Corn Seed Blend Proposal Containing Refuge 

Seed for Lepidopteran and Corn Rootworm 

Target Pests:

EPA Overview and IRM Considerations

Charge to the Panel

Authors: Jeannette Martinez, Ecologist

Alan Reynolds, Entomologist

Date: November 8, 2010
Section A: Biology of European Corn Borer, 

Southwestern Corn Borer, Corn Earworm, 

and Corn Rootworm

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Charge Question 1

European corn borer (ECB) has both local and long distance 

dispersal capability. Currently, the proportion and frequency of 

individuals in a population engaging in dispersal before or after 

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

aggregation sites near cornfields, mark-release-recapture studies 

in the U.S. have typically had a low recapture success (<1%). 

Recently it was suggested that 1-day old female ECB may engage 

in obligate pre-mating dispersal (Dorhout et al. 2008). 

Please comment on the uncertainties regarding ECB 

movement including mating sites, pre- mating dispersal, and 

the proportion of the population engaging in long- distance 

dispersal. How might these aspects of ECB movement affect a 

potential seed blend strategy?

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Charge Question 2

Scientific Advisory Panels (1998 and 2000) discouraged the Agency 

from the use of Bt seed mixtures to control lepidopteran target 

pests because substantial larval movement could be expected 

between Bt and non-Bt plants which could lead to more rapid 

selection of resistance. 

BPPD has reviewed new data developed by Dow and Monsanto 

simulating the effects of SmartStax on various instars of potentially 

mobile lepidoptera. These data provide evidence that SmartStax is 

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

survivability among 4th and 5th instars. While it has been 

established that ECB disperse as neonates, there is some 

uncertainty with respect to lepidopteran propensity for dispersal 

off non-Bt plants as later instars. BPPD notes that simulation 

models incorporating data on high larval mortality on SmartStax 

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

durable as structured refuges. 4
Charge Question 2 (continued)

Please comment on ECB larval plant- to- plant movement 

including uncertainties about late- instar movement and the 

potential effect on the durability of a seed blend strategy. 

5
Charge Question 3

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

southwestern corn borer (SWCB) are similar in many ways, ECB can 

serve as a surrogate for SWCB to address uncertainties regarding 

biology and genetics. The applicants’ efficacy data, however, 

suggest that SmartStax is somewhat less toxic to older instars of 

SWCB. Results of a larval exposure study by Monsanto showed that 

SWCB survival was higher than ECB on SmartStax. Should some 

SWCB larvae disperse as older instars, the rate of adaptation to 

SmartStax could increase in a seed blend deployment. BPPD 

concluded that simulation models should incorporate such 

information in their analyses. There is currently a lack of data on 

the propensity of SWCB larval plant-to plant movement and on how 

ECB and SWCB differ in this respect, if at all.

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Charge Question 3 (continued)

Please comment on the assumption that ECB is a suitable 

biological surrogate for SWCB and BPPD’s concerns that a 

SmartStax seed blend may affect SWCB differently than ECB.

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Charge Question 4

Corn earworm (CEW) was not considered in the applicants’ and 

EPA/ORD’s analyses for a 5% SmartStax seed blend based on the 

assumption that the insect does not overwinter in the Corn Belt 

where the blend has been proposed. BPPD is concerned, however, 

that there could be areas in the southern portion of the Corn Belt 

where CEW may be able to successfully overwinter, particularly in 

less severe winters. Such areas may need to be identified because 

they could contribute to increased selection for CEW resistance to 

Bt corn (including the proposed 5% SmartStax seed blend).

Please comment on the assumption that corn earworm does 

not successfully overwinter in the Corn Belt and poses less of 

a risk for resistance. If CEW can potentially overwinter in 

parts of the Corn Belt, should the insect be considered in the 

analysis of the proposed 5% SmartStax seed blend?

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Charge Question 5

To assess dose expression for corn rootworm (CRW) Bt toxins, the 

level of survival (adult emergence) is typically compared between 

artificially infested Bt and non-Bt corn plots. However, densitydependent mortality in non-Bt plots can potentially confound the 

comparison by reducing overall survival and adult emergence. 

(Density-dependent mortality is not expected in Bt plots due to 

effects of the toxin on young larvae.) To account for this effect, the 

dose calculation can be adjusted by removing density-dependent 

mortality from the control plots. This effectively increases the dose 

mortality estimate by raising the number of larvae present in non-Bt 

plots relative to the surviving larvae in Bt plots.

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Charge Question 5 (continued)

For the SmartStax toxins, Dow/Monsanto made a densitydependent adjustment to their dose estimates based on 

density/survival relationships developed by Onstad et al. (2006). 

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

Cry3Bb1 (99.75%), and Cry34/35Ab1/Cry3Bb1 pyramid (99.95%). 

On the other hand, BPPD has also considered separate work by 

Hibbard et al. (2010), which suggests that density-dependent 

mortality occurs at higher egg density levels than those assumed 

by Dow/Monsanto. In light of this research, BPPD recommended 

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

evaluated without a density-dependent adjustment. The nonadjusted dose profile for the SmartStax toxin is: Cry34/35Ab1

(94.2%), Cry3Bb1 (97.5%), and Cry34/35Ab1/Cry3Bb1 pyramid 

(98.2%).

Please comment on dose estimates for the SmartStax toxins 

(Cry34/35Ab1 and Cry3Bb1) targeting corn rootworm given 

the different interpretations of density- dependent mortality. 10
Charge Question 6

Northern and western corn rootworm studies have shown that 

male emergence in 5% seed blends can be variable and may be 

up to 60 times lower compared to emergence in non-Bt plots 

(Data submitted by Monsanto). This information was not 

included in any of the models used in the SmartStax seed blend 

analysis. The SAP (2009) concluded that a reduction in the 

number of males from Bt seed blends could have a negative 

impact on the effective refuge. BPPD is concerned with the 

potentially negative effects a reduction in male emergence 

might have on product durability.

Please comment on the potential effects of lowered male 

emergence of Northern and Western corn rootworm on the 

durability of the seed blend and whether this information 

should be incorporated into the risk assessment.

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Section B: Modeling of Resistance Evolution

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Charge Question 7

The durability of the proposed 5% SmartStax seed blend 

strategy was compared to the durability of a 5% structured 

refuge for lepidoptera and corn rootworm target pests. 

Monsanto developed a deterministic three locus model for 

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

CRW. Separate analyses were conducted using EPA/ORD’s two 

locus and three locus deterministic, probabilistic model to 

estimate the risk of resistance evolution with a 5% seed blend 

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

conservative assumptions, though of differing degrees, for 

parameters determined to be sensitive in the models. For 

example, more conservative initial resistance allele frequencies 

and fitness assumptions significantly lowered the time to 

resistance in EPA/ORD’s model for ECB and SWCB. In 

Monsanto’s modeling of ECB and SWCB, a greater degree of 

dispersal between compliant and non-compliant fields 

significantly affected the estimated time to resistance. 13
Charge Question 7 (continued)

Please comment on the appropriateness of the assumptions 

and inputs used for the following parameters in the 

Monsanto, Dow, and EPA/ORD models:

1. Initial resistance allele frequency for single traits 

Cry1A.105, Cry2Ab2, Cry1F, Cry34/35Ab1, and 

Cry3Bb1 for all modeled pests;

2. Survival/fitness for all modeled pests;

3. Dispersal for ECB and SWCB as modeled by Monsanto 

and EPA/ORD.

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Charge Question 8

EPA/ORD encountered challenges in the lepidopteran modeling 

with partitioning non-multiplicative interactions that occurred 

between more than two resistance genes since the mortality 

caused by each locus was not independent. With two gene 

pyramids this non-additivity can be assigned to the single two 

locus interaction, but in a three gene pyramid there are three 

possible two locus interactions. In the absence of data, this 

non-additivity was partitioned equally among the three two 

locus interactions. As more than two Bt genes are pyramided, 

this problem will have to be addressed so that resistance 

evolution in the target pests to these products can be more 

accurately simulated.

Does the Panel have any recommendations for distributing 

non- multiplicative interactions in models to evaluate 

multi- gene pyramided products? 15
Charge Question 9

Based on a review of the submitted simulation modeling, the 

preliminary conclusions are:

• For CRW, a 5% seed mixture and a 5% structured refuge 

had comparable durability in both the EPA and Dow 

models;

• For ECB, a 5% seed mixture was less durable than a 5% 

structured refuge in simulations with EPA’s model. 

However, ECB resistance did not evolve within 158 

generations in any of the simulations with the 5% seed 

mixture, similar to the level of durability predicted by 

Monsanto’s model. There was no difference in durability 

between the 5% seed mixture and the 5% structured refuge 

in Monsanto’s model. Resistance did not evolve to either 

refuge option within 100 generations (the extent to which 

the model was run); 

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Charge Question 9 (continued)

• For SWCB, a 5% seed mixture was less durable (78 

generations) than a 5% structured refuge (118 generations) 

in EPA’s model simulations. Conversely, with Monsanto’s 

model there was no difference in the prediction for 

durability between the 5% seed mixture and the 5% 

structured refuge. Resistance did not evolve to either 

refuge option within 100 generations (the limit of the 

model simulations).

Please comment on the reliability of the estimates of 

resistance evolution by each of the three models in light 

of the biological and parameter uncertainties identified 

by BPPD.

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