition rate was assumed to be 0.32 for worst case, benchmark, and best-case scenarios 

RR-oviposition rate was assumed to be 1 for worst-case and benchmark and 0.32 for best-case 1

Pre-female emergence (male movement), peak-female emergence (male movement), and post-female emergence (female 

movement) parameters were set to 5m/day. 

E. Pioneer Conclusions 

Pioneer concluded: “Depending upon the dimensions of the field, there can be areas of a 59122 

field inaccessible to refuge beetles. These distant ends of the 59122 field become zones of 

continuous selection that may have expected durabilities similar to the no-refuge scenario (five 

years durability). Thus, although the block refuge can have good durability in a best-case 

scenario, it may lead to unintentional selection for resistance in these "hot spots." Alternatively, 

with a 5% blended refuge, there is never a risk of hot spots with unintentional selection because 

susceptible CRW males emerging from refuge plants will be in close proximity (3.6 m) to any 
- 20 -

female beetles that emerge from 59122 plants anywhere in the field. Moreover, blending refuge 

ensures 100% compliance with CRW refuge requirements without the grower having to incur 

any extra work, planning or expense. 

In each scenario modeled, implementing a blended CRW refuge extends the durability of 59122 

maize compared to a 20% block refuge, including scenarios when 59122 maize has been in 

commercial use for four years. 

The results of all the scenarios modeled here demonstrate the superiority of a 5% blended refuge 

relative to block refuge deployment strategies for 59122 maize and corn rootworm. Based upon 

these results, a blended corn rootworm refuge, which ensures 100% corn rootworm refuge 

compliance, provides the most effective tool that the grower can use to delay the development of 

corn rootworm resistance to 59122 maize. Moving to a blended refuge as quickly as possible 

will extend the durability of existing 59122 products.” 

III. BPPD REVIEW AND CONCLUSIONS 

During the risk assessment process, the BPPD/IRM team routinely considers realistic case 

scenarios when evaluating simulation models. However, it can be very useful to look closely at 

scenarios with worst-case assumptions especially in cases when a degree of uncertainty is 

associated with important modeling parameters (e.g. dominance, initial resistance allele 

frequency, adult movement). 

Risk assessment conclusions are reached based on the best scientific information available to the 

Agency at the time of the assessment which is unique to any particular pest-crop system. In the 

case of CRW, where the larval life-stage inhabits a subterranean environment, there is less IRM 

knowledge than for above-ground pests of corn. For example, it has been more challenging to 

determine the dose of Bt toxins targeting immature CRW or to conduct larval behavioral studies 

than for Lepidopteran target pests. Such uncertainties have required that BPPD “bracket” the risk 

of putative resistance evolution. This goal has been accomplished by estimating risk using 

realistic assumptions based on available data and “defensible” worst-case assumptions in the 

model. With this approach, the ‘actual’ risk of resistance evolution may lie somewhere within 

this range of potential risk. If modeling results predict little to no-change in estimated risk of 

resistance with worst-case assumptions, then science conclusions will be more robust and 

defensible. 

For this current risk assessment, BPPD has been presented with a new opportunity to estimate 

the risk of resistance evolution with a recently developed simulation model. Deterministic 

simulations will be undertaken with the “USEPA population genetics two locus model – 

POPGEN”. A probability analysis will complement the deterministic simulations (conducted by 

Dr. Caprio) and based on a PERT distribution (min. value, most likely value, and max. value) for 

parameters of uncertainties. The Agency is now in a position to analyze a registrant submitted 

model, compare and assess durability results of two independent deterministic models, and 

estimate the probability of occurrence for a predicted durability result. 
- 21 -

In addition to the analysis of Pioneer’s model using POPGEN and conducting the probability 

analyses, a variation of a stochastic, spatially explicit model developed by Caprio et al. (2006) 

was used to assess the resistance evolution for two geographically distinct regions with different 

CRW biotypes. Resistance evolution was estimated in the western Corn Belt region where data 

from Nowatzki et al. (2003) suggest that male CRW may disperse on average 15m/d (field size 

average between 50-60 acres and max. 250-300 acres). Second, resistance evolution was 

estimated in the Red Zone where data indicate (Spencer 2009, Spencer -personal 

communication) that CRW adults disperse at much greater distances (between 30-200m/d; field 

size average between 40-80 acres). Given the uncertainty regarding CRW movement, potential 

differences in behavior need to be explored when estimating risk of resistance evolution. 

A. Review of Pioneer’s Modeling Assumptions 

Pioneer reported durability of 59122 corn as the number of years to product failure for all 

scenarios modeled (meaning resistance evolution in the target pest population). Durability of a 

trait can also be expressed as the ‘relative rate of adaption’ (RRA is typically used in public 

literature), which is the speed at which resistance evolves under certain conditions (i.e. % refuge) 

with respect to a baseline. One advantage of using RRAs is that they provide rates (rather than 

absolute values) and remind users of such data that modeling estimates should not be taken as 

absolute estimates. Pioneer stated that their results were presented in years because time to 

product failure was very short for all simulations and RRA’s would not have been 

distinguishable between different scenarios. The years of durability estimated by Pioneer and 

reported in this review should not be viewed as absolute time estimates of durability (or onset of 

resistance). 

1. Pioneer’s scenarios 

With respect to modeling scenarios, BPPD concludes that while the ‘no-refuge’ deployment 

strategy is informative, it represents an unrealistic and overly conservative scenario. The 

estimated years to product failure from this scenario should not be used to establish a realistic 

lower bound estimate for durability; however, a ‘no-refuge’ scenario does serve as a useful 

“negative” control. 

Pioneer focused their main risk assessment conclusions on results from the 5% seed blend 

modeling, not including prior use history of 59122 corn. BPPD believes that the 5% seed blend 

scenario by itself cannot be used to establish a realistic estimate for durability of 59122 corn. 

While it is informative to have this scenario included in the risk assessment, much like the “norefuge” scenario, it is more realistic to consider the prior use-history of 59122 corn. 

The registration for Event DAS-59122-7, also known as Herculex Rootworm (HXRW), was 

granted by the Agency in August, 2005 for commercial plantings beginning with the 2006 

growing season. Growers have since had the opportunity to plant four years of corn expressing 

the Bt toxin Cry34/35 (refuge options available to-date are 20% adjacent, block refuge or 20% 
- 22 -

infield strip refuge > 4 rows wide). CRW have been exposed to Cry34/35 for a period of time 

during which a putative major resistance gene could have increased its initial frequency in the 

pest population. Therefore, Pioneer’s modeling output with scenarios that considered prior-use 

history of DAS-59122-7 corn are likely to be more reflective of real world conditions. With 

respect to grower adoption of infield refuge option, BPPD has heard anecdotal accounts that 

approximately 30% of corn growers may be planting infield strip refuges. 

Scenario 

Number 

Refuge Option 

#1 No refuge 1

#2 20% Block (moved location twice) with 70% compliance 

#3 20% strip refuge with compliance ranging from 50-83% 

#4 20% Block (moved location once), 75% compliance followed by 5% seed blend after 4 years 

#5 20% strip refuge, 75% compliance followed by 5% seed blend 

#6 5% Seed blend1

1

 unrealistic scenario - not supported by use-history of DAS-59122-7 

2. Pioneer’s assumptions and uncertainties 

BPPD re-emphasizes that the numbers of years estimated under Pioneer’s different scenarios 

modeled represent relative values of durability and should not be taken at face value or as 

absolute estimates to predict durability of 59122 corn. Rather, output results from modeled 

scenarios should be compared relative to each other. However, the two worst-case scenarios 

using the most conservative parameter values resulted in relative short time estimates for trait 

durability for all scenarios. This may indicate that under worst-case assumptions, regardless of 

the scenario, (no refuge, structured refuge with low compliance rates, a combination of 

structured refuges and seed blends, or seed blends only) resistance evolution may develop 

quickly. 

Worst-case: Pioneer assumed for worst-case scenarios (as well as benchmark parameter values) 

that ss- genotypic survival was 2.5%. This genotypic survival value was not directly measured in 

the registrant’s experiment but rather represent the total average survival (0.025) measured on 

59122 corn in the field (MRID 473567-05). BPPD contends that ss-survival on Cry34/35 could 

be set higher for worst case assumptions because the average survival estimated by Pioneer may 

not be a good representation of what has been observed as maximum survival at some of their 

individual field sites. Storer et al. (2006) reported that Cry34/35 survival in the field could be as 

low as 4% (not adjusted for density-dependence). BPPD proposes the use 5% as a worst-case 

survival rate instead (see ‘worst-case - BPPD defined’, Table 10). 

A major resistance gene frequency of 0.01 appears to be a conservative value for this parameter 

although recent studies with WCR suggest that the resistance allele frequency for Bt toxins may 

be as high as 0.02 (Hibbard, 2010, personal communication). Because Cry34/35 is not a high 

dose toxin and allows for CRW survival on 59122 corn, BPPD concludes that a dominance value 

for the major resistance gene needs to be set higher than what Pioneer assumed in their worstcase simulations (0.1). BPPD recommends that a more conservative value for dominance, such 

as 0.2, be explored as a maximum value. 
- 23 -

Worst-Case – Pioneer defined 

R-allele freq = 0.01 Dominance = 0.1 SS-survival = 0.025 

Benchmark: The term ‘benchmark’ implies that values under this description may be the most 

realistic case. Pioneer chose an initial frequency of 0.001 for the putative major resistance gene; 

however, this is the value historically used for Lepidoptera. Considering how quickly resistance 

was selected for in the lab (Meihls et al. 2008; Lefko et al. 2008), 0.001 may be too low. 

Furthermore, Dr. Tabashnik (2009) stated (public comment submitted to SAP, 2009) that the 

data from the selection experiments reported by Lefko et al. (2008) demonstrated rapid evolution 

of resistance by WCR to 59122 corn, but with incomplete resistance. This rapid evolution of 

resistance may imply that either the major resistance allele frequency or dominance value in the 

natural populations sampled were greater than what was assumed for these benchmark parameter 

values. Pioneer’s choice of a benchmark value for dominance (0.1) may be adequate if resistance 

allele frequency was increased to greater than 0.001. Both dominance and resistance allele 

frequency have similar effects on the rate of resistance evolution. If either value is set high in the 

simulation model, resistance can be expected to evolve quicker than when they are both set at a 

lower frequency. If they both are set to a high value, resistance will evolve even faster in the 

model. Since field resistance has not been observed or reported yet for CRW, benchmark 

parameter values should only be increased for one or the other parameter. BPPD states that a 

reasonable range for the initial resistance allele frequency is probably between 0.005 and 0.001. 

A benchmark value of 2.5% for ss-survival due to toxin exposure and other causes may be 

adequate. Overall, BPPD considers Pioneer’s benchmark values as realistic best-case 

assumptions. 

 

Benchmark Parameters 

R-allele freq = 0.001 Dominance = 0.1 SS-survival = 0.025 

Best-case: The SAP (2009) concluded that in consideration of the limitations of Lefko et al.

(2008), an estimated value of 0.0015 for a major resistance gene was not justifiable. BPPD 

concurs with the SAP and concludes that a major resistance allele frequency in CRW may lie 

somewhere in the range of 1 in a 1000 (0.001) to 1 in 100 (0.01). As stated earlier in Section III 

of this review, BPPD considers Pioneer’s parameter values for “best-case” unrealistically high to 

establish an estimate of actual durability. 

Compliance: Pioneer assumed that a certain percentage of growers did not plant a refuge at all. 

In other words, when 70% compliance is modeled, it is implied that 30% of growers planted 0% 

refuge. After reviewing ABSTC’s (Agricultural Biotechnology Stewardship Technical 

Committee) data (2002-2008), BPPD reported that in 2007 and 2008 an apparent drop in grower 

compliance with refuge requirements for PIPs had occurred as compared to prior years (BPPD, 

Best-Case Parameters 

R-allele freq = 0.0005 Dominance = 0.025 SS-survival = 0.0125 
- 24 -

2009). Those particular results from the anonymous, 3rd party survey did not differentiate 

between different degrees of non-compliance, and reported non-compliance results per farm. The 

survey also assessed what percentage of growers planted at least a 15% refuge. BPPD specified 

that in 2007, approximately to 89% and 95% of the growers surveyed attested to having planted 

at least a 15% and a 10% refuge, respectively, while in 2008, approximately to 86% of growers 

planted at least a 15% refuge. Considering ABSTC’s data, BPPD concludes that Pioneer’s 

assumption of 30% non-compliance is too conservative since this would mean that 30% of 

farmers planted a 0% refuge. However, the Agency has received anecdotal evidence that certain 

counties/areas appear to have a greater amount of non-compliance than captured by the ABSTC 

report. This point will be incorporated and evaluated in BPPD’s independent analysis. 

Compliance is a sensitive parameter in Pioneer’s model; BPPD will carefully consider Pioneer’s 

outputs with overly conservative compliance percentages that aim to represent refuge planting 

behavior across the entire Corn Belt. 

Uniform oviposition: Pioneer structured its model in such a manner that oviposition of eggs 

occurred uniformly in each cell. Work by Branson (1986) suggests that CRW egg masses in 

fields are best described by a clumped distribution. This could indicate that female CRW are 

capable of detecting different host qualities. It is unclear what impact Pioneer’s assumption of 

uniform distribution of eggs may have on the estimated durability of 59122. 

Larval dispersal: Pioneer modeled larval dispersal as a one-time event occurring during the 

neonate stage and adjusted for density-dependent mortality. BPPD questions whether densitydependence is a factor for neonates leading to dispersal and whether the main focus of dispersal 

in the model should be on neonates. In Hibbard et al. (2004) density-dependent dispersal for 

neonates did not occur during a three-year field study even under high-density pressure (with 

artificial infestation). And Hibbard et al. (2005) found that late-instar movement occurred from 

sufficiently damaged non-Bt plants to surrounding transgenic plants. Work by Dr. Hibbard 

indicates that it is more realistic to assume high larval movement from late instars and little to no 

movement for neonates. However, statistically significant neonate movement from Bt plants to 

non-Bt plants was observed in one year of their field study (Hibbard et al. 2005). BPPD 

concludes that late-instar movement in response to resource limitations appears to be real and 

should be modeled. 

In addition, Pioneer demonstrated in a lab experiment with CRW larvae and Cry34/35 corn that 

when exposure to 59122 seedlings occurred and survival was measured at the neonate (0.41), 

second-instar (0.66), and third-instar stage (0.77) that a reduction to 0.002, 0.17, and 0.50, 

respectively, was apparent (MRID 473567-01). Pioneer’s data suggests that as CRW larvae 

mature, they become less susceptible to Cry34/35. Hibbard et al. (2005) stated that initial larval 

development on non-Bt roots followed by later movement to Bt roots could accelerate resistance 

evolution if heterozygote larvae (RS- genotype) experienced greater survival on Bt. This is 

supported by results of Meihls et al. (2008), where exposure of neonate larvae to Cry3Bb1 

followed by rearing on isoline corn did not result in the development of resistance in choice tests 

while transferring 2nd instars from isoline to Cry3Bb1 did eventually result in resistance, 

although the level of resistance was reduced and its development was slower than with complete 

rearing on Cry3Bb1 corn. 
- 25 -

Based on information provided by Dr. Hibbard (personal communication, 2009) and published as 

well as unpublished data (Hibbard et al. 2004 and 2005; Meihls et al., 2008; MRID 473567-01), 

BPPD concludes that larval movement at the later-instar stage is more realistic and important to 

model than movement at the neonate stage. 

Adult beetle movement: Pioneer simulated beetle movement using an exponential distribution 

(MRID 478879-01) relying on the data generated in a two-year, one-location (very limited 

geographic scope) rubidium study (Nowatzki et al. 2003), which showed that some male beetles 

moved as far as 220 ft/day (sample size 305) during pre-female emergence and 188 ft/day in 

2002 (sample size 125) and 152 ft/day in 2003 (sample size 47) during peak female emergence 

(see Table 2). Pioneer concluded that mean male movement should be around 15m/d while mean 

female movement was closer to 13m/d. The median for male ‘pre-female emergence’ movement 

ranged from 4.9-6.3m/d, for male ‘peak female emergence’ movement from 0-7m/d, and for 

male ‘post-female emergence’ movement from 0-7.9m/d. BPPD questions the uniform 

applicability of the reported ‘average m/d traveled. Dr. Joseph Spencer (University of Illinois) 

measured average dispersal of adult CRW in Illinois over the course of two planting seasons 

(ESA 2009; personal communication, 2009) in Bt corn fields and non-Bt soybean fields and 

reported that dispersal could be as low as 30m/d (average 50m/d) but as high as 80 m/d (very low 

CRW year in 2009). The prior year, Dr. Spencer measured adult male dispersal of up to 

>200m/d; he stated that when corn rootworm presence was abundant that adult dispersal was 

high. Dr. Hibbard confirmed the Spencer and Nowatzki dispersal data and agreed with the SAP 

(2002) that the adult movement behavior within the WCR was different in these two locations 

(distinct biotypes) (Hibbard, personal communication, 2009). BPPD concludes that these two 

CRW biotypes (NE region and Red Zone) appear to have real differences in dispersal behaviors. 

Short distance dispersal may be more appropriate for the Nebraska region while greater dispersal 

distances appear to be more appropriate for populations in the Red Zone. The stochastic 

modeling output in Section III.C.3 will address these differences in adult behavior and explore 

their effects on 59122 corn separately in these two regions of the Corn Belt. 

Probability of leaving non-Bt and Bt roots: Pioneer assumed that 50% and 62.5% of larvae leave 

a non-Bt and Bt host, respectively. However, Hibbard et al. (2004) reported that between 0.57% 

and 5.17% of larvae were recovered from nearby non-Bt expressing roots of artificially infested 

non-Bt plants. On the other hand, Hibbard et al. (2005) stated that the percentage of CRW larvae 

leaving Bt roots may be about 25% higher than what is reported for non-Bt roots. This suggests 

that Pioneer’s probabilities for larvae leaving non-Bt and Bt source may be too high. 

SS-Survival: Pioneer derived an overall survival parameter from their field experiment which 

measured emergence in pure stands of 59122 maize and refuge fields (Nowatzki et al., 2008). 

The mean survival value (0.05) was calculated by dividing mean emergence (across 

locations)/59122 plant by mean emergence (across locations)/refuge plant. Then Pioneer 

assumed that half of the survival was due to individuals being homozygous susceptible (0.025). 

There is some uncertainty with regard to the survival parameter chosen for the susceptible (ss) 

genotype. The experiment by Nowatzki was conducted in five different locations across the Corn 

Belt and over the course of one growing season (2007). The range of mortality in this timelimited study (one year) was 96.0-99.8% for WCR (64-94% for NCR). Mortality in this study is 

a function of dose of toxin as well as environmental factors. Hence, during any given year the 
- 26 -

range of mortality (or survival) could vary even if experimental locations remained the same. 

Mortality could in fact be lower if environmental conditions were favorable to CRW survival 

(hot and dry). Second, BPPD considered it important to include simulations with lower limits of 

observed survival in realistic worst-case scenarios rather than to rely on average 

survival/mortality rates alone and proposed to Pioneer to explore ss-genotype survival at 5%. 

Pioneer’s own sensitivity analyses showed that output results (durability of trait) were sensitive 

to changes in this parameter (see Table 5), which warrants exploring this parameter further. 

Mixing Technology: Pioneer originally reported that bags containing OAM1 had a 1.5% 

standard error of the mean; a 5% seed mix could contain percent refuge seeds ranging from 3.5-

6.5%. Pioneer has since analyzed data from a total of 32 commercial-scale seed blending runs to 

date (Rood, personal communication, 2009) and reported that seed bags containing the OAM1 

product have a blend rate ranging from 4.910% to 5.075% (95% confidence interval). The 

amount of seed in each run was enough to fill approximately 1000 bags of seed. BPPD will 

model the variation of % refuge seed from 4% - 6% because under high-production output, it is 

conceivable that the standard error of the mean could increase. 

Sensitivity Analyses: Sensitivity analyses are an important part of simulation exercises. They 

help determine the effects on output results when a range of possible values is considered for 

parameters of interest (i.e. reveal where uncertainty may be relevant in system). A modeling 

parameter is determined to be insensitive when changes in its value produce robust results (i.e. 

cause “little to no change” in predicted durability of the Bt trait). When outputs results are 

relatively insensitive to changes in values, then uncertainties may be less of a concern for risk 

assessment purposes. 

The sensitivity analyses conducted by Pioneer showed that for infield strip refuge, block refuge, 

and seed blend simulations, the output results were sensitive to changes in the following 

parameters: dose (survival), initial frequency of the major resistance gene, dominance, and (egg) 

overwintering survival. For strip and block refuge, output results were sensitive to changes in 

compliance rate. For block refuge alone, output results were sensitive to movement parameters. 

Main parameters of concern that have uncertainty are 1) initial frequency of the R-allele, 2) 

dominance, 3) larval survival on Bt corn, 4) % non-compliance with refuge, and 5) the adult 

CRW movement parameters chosen by Pioneer. In light of the uncertainty, BPPD believes it is 

better to incorporate more conservative, but still realistic, parameter estimates in model analyses 

for survival rate, R-allele frequency, dominance, % non-compliance, and adult movement for 

block refuges. 

The work of Godfrey et al. (1995) was the basis for the assumption that a 50% overwintering 

survival is appropriate for CRW, and therefore, a sensitivity analyses for this parameter may not 

be realistic to consider (Onstad testimonial, 2009). For grower compliance with refuge plantings, 

BPPD will consider a reasonable range of non-compliance derived directly from its review of 

ABSTC’s CAP reports (Bailey, 2008 & 2009; BPPD, 2009), which are also confirmed by a 

survey conducted in Illinois (Gray, 2009), and consider the percentages of growers who planted 

zero % and at least a 15% and 10% refuge. As stated in section II. A. 2, ABSTC reported that 

according to their third party anonymous survey approximately 95% and 89% of growers planted 
- 27 -

at least a 10% (between 10-20%) and 15% refuge (between 15-20%), respectively, in 2007; 

approximately 86% of growers planted at least a 15% refuge in 2008. BPPD concludes based on 

the 2007 and 2008 compliance data that a minimum, most likely, and maximum value for noncompliance (zero % refuge planted) should be 5%, 10%, and 60% (addressing anecdotal 

evidence of no-refuge planting in certain counties), respectively. BPPD does not concur with 

Pioneer’s universal assumption of 30% no-refuge non-compliance in their model; however, this 

percentage is within BPPD’s suggested range. Pioneer’s own sensitivity analysis showed that 

higher compliance with block refuges resulted in greater durability. When zero % compliance 

was set at 50%, 70%, and 100%, durability results were 8 years, 10 years, and greater than 20 

years. 

The sensitivity analysis for male movement prior to female emergence showed that this 

parameter was a very important and sufficiently sensitive when durability was estimated for 20% 

block refuges. When beetle movement was low (5m/d) or high (45m/d), Pioneer estimated that 

durability ranged from 9 years to >20 years for block refuges. Based on the different 

dispersal/movement data available from Nowatzki et al. (2003) and Dr. Spencer, as discussed 

earlier in this section, it will be important to further investigate and discuss the male movement 

parameter. 

B. BPPD Review of Pioneer’s Modeling Results 

The modeling outputs discussed in this section are cited in Table 10 of this review. 

Time to resistance using BPPD defined worst-case assumptions: 

The ‘no-refuge’ simulation represented the upper bound for risk in Pioneer’s modeling and 

predicted three years of durability for 59122 corn, which is less than the time the product has 

already been deployed. This result emphasizes that the time to product failure estimated by the 

‘no refuge’ scenario is likely unrealistically low and in addition emphasizes that years-toresistance should not be taken as absolute values. 

 

The ‘20% block refuge with 70% compliance’ predicted an estimated 4 years of product 

durability; this predicted durability is the same as for the strip refuge with 50% compliance. 

Since the two deployment strategies with either 50% and 70% of compliance resulted in the 

same amount of years estimated to resistance evolution, it appears that the strip refuge, which 

allows for better mixing between refuge and non-refuge beetles can handle a lower compliance 

rate compared to the block refuge. BPPD concludes that this may be the most likely explanation 

for the ‘no difference in durability’ between the two greatly differing compliance percentages of 

block and strip scenarios. 

A 20% strip refuge with 83% compliance proved more durable relative to a 20% block refuge 

with 75% compliance followed by a 5% seed blend. BPPD concludes that the strip refuge 

(allowing for better mixing between refuge and non-refuge beetles) and higher compliance rate 

fare better in Pioneer’s model than the block refuge with lower compliance followed by the seed 

blend. Introducing the seed blend after having had a block refuge for 4 years did not appear to 

improve durability relative to 1) the 20% strip refuge with variable compliance (50-83%) or 2) 
- 28 -

the 20% block refuge with 70% compliance. As previously stated, BPPD believes that the 

universal assumption of 30% no-refuge non-compliance is overly pessimistic and not 

representative of the actual numbers reported. Therefore, 20% block might have proven more 

durable with assumptions of higher compliance as is suggested by Pioneer’s sensitivity analysis 

in Table 7. 

The strip refuge with 75% compliance followed by a 5% seed blend was more durable, although 

just by one year (one CRW generation), relative to a 5% seed blend (assuming no prior 

deployment of 59122 corn as block or strip refuge). There was no difference in years of 

durability between 1) the 20% block refuge with 70% compliance, 2) the strip refuge with 50% 

compliance, 3) the block refuge with 75% compliance followed by a seed blend, and 4) the 5% 

seed mix. 

BPPD concludes that based on Pioneer’s modeling, the 20% strip refuge with or without a seed 

mix after four years appeared to be the most durable deployment strategies using BPPD’s worstcase assumptions. There was no difference in estimated durability between 20% block refuge 

scenarios (with or without seed blend) and 5% seed blend strategy (no prior deployment of 

59122 assumed). BPPD further concludes that the compliance percentages used (% growers 

planting no refuge) were too low. 

None of the scenarios predicted great durability for 59122 with BPPD’s worst-case assumptions 

in Pioneer’s model. 

No 

refuge 

20% Block, 

70% 

compliance 

20% Strip, 

50% -80% 

compliance 

20% Block, 75% 

compliance 4yrs, 

5% seed mix 

20% Strip, 75% 

compliance, 4 

yrs, 5% seed mix 

5% Seed 

mix 

BPPD 

worst case 3 yrs 4 yrs 4 yrs 

5 yrs 4 yrs 5 yrs 4 yrs 

Info extracted from Table 10 above 

Estimated time to resistance using Pioneer’s worst-case assumptions: 

Compared to results generated with BPPD defined worst-case assumptions, results for Pioneer’s 

worst-case differed only for the strip refuge with variable compliance and the 5% seed blend 

strategy. 

The strip refuge with variable compliance (50%-83%) resulted in 5 yrs and 6 yrs of durability for 

59122 corn compared to 5 yrs for the 5% seed blend (with no prior block or strip deployment). 

BPPD concludes that using Pioneer’s worst-case assumptions, a 20% strip refuge with more 

realistic compliance rates proved to be more durable relative to all other deployment options. 

There was no difference predicted in durability between either block refuge scenarios (with or 

without seed blends after 4 years). Using Pioneer’s model, the 5% seed blend fared better than 

both block refuge options and resulted in 5 years of durability rather than 4 years (block refuge 

deployment). 
- 29 -

No 

refuge 

20% Block, 

70% 

compliance 

20% Strip, 

50% -83% 

compliance 

20% Block, 75% 

compliance 4yrs, 

5% seed mix 

20% Strip, 75% 

compliance, 4 

yrs, 5% seed mix 

5% Seed 

mix 

Pioneer 

worst case 3 yrs 4 yrs 5 yrs 

6 yrs 4 yrs 5 yrs 5 yrs 

Info extracted from Table 10 above 

BPPD notes that under worst-case assumptions (BPPD defined and Pioneer defined), infield strip 

with 80% and 83% compliance (Table 10) was more durable (5-6 years) than any seed blend 

scenario modeled (4-5 years) when the same parameter values were used. While the universal 

assumption of ‘20% of growers planting no refuge’ is not supported by real data (Bailey 2008 & 

2009; BPPD, 2009), it could represent a worst-case, especially in counties with very low refuge 

adoption. The lowest percentage of growers potentially not planting any refuge appears to be 

around 15%. BPPD considers 15% a most-likely estimate for zero-% non-compliance across the 

Corn Belt. 

Estimated time to resistance using Pioneer’s benchmark assumptions: 

The ‘no-refuge’ scenario resulted in the least amount of estimated time to product failure; the 

strip refuge followed by seed blend, as well as the 5% seed blend scenario resulted in the greatest 

number of years to product failure. Since the 5% seed mix (without prior use-history for blocks 

or infield strips) is not a realistic scenario, BPPD concludes that with benchmark parameter 

values, the 20% strip refuge (75% compliance rates) followed by seed blend is the most durable 

scenario in Pioneer’s model compared with the scenarios accounting for prior use-history. BPPD 

notes that a comparison to a 20% strip refuge with variable compliance could not be made 

because Pioneer did not model this for benchmark parameters. BPPD speculates that the 20% 

strip refuge option might have provided the greatest durability compared to Pioneer’s other 

scenarios. Pioneer’s model did not predict a difference in durability between the two block 

refuge options with or without a seed blend after four years. 

No 

refuge 

20% Block, 

70% 

compliance 

20% Strip, 

variable 

compliance 

20% Block, 75% 

compliance 4yrs, 

5% seed mix 

20% Strip, 75% 

compliance, 4 

yrs, 5% seed mix 

5% Seed 

mix 

Benchmark 

Parameters 5 yrs 8 yrs Not modeled 8 yrs 10 yrs 10 yrs 

Info extracted from Table 10 above 

Estimated time to resistance using Pioneer’s best-case assumptions: 

The results from different scenarios fall into distinct time categories likely because best-case 

assumptions allow for greater modeling runs and hence durability. 

BPPD notes that Pioneer did not provide results for the 20% strip refuge with variable 

compliance when using its best-case assumptions. This scenario predicted the greatest durability 

among all scenarios when more conservative assumptions were used. From the scenarios 

modeled, the 5% seed mix was the most durable (>20 yrs) followed by the 20% strip (19 yrs) and 

block refuge (18 yrs) with variable compliance and seed blend. BPPD speculates that the 20% 

strip refuge option, had it been modeled (including more realistic non-compliance numbers), 

could have produced the greatest durability under Pioneer’s best-case assumptions. The 20% 

block refuge with variable compliance was predicted to be less durable (13 yrs) then when seed 
- 30 -

blends were introduced after the first four years of 59122 deployment. Even for this scenario, 

unrealistic non-compliance numbers decreased the durability for 59122 compared to seed blends. 

The ‘no-refuge’ option resulted in the least amount of time to product failure (9 yrs). 

BPPD is concerned with outputs generated with ‘best-case’ assumptions for the reasons 

discussed earlier in this review. BPPD believes that the dominance value and initial resistance 

allele frequency chosen may not be justifiable based on what is known about Cry34/35 toxicity 

and history and that these values could represent unrealistic best-case values for estimating 

durability. Rather, BPPD considers Pioneer’s benchmark parameter values as defensible ‘bestcase’ assumptions. 

No 

refuge 

20% Block, 

70% 

compliance 

20% Strip, 

variable 

compliance 

20% Block, 75% 

compliance 4yrs, 

5% seed mix 

20% Strip, 75% 

compliance, 4 

yrs, 5% seed mix 

5% Seed 

mix 

Best-case 9 yrs 13 yrs Not modeled 18 yrs 19 yrs >20 yrs 

Info extracted from Table 10 above

BPPD’s concluding remarks regarding Pioneer’s results: 

BPPD questions Pioneer’s conclusions that the use of a blended refuge following a block refuge 

extends the durability of 59122 maize relative to continuation of a block refuge, and that, 

therefore, moving to a blended refuge as quickly as possible would extend the durability of 

existing 59122 products. 

1. Pioneer’s conclusion that the addition of an in-field movement parameter makes the 5% seed 

blend (OAM1) always superior to the 20% block refuge is not supported in all model 

simulations. Pioneer’s modeling output shows that the 20% block refuge alone and followed by a 

5% seed blend resulted in the same amount of estimated durability for 59122 corn (8 yrs) when 

Pioneer’s benchmark parameters (and unrealistic non-compliance rates) were used. Also, 

Pioneer’s model supports the conclusion that strip-refuges are more durable than seed blends 

under worst-case (less conservative) assumptions. 

2. Pioneer has modeled universal adult CRW movement based on data developed by Nowatzki 

et al. (2003), which indicate that adult movement is limited (average 15m/d for males). Spencer 

(2009) found that adult CRW movement may depend on population density and reported that 

movement ranged from 30m/d to over 200m/d. Even with Spencer’s data (2009), there is still 

uncertainty regarding what constitutes reasonable adult CRW movement and whether different 

biotypes display different movement behavior. 

Pioneer’s sensitivity analysis (using benchmark assumptions) showed that greater male dispersal 

prior to female emergence increased the durability 59122 corn with block refuge deployment to 

>20 years. Hence, assessing the effects of adult dispersal as reported in the Illinois region is of 

great importance (see BPPD’s own analysis in Section IV.C.3). 

3. The assumption that 30% of corn growers across the Corn Belt plant no refuge is not 

supported by ABSTC’s data and underestimates non-compliance (see BPPD’s own analysis in 

Section IV.C.2 of this review). Pioneer’s sensitivity analysis showed that when the percent of 
- 31 -

growers who complied 100 % with refuge requirements was increased from 70% to 100% that 

durability was increased from 10 years to >20 years (refuges not moved and benchmark 

parameters assumed otherwise). Hence, durability for 20% block refuges using Pioneer’s model, 

should be between 10-20 years rather than 8 years (refuges moved) as reported. In this case, 

durability for blocks would be greater than for seed blends. 

4. Field size is an important consideration that will affect the durability of 59122 corn in a block 

refuge configuration. The greater the distance between the non-Bt portion and the opposite end 

of the Bt field, the greater the probability that a refuge male, assuming limited dispersal, may not 

find the potentially resistant female emerging from the Bt corn to mate with, and vice versa. 

However, if dispersal is greater in some regions of the Corn Belt (Spencer, 2009) than reported 

by Nowatzki et al. (2003), then field size may not be of such great important anymore. Section 

IV.C.2.a in this review will explore the different planting behaviors by growers, CRW 

movements encountered in the Corn Belt as well as the effect of egg densities in refuge and Bt 

fields. 

5. If larval movement had been assumed for later instars (rather than neonates) while including 

lower mortality and greater tolerance for Cry34/35, estimated durability of 59122 in Pioneer’s 

model might have been reduced for seed blends. Late-instar dispersal could allow for 

heterozygous resistant survival, which might speed up evolution of resistance in seed blends 

using a single, non-high dose toxin. Mallet and Porter (1992) concluded that when larvae were 

able to move from plant to plant within a Bt seed mixture that resistance could occur faster than 

compared to pure stands of Bt. 

6. Pioneer has set model conditions/parameters that are unfavorable to blocks (compliance, CRW 

adult movement, refuge placement, and field size) and that are more favorable to blends. 

7. BPPD concludes that with Pioneer’s realistic and more conservative assumptions, seed blend 

and block scenarios predict about equal durability for 59122 corn. Seed blends are not superior; 

but Pioneer’s modeling shows they are not worse either. Seed blends fare better when prior-use 

history is ignored and best case assumptions are modeled. Seed blends do not fare well under 

most conservative (worst-case) assumptions. 

C. EPA/ORD’s ANALYSES OF A 5% SEED BLEND AS PROPOSED IN 

PIONEER’S SUBMISSIONS 

1. Purpose 

For the first time, BPPD has access to an EPA-generated model to simulate resistance evolution 

in a target pest in response to pesticide exposure (“USEPA population genetics two locus model, 

POPGEN”). In this particular section of the risk assessment, the main objective was to determine 

whether Pioneer’s conclusions are supported by using an independent model. This includes the 

important claim that the adult infield movement parameter in a 5% seed blend would lead to 

greater random mating between adult CRW emerging from Bt and non-Bt plants and 

consequently extend the durability of 59122 corn over the currently approved, 20% block refuge. 
- 32 -

Further objectives of EPA’s modeling efforts were to 1) assess whether Pioneer results could be 

replicated using their benchmark assumptions, 2) use BPPD’s and ORD’s assumptions for likely 

more realistic and conservative parameter values and assumptions and incorporate a probability 

analysis to assess which generated results (estimated durability for block or seed blend) would be 

more likely to occur, and 3) test alternate hypotheses. 

To understand the relative risks related to the evolution of resistance in western corn rootworm 

between a block refuge deployment (as currently mandated by EPA) and a 5% seed-mixture 

deployment as requested by Pioneer, EPA/ORD developed two models. The first was a 

modification of a spatially-explicit stochastic model (Caprio et al. 2006), and the second was a 

simpler, frequency-based deterministic model. The latter could be run in a single simulation 

mode with a graphical interface to enter parameters or in a risk-assessment mode capable of 

running thousands of simulations to estimate the effects of parameter uncertainty. The stochastic 

model was used to develop realistic estimates for several parameters used in the deterministic 

model and to assess the effect of different dispersal rates and field sizes on the durability of a 

20% block refuge strategy. 

2. Deterministic Model (POPGEN) and Probability Distributions 

(Taken out of Caprio & Glaser, 2010)

a. Model Description

The deterministic model is a frequency-based model that incorporates non-compliance, nonrandom mating in compliant fields, and larval movement. It includes a switch to vary between a 

block-refuge design and a seed-mixture design. Appendix A (Caprio & Glaser, 2010) includes 

descriptions of the different parameters. No refuge was planted for non-compliant fields. CRW 

in ‘non-random mating populations’ came from fields with refuges. Non-compliant populations 

increase the amount of non-random mating and reduce refuge size. As noted in the Pioneer 

submission, a critical component of their modeling was that refuges were moved every 

generation. While EPA/ORD believes that this may overestimate grower placement of refuges, 

this was incorporated into the model by defining the refuge size not as the physical area devoted 

to the refuge, but rather the proportion of the eggs in a field that started out underneath the new 

refuge. These small refuge sizes were independent of non-compliance and resulted from growers 

moving their refuge to areas of the field were few eggs were oviposited the previous year. To 

incorporate this important parameter, the stochastic model was used to estimate egg densities at 

the start of a year across the model’s four different field sizes (Figures 1a-e). In each case, a best 

and worst scenario was estimated. In the best case scenario, the grower moved refuges to the 

opposite side of the field next to where the refuge would have been planted in the neighboring 

field the previous year (these simulations were run with all growers planting refuges to the same 

sides of their fields and swapping them every year). The worst case scenario was based on 

planting the refuge in the area of the field where the egg density was the lowest. Because the egg 

density distribution across fields has never been measured, it is unlikely that growers could 

accomplish this, but it does represent a theoretical lower bound for effective refuge size. The 

effective refuge sizes for the four different field sizes were (from smallest to largest field size): 

15%, 11%, 3% and 0.055%. As noted by Pioneer, few insects were moving into the center of 
- 33 -

large fields as reflected by the effective refuge size of 0.055% for 144 ha fields. Other estimates 

for dispersal were higher and might depend on the region of the Corn Belt. To test the effect of 

higher dispersal rates, the vector of daily movement was doubled (to 29m/day) and the large field 

size simulations were repeated (Fig. 1e). In this case the density of eggs in the center of the field 

was fifty times greater under the worst case scenario and the effective refuge size was 1%. Most 

fields in the Corn Belt, however, are smaller, in the range of 30-50 ha, and EPA/ORD decided to 

use a lower limit of 1% for this parameter. 

Figure 1a. Simulated egg densities in transects across Bt-corn and associated refuges: a small 2.25 hectare 

(approximately 5.56 acre) field size. There is a 30m row of refuge along the edge of a 120m wide field of 

transgenic corn. The refuge is always planted along an edge and is switched every year. The previous years, 

refuge was in strip 5, while the current one is in strip 1. In the model, growers always plant to the same side 

of their fields, so the reason there is a larger egg population under strip 1 than in strip two is because there 

was a refuge to the left in another field. In this case, if growers planted in strip one, the new refuge would get 

19.8% of the overwintering eggs. On the other hand, if they planted in the area with the lowest density, the 

refuge would have 15% of the overwintering eggs. 

Figure 1b. Simulated egg densities in transects across Bt-corn and associated refuges: a 9 hectare (22.24 acre) 

field. The highest density is under the previous year’s refuge, but we continue to get “drift” from 

neighboring fields. In this case 18.6% of the eggs would be under the refuge in the default planting pattern, 

12345

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- 34 -

but the refuge would only get 11% if growers planted the refuge in the area with the lowest density of eggs. 

The refuge in this case was 2 strips (60m) wide.

Figure 1c. Simulated egg densities in transect across Bt-corn and associated refuges: A 36 hectare field (88.96 

acres, about an average field for many parts of the U.S.). With the default planting pattern, the refuge would 

receive 20% of the eggs, but only 3% if growers planted over the area with the lowest density. The refuge in 

this case was 4 strips wide (120m). 

Figure 1d. Simulated egg densities in transect across Bt-corn and associated refuges: 144 hectare (355.82 

acre) field size. At this size, the default planting would result in 23% of the eggs in the new refuge, but the 

worst case drops to 0.055% (if growers plant in the middle). The refuge in this case was 8 strips wide (240m). 

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- 35 -

Figure 1e. Simulated egg densities in transect across Bt-corn and associated refuges: doubled dispersal 

distance in a 144 hectare field. This figure has a column for each 30x30m block rather than an average of 3 

transects as in the other four figures.

b. Model Assumptions 

Asymmetrical movement: Asymmetrical movement is defined as the probability that larvae that 

have survived exposure on a Bt-plant will move. This movement was included in the ORD’s 

seed mixture model, though the amount movement of susceptible larvae off of transgenic corn is 

uncertain (Hibbard et al., 2005). When larvae moved, they moved onto plant genotypes 

according to the ratio of those genotypes in the field. In this case, a larva leaving a transgenic 

plant had a 95% probability of moving onto another transgenic plant. As with other rates in the 

model, this movement rate was converted into a daily rate (depending on the days of larval 

movement parameter). 

Developmental delays: To replicate the developmental delay of larvae on transgenic plants in the 

Pioneer model, the ability to delay insects was added by holding back a portion of each age 

group by a fraction. Normally, all the individuals in an age group would age one day in the 

model per time step. In the ORD model, 5% were held back from each age group (though they 

did experience mortality) and did not age. This resulted in a mean developmental delay of 2.3 

days. 

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- 36 -

Prior use-history: the deterministic version of the ORD model was run for four years as a 20% 

block refuge deployment. After the initial four years (simulating prior use-history of 59122 corn) 

had passed, simulations were continued with a 5% seed blend strategy. For the 20% block 

refuge, no changes occurred for further simulations. 

 

Uncertainty (probability) distributions: To incorporate the uncertainty in the parameters used in 

the deterministic model, distributions were developed based on data for the lowest and maximum 

possible values, as well as a most likely values parameters might take (as determined jointly by 

BPPD and EPA/ORD). These values were used in PERT distributions based on a beta 

distribution with the most likely value weighted by 4 (Vose 2001). This was done for both the 

block refuge model (Table 12) and seed blend (larval movement) model (Table 13). Each model 

was run 2000 times, each time drawing parameter values from the relevant PERT distributions 

and using the sampled parameters as inputs to the model. The time it took for the resistance allele 

to exceed 50% was recorded for each simulation. EPA/ORD measured the mean, 25th, and 75th

percentiles of the time to resistance and determined the proportion of simulations in which the 

time to resistance exceeded 10 years (i.e., 11 years or longer). Eleven years was chosen as a 

cutoff point because the gene has already been in use for 4 years, and previous EPA-SAPs have 

suggested 15 years as a reasonable minimal goal for the lifetime of single gene products. 

The uncertainty distributions are described in Table 11 and Table 12 for the block refuge and the 

seed blend model. 

Common parameters: 

1. Fitness of susceptible homozygotes on transgenic plants (Wss): The Pioneer estimate for 

this parameter was 0.0125. In the past, however, as transgenic events have been exposed 

to a wide variety of environments, Wss has often been found to exceed initial estimates. 

2. Dominance (h): In some cases, dominance associated with events that are not a true highdose is considerably greater than the Pioneer benchmark estimate. BPPD/ORD’s estimate 

was centered on the Pioneer estimate but included the possibility that it could be 5x less 

or 5x greater. 

3. Initial resistance allele frequency (IRF): While many estimates of the initial resistance 

allele frequency for loci providing resistance to Bt toxins are in the range of 0.001 (for 

Lepidoptera), recent studies with western corn rootworm have suggested they may be 

more common, perhaps as high as 0.02 (Hibbard 2010, personal communication). These 

two values were used as our maximum and minimum values, with 0.005 used as the most 

likely value. 

Block refuge parameters: 

1. Effective refuge size: In the absence of non-compliance, the physical refuge size was 

always assumed to be 20% of the area. Non-random oviposition and refuge placement 

could, however, lead to that 20% of the area having more or less than 20% of eggs 

initially (see Figures 1a-e). This is a complex parameter that depends on the size and 

shape of fields, how growers physically place the refuges in the fields, how they move 

the refuges between years, and on corn rootworm dispersal rates. It is likely that this 

parameter is responsible for many of the differences between the results of this model and 
- 37 -

the model used by Pioneer. The effective refuge size varied from 1% to 20%, with 15% 

as the most likely value. 

2. Non-compliance: Non-compliance must be measured on a field by field basis, and 

BPPD’s interpretation is that a non-compliance rate of 30% (meaning that 30% of 

growers planted 0% refuge) is an overestimate. There may be localized areas, however, 

where non-compliance may be greater than 30%, and to incorporate the effects of these 

areas, the maximum value of non-compliance was set to 60%. The minimum value was 

5% with a most likely value of 10%. 

3. Non-compliance mixing: This parameter is related to the rate of mixing per generation 

between compliant and non-compliant fields and may be seen as a measure of persistent 

non-compliance versus growers that are accidentally non-compliant in one or more fields. 

Bailey (2008 & 2009) suggested 5-7% of growers were persistently non-compliant, so 

93% was chosen as a most likely value, 95% as the maximum and 85% as a conservative 

minimum value. 

4. Non-random mating: This parameter is an estimate of the proportion of the population 

emerging in the transgenic fields that mates among themselves with no contribution from 

refuge insects. Based on the simulated dispersal experiment, it appears as though as many 

as 50% of these individuals would be unlikely to mate with refuge individuals in the 

largest field sizes. Because refuges are moved each year, these non-random populations 

are merged with the random mating populations for reproduction. The maximum value 

used was 50%, with a minimum of 5% and a most likely value of 30%. 

Seed blend model parameters: 

1. Base larval movement: Data suggest that there is limited larval movement among nontransgenic plants in the absence on high larval densities (unlikely to be found in a 5% 

seed-mixture). The range for this parameter was from 1% to 5%, with a most likely value 

of 3%. 

2. Days of larval movement: Based on Mallet and Porter (1992), larval movement affects 

the evolution of resistance by shifting dominance. This shift occurs gradually over a 

number of days. The range of time over which larval movement occurred ranged from 1 

to 18 days, with a most likely value of 10 days. The mortality rate was assumed to be 

constant over this period, with the overall mortality rate equal to 1 – w, the relative 

fitness of the genotype. 

3. Asymmetrical movement: Food choice bioassays in many species suggest that larvae will 

tend to taste and then move off of transgenic tissue at a greater rate than non-Bt tissue. 

Hibbard et al (2005) found, in a two year study, that this rate varied in those two seasons 

between 0 and 30%. Because of the limited data on this parameter, the range of this 

parameter was 0 to 50% with a most likely value of 30% (30% of larvae will leave initial 

Bt host plant). 
- 38 -

Table 11. Uncertainty parameter values used for the block refuge simulations 

Min Most Likely Max

Wss 0.01250 0.04000 0.08000

Dominance 0.01000 0.05000 0.20000

IRF 0.00100 0.00500 0.02000

Refuge 0.01000 0.15000 0.20000

Non-compliance 0.05000 0.10000 0.60000

Non-compliance mixing 0.85000 0.93000 0.95000

Non-random mating 0.05000 0.30000 0.50000

Table 12. Uncertainty parameter values used for the seed mixture simulations 

Min Most Likely Max

0.01250 0.04000 0.08000

Dominance 0.01000 0.05000 0.20000

IRF 0.00100 0.00500 0.02000

Refuge 5.00000 5.00000 5.00000

Base larval movement 0.01000 0.03000 0.05000

Days of larval movement 1.00000 10.00000 18.00000

Asymmetrical movement 0.00000 0.30000 0.50000

Wss

c. Sensitivity Analyses

ORD generated effects-plots for relevant parameters to determine the change in estimated time to 

resistance evolution when a range of values used in EPA’s model was explored. The graphs 

below (Fig. 2) allow side-by-side comparisons of which parameters created greater changes in 

durability (i.e. were more sensitive). The y-axis represents the estimated time-to-resistance in 

years; the x-axis represents the numerical value of the parameter of interest that was varied. 

It becomes clear from viewing the plots in Figure 2 that dominance, % refuge planted, and initial 

resistance frequency were the most sensitive parameters and predicted the greatest changes in 

estimated durability in the model. Non-compliance had an intermediate effect on durability, and 

non-random mating, fitness, and non-compliance mixing had very little effect on the predicted 

durability. Other parameters, such as random mating, were less sensitive and did not affect the 

estimated years to resistance evolution as much as predicted by Pioneer. 
- 39 -

Figure 2. Effects of changes in parameter values on the time to resistance evolution 

e. Modeling Results 

The IRM scenarios that were compared with the deterministic version of the ORD model were a) 

the 20% block refuge with non-compliance, non-random mating, and b) the 5% seed blend (also 

referred to as the larval movement model) incorporating asymmetrical larval movement. The 

distributions of the time to resistance for the block refuge strategy and the larval movement 

strategy with and without asymmetrical movement are presented in Figure 3. Because the bars 

each represent a year, the density of the bars is also the proportion of simulations that resulted in 

resistance evolving in that generation. The mean time to resistance for the block refuge strategy 

was 11.95 years, and 56.55% of the simulations exceeded 10 years. For the seed mixture (larval 

movement) model, the mean time to resistance was 8.1 years and 13.5% of the simulations 

exceeded 10 years. 

Non.compliance.mixing effects plot Non.random.mating effects plot Refuge.effects plot

Non.compliance.effects plot

Non.compliance.mixing Non.compliance.mixing effects effects plot Non.random.mating plot Non.random.mating effects effects plot Refuge. plot Refuge.effects effects plot

Non.compliance.effects plot
- 40 -

Figure 3. Distribution of time to resistance for a) the block refuge model and b) seed mixture model 

Table 13. Regression coefficients for the regression on the results from the block refuge model 

Coefficients Estimate Std Error t-value Pr(>|t|) 

Intercept 11.6958 2.2321 5.240 1.78e-07*** 

Dominance -84.9810 1.3097 -64.887 <2e-16*** 

IRF -629.7121 14.7731 -42.626 <2e-16*** 

Non-compliance -8.8690 0.4426 -20.037 <2e-16*** 

Non-compliance 

mixing 

1.5415 2.4028 0.642 0.521 

Non-random mating -0.7173 0.5336 -1.344 0.179 

Refuge 54.6024 1.2589 43.374 <2e-16*** 

Wss 61.5433 3.4862 17.653 <2e-16*** 

Signif.codes: 0 ‘***’, 0.001 ‘**’, 0.01 ‘*’ 

Residual standard error: 1.797 on 1992 degrees of freedom 

Multiple R-squared: 0.8095, Adjusted R-squared: 0.8088 

F-statistic: 1209 on 7 and 1991 DF, p-value: <2.2e-16 

3. Stochastic Model (Taken out of Caprio & Glaser, 2010)

a. Model description

The stochastic model was a modification of a previous western corn rootworm model (Caprio et 

al. 2006). Biological parameters were primarily derived from that model with the exception of 

mortality rates, block size and dispersal rates. 

The stochastic model used the same mortality rates on transgenic corn as were applied in 

Pioneer's submission. Two different dominance values (h) were utilized, 0.05 and 0.2. If Wss is 

the likelihood that a susceptible individual will emerge from transgenic corn relative to non-Bt 

corn, and Wrr is standardized as 1, then the fitness of heterozygotes (Wrs) is estimated as (Wrr – 

Wss)*h+Wss. When h=0.05, heterozygotes were approximately 5x more likely to survive than Density of total simulations

Generations to Resistance for Block Refuge 

Model

Generations to Resistance for Seed Blend 

Model

Density of total simulationsDensity of total simulations

Generations to Resistance for Block Refuge 

Model

Generations to Resistance for Seed Blend 

Model

Density of total simulations
- 41 -

susceptible genotypes and approximately 20x more likely to survive when h=0.2 and when Wss = 

0.01. Those rates decreased as the fitness of susceptible homozygotes increased (a reflection of 

the difficulty in using dominance rather than the Wrs:Wss ratio). 

The simulated universe consisted of a 40x40 matrix divided into 30m x 30m blocks, or a total of 

144 ha. Four different types of simulations were run with different field sizes. Because the 

current refuge strategy requires that refuges be associated with each field, the four different 

scenarios were: 

• Field size of 5x5 blocks (each block is 30m x 30m, for a total of 2.25 ha); 

• Field size of 10x10 blocks (9 ha field size); 

• Field size of 20x20 blocks (36 ha field size); and 

• One single field of size 40x40 block field of 144 ha. 

Mean field sizes in the U.S. range from as low as 2.5 ha/field to as high as 150 ha/field, with 

most fields in the 30-50 ha/field range. In each case, a 20% refuge was planted along one edge 

of the field. In the case of the 5x5 block fields, this refuge consisted of a single row of 5 blocks 

only along one edge. In the 10x10 block fields, the refuge was 2 rows wide and 10 blocks long. 

The refuge widths for the remaining scenarios were, of course, 4 and 8 rows wide, maintaining in 

each case a 20% refuge (by area). 

Pioneer maintained that a square field is a best-case scenario (Flexner, 2009), but EPA feels that 

the opposite is true. It is not completely clear from the Pioneer submission, but it appears that the 

registrant simulated rectangular fields with refuges planted at the short ends of these fields. It is 

EPA’s belief that most growers plant lengthwise along such fields to minimize the number of 

turns that would have to be made with planters. Corn entomologists (Meinke 2010, Hibbard 

2010, personal communication) confirmed EPA’s assumption and felt that most growers would 

plant refuge fields along one long edge of the field. If dispersal of adults is in question, then as 

fields become more rectangular (length:width ratio increased), refuges become thinner, but the 

maximum distance between any Bt-plant and the refuge decreases. 

Under these conditions a square field (as EPA/ORD used) maximized the distance between 

refuge and Bt plants and can be seen as a worst case. Pioneer's model appeared to maximize the 

distance between refuge and Bt plants, but again, this did not seem consistent with typical 

grower practices. 

b. Assumptions

Adult dispersal: This was modeled based on Pioneer's dispersal data. EPA/ORD combined the 

data from Nowatzki et al. (2003) (Fig. 5) and built probability distributions based on the 

frequency of beetles moving different distances. In short, if the total height of the bars in Figure 

5 summed to 1, then the individual bar heights would represent the likelihood that a beetle would 

move that distance. However, Pioneer’s data included beetles that did not disperse, and the zero 

movement class consisted of 45% of the beetles. EPA/ORD made one change from how Pioneer 

defined ‘adult dispersal’. EPA’s stochastic model assumed that 45% beetles did not disperse (on 

a daily basis), while the remainder moved according to the Nowatzki data. The difference in 

terminology of ‘dispersal’ may be important. Pioneer's interpretation of the data is that beetles 

move an average of 14.5m/day. Using the same data, but only looking at beetles that move, the 
- 42 -

average movement rate is ca. 29m/day. This is purely a difference based on definition. In 

EPA/ORD’s stochastic model, at the beginning of each day a beetle was assigned a random 

position in the 30x30m block before it moved, upon which it chose a random direction. The 

model determined the distance the beetle had moved (based on Nowatzki’s data) and placed the 

insect in a new block. However, if the dispersing beetle moved only a short distance, then it may 

not have changed the 30x30m block it originated in. 

To determine how well the stochastic model dispersal kernel matched the Nowatzki data, a 

simulation was run in which one block was populated with adults and the remaining blocks were 

empty. The number of beetles along a transect through the fields was counted every 3 days, 

corrected for the number of cells at a distance, and multiplied by the distance they had dispersed 

to estimate the mean dispersal rate (per Nowatzki) at different times. The dispersal rate estimated 

after 3 days was 17.6 m/day, which closely matched the dispersal estimate by Nowatzki et al. 

Given that EPA’s model used Pioneer’s actual data to parameterize the dispersal, EPA/ORD felt 

that this was a suitable match. 

Interestingly, EPA/ORD noticed that the estimated mean dispersal rate dropped over time. This 

occurred because beetles did not move in a straight line day after day. Brownian diffusion (based 

on a random walk) is usually assumed for insect dispersal. By confounding measurements over 3 

days and using that data in a daily time-step model, Pioneer may have underestimated the daily 

dispersal rate of western corn rootworm. To demonstrate this process, a simple spreadsheet was 

generated (see Table 14), where 10 individuals were simulated that all moved 14.5 m/day. At the 

start of each day, a new direction was randomly drawn, and the beetles moved 14.5 m in that 

direction. The right-hand column estimated the dispersal rate using the Nowatzki method. Only 

the estimates from the first day accurately estimated dispersal rate/day (because all the 

individuals were moving away from the release point). Estimates based on beetles’ displacement 

on subsequent days tended to underestimate daily movement. After three days of movement the 

estimated dispersal rate/day was less then half the actual value (spreadsheet used random 

numbers, hence values would differ from ‘run to run’). Unfortunately, because EPA/ORD did 

not have access to the original data and did not know the capture distributions for beetles of 

different ages, the problem could not be corrected. 

Table 14. Beetle Count over Time in a Simulated Dispersal Experiment 

Dispersal Distance from Initial Release Point (m) Days 0 30 60 90 120 150 180 210 240 270 300 330 360 Sum m/day 

1 0 130080 114240 43200 7680 18000 0 0 0 0 0 0 0 313200 27.22 

4 0 92640 174720 138240 72960 72000 34560 35280 46080 38880 0 0 0 705360 17.64 

7 0 40560 95040 155520 142080 90000 77760 23520 0 0 96000 0 0 720480 13.66 

10 0 24720 57600 123120 115200 102000 112320 82320 0 0 24000 0 34560 675840 11.09 

13 0 14640 37440 75600 96000 54000 60480 58800 30720 38880 0 0 34560 501120 9.28 

16 0 7440 29760 56160 107520 108000 43200 0 15360 77760 72000 29040 34560 580800 8.98 

19 0 6240 19200 28080 49920 54000 34560 35280 15360 19440 24000 0 0 286080 6.81 
- 43 -

In retrospect, analysis of the data using a diffusion model may have been more appropriate. 

EPA/ORD estimated that Nowatzki et al. underestimated the daily dispersal rate by a factor of 

0.68. Using a definition of estimating the dispersal as the distance a beetle moves when it moves 

(this is purely semantics but critical to define), and correct by the above factor, the resultant 

dispersal rate is ca. 41m/day. It should also be noted that, because a different definition of 

dispersal was used, this rate is not the same as the 45m/day dispersal rate used in the sensitivity 

analysis in the Pioneer submission. 

Figure 4. Beetle Position by Day after Release (using Nowatzki et al. (2003) Data)

Refuge movement: Several scenarios were explored for refuge movement. One scenario, used for 

the transect experiments presented below, used refuges that were moved every generation. All 

growers planted their refuge on the west side of their fields in one generation and moved them to 

the east side in the next. Because the east side of one field is adjacent to the west side of a 

neighboring field, this scenario resulted in refuges always being planted adjacent to a refuge 

from the previous year. Ultimately, random placement of refuges each year was chosen as the 

most likely grower behavior. This means that in some cases refuges are not moved, while in 

other cases they are moved to relatively isolated areas. 

c. Results

EPA/ORD’s stochastic model was run for four different field sizes and two different adult 

dispersal rates (dispersal (1) represents Nowatzki et al.’s (2003) data; dispersal rate (2) is double 

Nowatzki’s rate) (Fig. 6). Because the largest field size took the entire simulated universe, it was 

0 30 60 90 120 150 180 210 240 270 300 330 360

0.5

5

50

500

5000

1

4

7

10

13

16

19

Beetle Position By Day After Release

1

4

7

10

13

16

19

Distance (m)

Number of beetles

DayPost-Release
- 44 -

not possible to incorporate non-compliance. To address this issue, the model was also run in an 

80x80 cell mode (6400 cells, each 30x30m) for this field size, allowing simulation of 4 144 ha 

fields. These larger simulations were used in Figure 6 and in the data analysis. 

Analysis of variance showed no significant impact of dispersal rate, but field size was highly 

significant (p << 0.001). When run with a single 144 ha field and non-compliance excluded, the 

times to resistance were much longer than when run in a comparable fashion to all the other 

stochastic simulations (mean durability = 3292 d, approx. 23 yrs). It was not expected that 

doubling the dispersal rate would have so little impact on the time to resistance, and it suggests 

that the model is relatively insensitive to this parameter. 

Using Pioneer's 40 ha field size as a benchmark, the stochastic model predicted an increase in 

durability of 3.94 fold for 20% block refuges compared to simulations with no refuge (see 

discussion). This is much higher than the 1.4 fold reported for this field size in the Pioneer 

simulations. The simulations presented here predict that a block refuge with 40 ha fields would 

last 13.9 years, almost double the time predicted by Pioneer. The 144 ha simulations showed 

much lower durability (8.4 yrs, or 2.4 fold longer than without a refuge). It may be relevant that 

the dimensions of these fields had a 240m wide refuge and a 960 meter wide strip of Bt-corn. 

Pioneer's simulations may have been run with a 200m wide refuge and an 800 m wide strip of 

Bt-corn (this occurs in a 40 ha field size in the Pioneer model and in a 144 ha field in the current 

model). 

To determine further the sensitivity of the model to the rate of dispersal, the 36 ha field size was 

run with 2x, 4x and 8x dispersal. Dispersal was again not significant in an ANOVA (p=0.058). 

The 36 ha field size had only 4 refuges in the default 144 ha system, and this increased the 

variance when in any given year each field had a 30% chance of being non-compliant. There is a 

24% chance that all Bt fields would have a refuge in any year and a 0.8% chance that all four 

fields would not have any refuge in any year. In a larger system, the mean number of noncompliant fields would remain more constant over time (law of larger numbers), and there might 

be a better opportunity to investigate how dispersal interacts with refuge size and placement. 

It is possible that dispersal has less effect than might have been expected because there are 

several different situations that arise in the model. When a refuge is not moved, lower dispersal 

rates result in a larger effective refuge. Simulations suggest this frequently delays resistance 

evolution, although it reduces the probability that an individual beetle will move and mate at 

random with beetles emerging in Bt-fields (Caprio 2001). Similarly, if the refuge is moved next 

to the location of where a refuge had been planted in a neighboring field in the previous year, 

lower rates of dispersal might also maximize effective refuge size. In contrast, when refuges are 

moved to areas relatively distant from the locations of the previous years refuges, higher rates of 

dispersal would maximize the effective refuge size. It is clear, however, that these results suggest 

that the model is less sensitive than expected with respect to dispersal rates. Discussions over 

whether the Nowatzki data over or underestimated movement rates for the entire Corn Belt may 

perhaps be less critical than expected. 
- 45 -

Figure 5. Means of four block simulations at each field size (ha). Simulations used for the dark line used 

Nowatzki's data (15m/d); simulations used in the red line doubled that dispersal rate (30m/d). 140 days/year 

are used; hence, 1400 days would represent 10 years to resistance. 

4. Discussion for Stochastic and Deterministic Results (Taken out of 

Caprio & Glaser, 2010)

Pioneer presented in Table 1 of its submission (MRID 478710-01) the results from their model 

using the benchmark parameters listed in Table 3 of this review. Using the deterministic model 

presented here with similar parameter values, similar but not identical durabilities were found 

(Table 15). 

In the case with no refuge, Pioneer's model suggested resistance would evolve in 5 years, while 

ORD’s deterministic model suggested durability would be 3 years. The stochastic model 

predicted a durability of 3.5 yrs in the absence of refuges. For comparison's sake, other rates 

were measured relative to these baseline numbers. For the block refuge model, Pioneer suggested 

resistance might evolve in 7 years, or 1.4x the time without a refuge. Assuming EPA’s most 

likely value of a 15% effective refuge (and otherwise using Pioneer's benchmark values), a 

predicted durability of 14 years was obtained with ORD’s deterministic model, 4.7 fold longer 

than the simulation without a refuge. One possible reason for the large discrepancy is the Mean Time to Resistance (days)
- 46 -

effective refuge size. In order to duplicate the 1.4x durability increase noted by Pioneer, the 

effective refuge size would need to be decreased to 1%. This was the minimum value in 

EPA/ORD’s uncertainty distribution for this parameter. This parameter value (1%) was felt to be 

the minimum because it would require the following likely unrealistic assumptions: 

• Large fields (on the order of 150 ha); 

• Dispersal rates used by Pioneer were not underestimated; 

• Movement rates were typical of rootworm dispersal rates throughout the Corn Belt (Data 

by Spencer (2009) suggests otherwise); and 

• Growers consistently moved refuges to regions of their fields with the lowest egg density 

(would require that growers had a mechanism to detect areas of low egg density). 

Table 15. Years of Durability Predicted by Pioneer's and EPA/ORD's Model using Benchmark Parameter 

Values 

Durability (time to resistance) Refuge Scenario Pioneer* ORD deterministic model1 ORD stochastic model2

No Refuge 5 years 3 years 3.5 years 

20% Refuge 7 years 14 years 13.9 years 

5% Seed Blend 10 years 9 years Not modeled 

*using Pioneer’s benchmark values (Table 1, MRID478710-01) 1

 using Pioneer’s benchmark values but 15% effective refuge as most likely value and fixed 30% non-compliance 2

Stochastic model used Nowatzki’s dispersal data, Pioneer’s survival data, and otherwise different assumptions: simulated typical 

field size a) 40 ha and outlier field size b) 144 ha; refuge placement random; dominance values of 0.05 and 0.2. 

The overall risk profile from the uncertainty simulations for block refuges was also impacted by 

other parameters, which deviated from the Pioneer benchmark values. Although relative fitness, 

initial frequency and dominance used the same uncertainty distributions for both the block and 

seed blend simulations and were likely to affect both models in a similar manner, these 

parameters could shift both distributions in time. The inclusion of the possibility of higher levels 

of dominance, in particular, could lead to shorter predicted durability estimates. Other 

parameters specific to the block refuge model could lead to the longer durability predicted in 

these simulations compared to Pioneer's simulations. ORD’s distribution for non-compliance was 

weighted more heavily towards lower values (with a most likely value of 10% non-compliance) 

than reported by CSPI (2009); based on BPPD’s analysis, that report overestimated noncompliance. EPA also assumed that most non-compliant growers were “partially” noncompliant, perhaps due to miscalculations or mistakes during planting. Only 5% of noncompliant growers were assumed to be persistently non-compliant year after year. 

For ORD’s seed blend (or larval movement) model, the analysis predicted a durability of 9 years, 

or a 3-fold increase in durability compared to ‘no refuge’. This compared reasonably to the 

Pioneer estimate of 10 years, or a 2-fold benefit. The values both compared quite well to the 

mean of EPA’s risk profile, 8.1 years. 

These data suggest that although there were minor differences between the seed blend (larval 

movement) results generated by the Pioneer model and the model presented here, the major 

differences lie in the block refuge model. Although Pioneer suggested that non-random mating 
- 47 -

was important in their results, EPA/ORD did not find that to be the case in their simulations 

(Table 13). The parameter most likely to be responsible for the differences between the two 

models is the effective refuge size. EPA agrees with the Pioneer assertion that it is possible that 

areas of large fields could have very low egg densities, and should growers manage to locate and 

plant refuges over those areas, the block refuge strategy could be severely compromised. 

A complete understanding of the differences in the results is limited by details missing in the 

Pioneer data submission. The registrant simulated a size of 4x10 cells (each cell as 100m 

x100m), or 40 ha. It was unclear if this was a single field or represented multiple smaller fields, 

but given their results, a single field is likely. The exact placement of the refuge in the 4x10 cell 

universe is also important and not stated. Based on the Entomological Society of America 

presentation by Flexner (2009), it appears as though the refuge was located to one “thin” edge of 

this system (perhaps two 4 cell rows to one edge), which would maximize the furthest distance 

between transgenic plants and the refuge but would seem to be at odds with common grower 

behavior (this is also supported by the fact that one cannot get 20% of the cells by planting along 

the long edge in this size). Again, based on the same information by Flexner (2009), it appears as 

though the only way for insects to reach the plants to the right of the system (when the refuge 

was planted on the left side) was to move across the intervening 800m. In actual field systems, 

however, there are other fields surrounding the simulated field with refuges. Those refuges 

would on average be adjacent to the current field, and the mean distance between the most 

distant field and the refuge would be 400m. Most simulations were designed as toruses (an insect 

moving off the left side of the system arrives at the right side as if the system were a donut) to 

overcome this problem, but the animation presented at ESA was not designed in this manner. 

This type of model minimizes the effects of dispersal and would tend to lower Pioneer's estimate 

of the durability of a block refuge system. 

In the current stochastic model (discussed in Section C.3), EPA/ORD simulated multiple fields 

arranged in a large torus. Refuges were placed randomly within fields, and non-compliant fields 

were also chosen at random on a yearly basis. In these simulations, in some cases refuges in 

adjacent fields would be side-by-side, while in other cases they would lie at opposite ends. In the 

latter case there would clearly be an area with low egg densities between the two refuges, but 

that was apparently offset by refuges that must logically be more closely spaced in other areas. 

Both the Pioneer model and EPA’s dispersal kernel, based on the Pioneer dispersal data, ignored 

the potential of longer distance dispersal (Storer 2003). Although the contribution from such 

beetles is likely to be small, in areas of fields were there are essentially no eggs (see Figure 1d), 

the contribution of these few individuals could be significant. 

There are a number of explanations for the differences in the predicted longevity of the block 

refuge strategy between the Pioneer simulations and those presented here. In most cases 

important details of the Pioneer model were not presented, which made it difficult to determine 

how certain key elements were modeled. The Pioneer model used a single 40 ha block of 

100x100m cells arranged in a rectangular fashion (4x10). In discussions with corn 

entomologists, they felt that most growers would plant lengthwise in such a field, and the refuge 

would be planted along either one of the long edges (here termed the east and west edges). 

Simulations of refuges planted to the short ends of these fields (the north and south edges) would 

not seem to be the normal grower practice and would tend to increase the mean distance between 
- 48 -

refuges and Bt plants. This choice, if made, would increase the likelihood of the “holes” or 

“pockets” in random mating mentioned by Pioneer in their submission and reduce predicted 

durability of the block refuge design. 

Pioneer also provided little detail in the spatial aspects of their simulation. Was a single field 

simulated with all the refuge gathered in one place? Or were smaller fields simulated with 

multiple refuges? When the refuge was moved, where was it moved to? Was it moved to a 

random location, or was it always moved to the opposite end of the field from its previous 

location? The latter choice might lead to a significant decrease in the effective refuge size and 

more rapid resistance development for the block refuge. 

How were boundary conditions treated? When an insect moved off of the field to the west side, 

for example, what happened to it? Did it die, was it reflected back into the system, or was a torus 

simulated? The choice here may be important as it can again impact the mean distance between 

refuge and Bt plants and ultimately effective refuge size. As an example, if a string of 4x10 

fields is considered, each 1000m long, each with its own 200m wide refuge, then the maximum 

distance between any two refuges would be 1600m, and the maximum distance between a refuge 

and a Bt plant would be 800m. The average distance would be 800m between refuges and 400m 

between Bt plant furthest from a refuge and the nearest refuge. If Pioneer chose not to use a torus 

design, and simulated a single field with a refuge at either the west or east end, the maximum 

distance between a Bt plant and the nearest refuge would be doubled (800m). In a simulation of a 

torus, insects leaving the east edge of the field will end up in the west edge, in essence creating a 

long string of identical fields. This design would correctly model the mean distance but without 

any variance. EPA/ORD simulations included multiple fields with random placement of refuges 

to either the east or west edge of a field, which seems to best mimic grower behavior. This 

design meant that in some cases the refuges would be at opposite ends of adjacent fields, while in 

other cases they might be side-by-side; simulations of this kind allowed for the correct mean 

distance between refuges but also for variance in this distribution.

EPA/ORD’s interpretation of the data indicates that rather than non-random mating being the 

threat to a block refuge system, it appears that non-random oviposition and refuge placement are 

most important. This suggests a potentially plausible method that Pioneer might use to bolster 

their claims that failure of the block refuge system is imminent. If the relative density of eggs 

could be estimated in transects across fields of different sizes (similar to Fig. 3a-e), and if more 

accurate estimates of how growers plant and move their refuges over years were available, then 

major differences between the two models could be resolved. Such a study might also provide an 

estimate of the contribution of beetles dispersing long distance. 

5. Simulating Other Deployment Strategies (Taken out of Caprio & 

Glaser, 2010) 

a. Different seed blend proportions: 

The effect of the proportion of non-Bt seeds in a seed-blend was determined by varying this 

proportion from 0.025 to 0.10 in increments of 0.025. At each non-Bt proportion, 3 replicates of 

2000 simulations were run. Each simulation used random draws from the uncertainty 
- 49 -

distributions outlined in the sections above. From each replicate, the mean of the distribution was 

obtained and the Standard Error of Mean (SEM) determined based on the means from the three 

replicates. The percent of the simulations that exceeded 10 years for each non-Bt proportion was 

estimated from the first run for a non-Bt class. 

These results suggest that, at least in the modeled range, the more non-Bt seeds included in a 

seed-blend, the longer the expected durability of that seed-blend will be. This relationship in this 

case was very close to a linear relationship (adjusted R2

 = 0.9996). 

When compared with the durability predicted by ORD for a 20% block refuge (11.95 yrs, 56.6% 

of the simulations exceeding 11 yrs), a 10% seed blend was still slightly inferior (11.3 yrs, 51.6% 

of simulations exceeding 11 yrs). However, a comparison between the blends modeled and the 

20% block refuge show that the 10% seed mix result is most similar to the block refuge strategy 

risk profile discussed earlier. 

 

Table 16. ORD's Risk profile for Cry34/35 deployed in different seed blend percentages based on BPPD and 

ORD assumptions and probability analysis 

2.5% Blend 5.0% Blend 7.5% Blend 10.0% Blend 20% Block 

Mean Durability 6.6 yrs 8.1 yrs 9.7 yrs 11.3 yrs 11.95 

SEM 0.014 0.013 0.005 0.028 Not provided 

% of simulations 

exceeding 11 years 3.4% 13.5% 30.5% 51.6% 56.6%* 

*Exceeding 10 years 

b. Resistance simulation for dual-gene (pyramided) product: 

A resistance simulation was run for a two locus seed-mixture model. In this case the fitness of 

the susceptible genotyped (SS) and the dominance of resistance at each locus were independently 

determined according to the PERT distributions from the single locus model. This distribution is 

informative because it estimates the relative longevity of two genes stacked into the same plant 

versus using the same two genes sequentially in single gene plants. The PERT distribution is a 

version of the Beta distribution and requires the same three distributions as the Triangle 

distribution, the minimum, the most likely value (or mode), and the maximum. This is relevant 

because introducing a single gene event into a landscape with dual gene events may lead to rapid 

resistance at the first locus and essentially turn all dual gene events that share the toxin into 

single gene events. 

The results from the two locus deterministic model showed that, even though the second toxin 

has identical uncertainty distributions as the first toxin, the mean expected lifetime of the dual 

gene product is 52.6 generations for a 5% seed blend, a 3.25 fold increase from the sequential 

use of two genes (16.2 years). Single gene products that share toxins with dual gene events can 

lead to a pseudo sequential use because the single gene event selects rapidly for resistance to one 

gene, and once this occurs the dual gene events are effectively rendered single gene events 
- 50 -

Figure 6. Distribution of time to resistance from 2000 simulations of a 5% seed blend deployment with two 

resistance loci. The mean time to resistance was 52.6 generations. 

IV. BPPD Risk Assessment Conclusions 

A. BPPD’s overall conclusions

Pioneer submitted new modeling information in which they determined that the addition of an 

adult CRW infield-movement parameter in Bt seed mixtures allowed for the reduction of refuge 

from 20% block to 5% seed blend. Pioneer concluded that a 5% seed blend would extend the 

durability of Bt corn expressing Cry34/35 over the current refuge requirement of a 20% block 

refuge. 

BPPD has reviewed Pioneer’s submissions for Optimum AcreMax1 and in addition conducted an 

independent modeling analysis of Pioneer’s proposal using a model (deterministic and stochastic 

version) developed by EPA/ORD. After careful review of Pioneer’s modeling assumptions and 

based on the analyses conducted by ORD (modeling efforts and probability analysis), BPPD 

concludes that the registrant’s assertion that a 5% seed blend is always superior to the current 

block refuge deployment strategies for OAM1 and corn rootworm” is not scientifically 

supported. 
- 51 -

B. BPPD conclusions regarding Pioneer’s modeling

Pioneer’s model made a number of assumptions on CRW movement, refuge location, and 

percent non-compliance. In their model, when these assumptions were included, a 5% seed blend 

was more durable than a 20% block refuge. However, BPPD has concerns that some of Pioneer’s 

assumptions may not be realistic or conservative enough to support a reduced refuge for OAM1. 

During the risk assessment process, BPPD makes an effort to consider realistic case scenarios 

and parameters when evaluating simulation models. However, it can be very useful to look 

closely at scenarios with worst-case assumptions especially in cases when a degree of 

uncertainty is associated with important modeling parameters (e.g. dominance, initial resistance 

allele frequency, adult movement). Risk assessment conclusions are reached based on the best 

scientific information available to the Agency at the time of the assessment which is unique to 

any particular pest-crop system. This often includes a degree of uncertainty. Such uncertainties 

have required that BPPD “bracket” the risk of putative resistance evolution. This goal has been 

accomplished by estimating risk using realistic assumptions based on available data and 

“defensible” worst-case assumptions in the model. With this approach, the ‘actual’ risk of 

resistance evolution may lie somewhere within this range of potential risk. If modeling results 

predict little to no-change in estimated risk of resistance with worst-case assumptions, then 

science conclusions will be more robust and defensible. 

The following is a summary of Pioneer’s modeling parameter values and assumptions which 

were not supported by BPPD’s and ORD’s analysis. 

 

1. Random mating: Pioneer reported that the addition of an infield movement parameter for 

adult CRW in the 5% seed blend increased the random mating between refuge and 

putative resistant beetles emerging from Bt plants. EPA’s analysis however showed that 

random mating is not as sensitive a parameter as stated by Pioneer, and that it has little 

effect on the resistance evolution (see Fig. 2). In fact, EPA’s modeling revealed that egg 

density/distribution in corn fields is a much more important (sensitive) parameter that 

affects the rate of resistance evolution. EPA determined that in ≥150 ha fields (366 

acres), ‘dead zones’ could occur without any refuge beetles moving into those parts of the 

fields (as was pointed out by Pioneer) and without eggs present. It is important to recall, 

however, that the average field size in the Western Corn Belt is around 32 ha (80 acres) 

and in the Red Zone (WI, IL, IN, and MS) between 16 ha to 32 ha (40-80 acres). For such 

fields of average size, these ‘zones of continuous selection’ did not occur with EPA’s 

model. 

2. Neonate dispersal: If larval movement had been assumed for later instars (rather than 

neonates) while including lower mortality and greater tolerance for Cry34/35, estimated 

durability of 59122 in Pioneer’s model might have been reduced for seed blends. Lateinstar dispersal could allow for heterozygous resistant survival, which might speed up 

evolution of resistance in seed blends with a single, non-high dose toxin. Mallet and 

Porter (1992) concluded that when larvae were able to move from plant to plant within a 

Bt seed mixture that resistance could occur faster than compared to pure stands of Bt. 

3. Adult movement: Pioneer used data from Nowatzki et al. (2003) to calculate the mean 

adult movement and included 45% of non-moving individuals into this calculation, which 
- 52 -

is not how dispersal is typically calculated. For example, Storer (2003) only included 

individuals that actually dispersed into his movement calculations. Pioneer’s approach 

decreased the estimated mean dispersal to 15m/day, which is a much more conservative 

value than what would have been obtained otherwise (28m/day). This approach of 

calculating dispersal puts block refuges at a disadvantage over seed blends. EPA/ORD 

estimated that Nowatzki et al. further underestimated the daily dispersal rate by a factor 

of 0.68. Using a definition of estimating the dispersal as the distance a beetle moves 

when it moves, and correcting by the above factor, the resultant dispersal rate is ca. 

41m/day, which is similar to what was reported by Spencer (2009). 

4. Planting location of block refuges: Pioneer stated in their submission that farmers moved 

their refuges to a new location every year. A critical component of their modeling was 

that refuges were moved every generation. EPA/ORD believes that this may overestimate 

grower placement of refuges. This refuge planting practice would tend to decrease the 

number of beetles generated each year and making refuges less effective (Storer 2003). 

Pioneer’s assumption that growers move their refuges each year may be unrealistic and 

certainly made block refuge deployment less durable. Fixing refuge locations and aiming 

for uniform distribution of refuges should help delay resistance evolution (Sisterson et al., 

2005). Second, Pioneer appeared to have planted refuges at the far and narrow end of 

fields in their model rather than along the long side of the Bt portion of the field. Most 

growers, however, plant lengthwise along such fields to minimize the number of turns 

tractors must make. Pioneer's model maximizes the distance between refuge and Bt plants 

but is not consistent with typical grower practices. 

5. 0% non-compliance: Pioneer explored a range of compliance from 50-100% and used 

70% and 75% for block refuge simulations. This assumption translates into 25-30% of 

growers planting 0% refuges. This percent of non-compliance is an overestimate. Actual 

data indicates that up to 89% of growers planted more than 15% of the required refuge, 

and that only 5%-15% of growers planted less than 10% refuge in 2007 and 2008. 

6. Lepidopteran refuge: Pioneer proposes that the refuges adjacent to the Bt fields are 

planted with Herculex RW seed (100% CRW control). BPPD contends that this would 

further reduce the proposed refuge from 5% to 4.1% and possibly further increase the rate 

of resistance evolution. 

In light of the concerns identified above, BPPD concludes that model conditions/parameters in 

Pioneer’s model (compliance, CRW adult, neonate movement rather than later instar movement, 

refuge placement to maximize distance between refuges) are unfavorable to blocks while being 

more favorable to blends. This has the effect of biasing Pioneer’s modeling analysis towards 

more durable seed blends and less durable block refuges. 

C. EPA/ORD’s deterministic and stochastic analyses

BPPD has collaborated with ORD on independent modeling efforts to assess the 5% seed blend 

strategy as proposed by Pioneer. As part of this effort, BPPD 1) modeled Pioneer’s data and 

assumptions (i.e. side-by-side comparison) using ORD’s deterministic version of their model and 

2) used two separate models (deterministic and stochastic version) with parameter estimates that 

BPPD believes are likely more realistic or conservative. 
- 53 -

ORD and Pioneer models: a side-by-side comparison of modeling output using Pioneer’s 

benchmark parameter values 

A side-by-side comparison (using Pioneer’s benchmark parameter values) between the ORD’s 

and Pioneer’s deterministic models showed that the ORD model predicted greater durability for 

blocks than for seed blends (see Table 17). Predicted durability estimates were similar between 

the two models except for the 20% block refuges. 

Table 17. Side-by-side comparison between predicted durability of ORD and Pioneer model with Pioneer’s 

benchmark parameters 

Durability (time to resistance) Refuge Scenario Pioneer EPA/ORD1

No Refuge 5 years 3 years 

20% Block Refuge 7 years 14 years 

5% Seed Blend Refuge 10 years 9 years 

1

15% effective refuge was chosen in ORD’s model as most likely value and fixed 30% non-compliance 

The following is a likely explanation as to why ORD’s deterministic model predicted greater 

durability for 20% block refuge strategy than Pioneer: 

BPPD and ORD chose the effective refuge to be 15%, meaning it was assumed that 15% of the 

eggs were laid in the refuge due to non-random oviposition and refuge placement by growers 

(otherwise Pioneer’s benchmark parameters were used). In order to duplicate Pioneer’s block 

refuge result (7yrs), the effective refuge size would need to be decreased to 1%. This parameter 

value (1%) was felt to be unrealistically low for a universal assumption because it would require: 

• Large corn fields of 150 ha (but average field sizes are on the order of 30-50 ha); and that 

• Pioneer’s dispersal rates are not underestimated (however, non-moving insect were 

included in the dispersal calculation lowering the mean movement rate); and 

• Pioneer’s assumed movement rates are typical of corn rootworm throughout the Corn 

Belt (Data by Spencer (2009) suggests otherwise); 

• Growers consistently move their refuges to regions of their fields with the lowest egg 

density (would require that growers had a mechanism to detect areas of low egg density). 

ORD’s risk profile based on more realistic or conservative assumptions:

a) Deterministic Model

Considering average movement data as reported by Spencer (2009) and as calculated from 

Nowatzki’s data aver correcting for errors, ORD’s deterministic model predicted that the mean 

time to resistance for the block refuge strategy was 11.95 years (56.55% of the simulations 

exceeded 10 years). For the seed mixture simulations, the mean time to resistance was 8.1 years 

(13.5% of the simulations exceeded 10 years) (see Table 18). 
- 54 -

Table 18. ORD's risk profile for Cry34/35 deployed in a 20% block and 5% seed blend based on more 

realistic assumptions and probability analysis 

20% Block 5% Blend 

Mean Durability 11.95 yrs 8.1 yrs 

% of simulations 

exceeding 10 years 56.6% 13.1% 

 See Section C.2.b and Table 11 and Table 12 for assumptions and parameter values 

BPPD concludes that the 20% block refuge extends the durability of Cry34/35 with greater 

probability than Pioneer’s proposed 5% seed blend when more realistic assumptions for the 

following are made: 

• Greater rates of adult CRW movement as suggested by Spencer (2009) and (after ORD 

corrected error in movement calculations) by Nowatzki et al. (2003) data; 

• Refuge placement by growers (at the long side of Bt fields); and 

• Compliance percentages (between 1-15% of no refuge planting but with maximum of 

60% to address anecdotal communication of non-compliant counties in some regions of 

the Corn Belt); 

b) Stochastic Model

ORD’s stochastic model predicted greater durability (like ORD’s deterministic version) for the 

20% block when Pioneer’s benchmark parameters were used (see Table 19). ORD modeled Bt 

fields of square shape rather than rectangular (as Pioneer), which is less favorable for block 

refuges. 

Table 19. Risk Profile of ORD's stochastic model for 20% block refuges 

Durability (time to resistance) Refuge Scenario Pioneer deterministic model ORD stochastic model 

No Refuge 5 years 3.5 years 

20% Block Refuge 

(40 ha fields) 7 years 13.9 years 

20% Block Refuge 

(144 ha fields – atypical 

field size) 

Not modeled 8.4 years 

The stochastic results predicted that durability for Cry34/35 deployed with a 20% block refuge 

would last approximately 13.9 years for fields of typical size; for large 144 ha fields (which are 

not average size but outliers), durability is predicted to be around 8.4 years. The stochastic model 

also analyzed the effect of two different dispersal rates on the durability of Cry34/35 when 

deployed in a 20% block refuge. These two different dispersal scenarios were explored because 

there are reported differences in adult CRW movement from Nebraska (Nowatzki et al. (2003)) 

and Illinois (Spencer 2009) implicating that CRW movement may be different throughout the 

Corn Belt. ORD did not find that doubling the dispersal rate used by Nowatzki et al. (2003) 

increased the durability significantly. However, long-distance dispersal was not modeled (i.e. 

>200 m/d) and may be an important factor for larger, atypical field sizes with areas of low egg 
- 55 -

density. Discussions over whether the Nowatzki data over or underestimated movement rates for 

the entire Corn Belt may perhaps be less critical than expected. An analysis of the stochastic 

results suggests that there may be an interaction between dispersal and effective refuge. 

Summary of ORD’s modeling (deterministic and stochastic)

ORD’s predicted durability for block refuges ranges from 12 to 14 years in both the deterministic 

and stochastic simulations. Three different approaches were used to in the process of the 

analysis: 1) Pioneer’s benchmark assumptions were incorporated (side-by-side comparison, see 

Table 17) and 2) BPPD’s and ORD’s more realistic parameter assumptions were considered for 

the deterministic simulations, and 3) the stochastic model was used to predict block refuge 

durability for two different dispersal rates and different field sizes (typical and outlier). 

EPA/ORD’s analysis also included a probability distribution for the deterministic part of the 

assessment to establish which durability results were more likely to occur. BPPD believes that 

these factors have allowed ORD to conduct a more comprehensive analysis than was submitted 

by Pioneer. 

At the present time, based on the available data and independently generated evidence discussed 

in this risk assessment, BPPD cannot recommend a registration for OAM1, a 5% seed blend 

expressing Cry34/35 to control CRW, because ORD’s modeling supports that such an action 

would increase the risk of unreasonable adverse effects (i.e. resistance evolution). 

D. Strip refuge configuration

In Pioneer’s assessment, predicted durability for infield strip refuges was assessed and compared 

to blocks and seed blends. For the most part, infield strip deployment was superior to the 

predicted durability of 20% block refuges and 5% seed blends. ORD’s preliminary exploration 

suggest that predicted durability for infield strips is not as straight forward and may be affected 

by effective refuge size which could make the strip refuge deployment either inferior or superior 

to blocks. ORD determined that evaluation of durability for strips needed further analyses. 

E. Options

1. Pioneer’s conclusions that a 5% seed blend is superior to the 20% block refuge were the 

result of a number of possibly unrealistic assumptions that would have to hold true for the 

entire Corn Belt. These include: 

• Refuges would have to be planted consistently along the shorter side of the Bt fields. 

This is against grower practice. 

• Growers would have to move their refuge locations yearly. This is believed to be 

unlikely. 

• Growers would have to plant refuges consistently over areas of lowest egg densities, 

which meant that they would have to be able to somehow determine where egg 

densities were lowest. This is probably not a realistic assumption. 
- 56 -

• Adult male movement would have to be limited to Pioneer’s estimate of 15 m/d. 

ORD’s analysis demonstrated that Pioneer underestimated dispersal; after having 

corrected for errors, adult movement was similar to what was reported by Spencer 

(2009). Data by Spencer suggested that adult CRW movement occured on the order 

of 30 m/d to greater than 200 m/d (average 50 m/d) and further suggested that adult 

movement may be positively affected by greater population density. 

EPA/ORD’s interpretation of the data suggest that non-random oviposition and refuge 

placement are most important rather than non-random mating being the threat to a block 

refuge system. This suggests a potentially plausible method that Pioneer might use to 

bolster their claims that failure of the block refuge system is imminent. If the relative 

density of eggs could be estimated in transects across fields of different sizes, and if more 

accurate estimates of how growers plant and move their refuges over years were 

available, then major differences between the ORD and Pioneer models could be 

resolved. Such a study might also provide an estimate of the contribution of beetles 

dispersing long distance. 

2. ORD explored the durability for different proportions of seed blends (0.025, 0.05, 0.075, 

and 0.10) and reported that predicted durability for the 10% seed blend was 11.3 yrs 

(with 51.6% of the simulations exceeding 11 years). While this is still lower than the 

predicted durability for a 20% block refuge, the 10% seed blend came closest to the 

predicted block durability. 

3. BPPD has considered temporally/geographically-limited registrations in the past because 

they may reduce the risk for resistance evolution. These registrations are typically limited 

by total U.S. acreage (usually <250,000) and have county-level acreage caps (usually 

<20,000). However, the modeling analyses conducted by Pioneer and ORD were not 

designed to address the risk of a time limited registration for OAM1. Therefore, BPPD 

cannot specifically quantify the risk of resistance evolution for such an option. 

4. BPPD notes that models predict that pyramids of two PIP toxins can increase the 

durability of the traits and allow for a reduction of the percentage of refuge. ORD’s 

modeling showed that if two traits with the characteristics of Cry34/35 were pyramided 

and deployed as a 5% seed blend that durability would increase by more than three fold 

(52 years) over the sequential use of the two single toxins. Pioneer could consider the 

addition of a second CRW-active toxin to OAM to improve the durability and potentially 

justify a 5% seed blend refuge. 

 
- 57 -

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Protection, also known as Herculex XTRA(Cry34/35+Cry1F, EPA Reg. 29964-A) and Herculex 

I (Cry1F). Memorandum from J.C.Martinez to M. Medelsohn on December 17, 2008. 

BPPD. 2009. EPA review of ABSTC’s 2007 and 2008 corn insect resistance management 

compliance assurance program. Memorandum from J.C. Martinez to M. Mendelsohn on April 

15, 2009. 

EPA Scientific Advisory Panel Meeting. 2002. Corn rootworm plant-incorporated protectant 

non-target insect and insect resistance management issues, part B: insect resistance management 

issues. http://www.epa.gov/scipoly/sap /2002/august/august2002final.pdf

EPA Scientific Advisory Panel. 2009. Transmittal of meeting minutes of the FIFRA Scientific 

Advisory Panel meeting held February 23-24, 2009 on the evaluation of the resistance risks from 

using a seed mix refuge with Pioneer’s Optimum AcreMax1 corn rootworm-protected corn. 

Memorandum from J.E. Bailey (DFO) to Dr. D. Edwards (Director of OPP) on May 6, 2009.