Nicholas P. Storer

DOW AGROSCIENCES APPROACHES TO 

RESISTANCE MANAGEMENT OF CORN 

ROOTWORM-PROTECTION TRAITS

US EPA Scientific Advisory Panel 

on Corn Rootworm Resistance 

Monitoring for Bt corn

4 – 6 December 2013
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SmartStax® Anatomy

MON 89034 x TC1507 x MON 88017 x DAS-59122-7

Cry1A.105

Cry2Ab2

Broad-spectrum Lep

Control

Dual Effective Dose 

(high dose) for corn 

borers

Cry1F

PAT

Broad-spectrum Lep

Control

High dose for corn 

borers

Glufosinateammonium 

tolerance

Cry3Bb1

EPSPS

Corn rootworm Control

Less than high dose

Glyphosatetolerance

Cry34/35Ab1

PAT

Corn rootworm Control

Less than high dose

Glufosinateammonium tolerance

+ =

*SmartStax®, the SmartStax Logo, Roundup Ready®, Genuity®, VT Triple PRO®, YieldGard® ,and YieldGard® are trademarks of Monsanto 

Technology LLC. SmartStax® multi-event technology developed by Dow AgroSciences and Monsanto. 

Refuge Advanced® and Herculex® and the Dow Diamond are registered trademarks of The Dow Chemical Company (“Dow”) or an affiliated 

company of Dow. Herculex Insect Protection technology by Dow AgroSciences and Pioneer Hi-Bred International, Inc. 
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● Bt corn is a powerful and convenient technology for managing 

important pests and protecting yield

● Pyramided products offer improved performance and robust durability

● Simplified refuge through seed blends promotes resistance 

management

Extended Durability; Simplified Refuge Management

Reduced 

Refuge

Blended 

Refuge
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Performance of corn rootworm protection traits over time 

Heavy rootworm pressure in areas with and without prior 

unexpected damage to Cry3Bb1 corn

0.0

0.5

1.0

1.5

2.0

2.5

3.0

ISU Node Injury Scale (0

-3)

Iso-genetic null hybrid Cry3Bb1 hybrid

Cry34/35Ab1 hybrid (Herculex®) SmartStax® hybrid 

2007-11 

locations without reports of 

unexpected damage to 

2012-13 

locations with prior Cry3Bb1 

or mCry3a unexpected 

damage

2012-13 

locations without reports of 

unexpected damage to 

Cry3Bb1 corn
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Practical Definitions of Resistance

5

● Laboratory resistance: a genetically-mediated reduction in sensitivity 

of a target pest to Bt toxins, either in artificial diet or plant-based 

bioassays

● Field resistance: a genetically-mediated increase in the ability of a 

target pest to survive on Bt corn under field conditions

● Population resistance: occurs when a large portion of a pest 

population is field-resistant and causes the Bt corn to fail to confer 

economic control of the population

 A population is a priori believed to contain individuals with resistance 

alleles. Identification of such individuals does not constitute identification of 

population resistance
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Response to Selection Pressure ≠ Population Resistance 

Insects that were killed on Bt corn 

were on average more sensitive to the 

Bt protein than those that survived Proportion of population

Bt tolerance

Generation 0

Proportion of 

population

Bt tolerance

Generation 1

Depending on the heritability of survival 

on Bt corn, the mean tolerance of the 

surviving population is greater than that 

of the original population

 We assume this happens to corn rootworms in every field of Bt corn

 We need to distinguish field-relevant population resistance from 

simple response to directional selection
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Key considerations for field-based monitoring

● Effective: High probability of detecting changes in populations that 

are expected to lead to changes in pest management (population 

resistance)

● Efficient: Avoid spending time and resources on enquiries that are not 

related to resistance

● Relevant: Detect emergence of field-relevant population resistance, 

not simply selection pressure

● Adaptable: Continued experience with SmartStax and refuge-in-thebag products should inform on-going development of monitoring 

schemes

● Transparent: Key stakeholders kept informed where resistance may 

cause reduced performance

7
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Field resistance monitoring process

8

Performance 

Inquiry

• Trigger

• Immediate 

response

• Investigative 

Steps

Unexpected 

Damage

• Trigger

• Immediate 

response

• Investigative 

Steps

Suspected 

Resistance

• Trigger

• Remedial 

action

• Investigative 

Steps

Confirmed 

Resistance

• Trigger

• Remedial 

action
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Customer Performance Inquiry

Customer, crop consultant, extension agent 

etc contacts Dow rep with a concern about 

the performance of Bt corn product

Visit relevant field(s) to assess damage and 

investigate causes

Confirm presence of Bt trait(s) and planting 

layout (ELISA tests, planting records)

Determine if crop damage meets 

“Unexpected Damage” definition

Recommend rootworm best management 

practices
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Unexpected Damage

1. Is the field planted to Dow’s Herculex® or SmartStax® product? 

(planting records, ELISA strip tests)

2. Is lodging > 25% of plants in an area of the field > 1 acre?

3. Is the damage caused by corn rootworm larval feeding?

4. Is the nodal injury score > ≈ 0.5 – ≈0.75* on at least six of ten roots 

dug from arbitrarily chosen lodged plants in the affected area?

5. For blended refuge (RIB) products, are the damaged plants SSX 

(not refuge plants) and are they non-adjacent to refuge plants?

“Unexpected Damage ” if is YES to all of these

Lower end of the range is used in conditions that lead to lower expected damage; higher end is used 

in conditions that lead to higher expected feeding damage.
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Investigations into Unexpected Damage 

Sample 5 plants from each of at least 3 additional areas in 

the field to determine extent of damage

Collect healthy plant tissue (if available) for quantitative 

ELISA

Collect adult corn rootworm beetles (if available) for 

bioassay

Bioassay progeny (after diapause) using standard 

laboratory techniques

Recommend grower implements additional rootworm 

management the following year

Review CRW management practices the following year
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Suspected Resistance

Suspected Resistance is defined as 

● Unexpected Damage (if tissue available for testing, confirmed to 

express CRW proteins at the normal level), 

and

● Insects show significantly different response from 

baseline/controls in bioassay; 
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Response to Suspected Resistance

• Emphasis on crop rotation

• Use of an additional rootworm control tool in combination 

with or in place of SmartStax (e.g. soil insecticide, 

chemigation, seed-applied insecticide, adulticide, 

alternative CRW trait)

• Follow up in the following year to assess the success of the 

remedial actions in terms of insect susceptibility and 

product performance

Recommend alternative corn rootworm 

management

• Change in response is heritable and provides selective 

advantage

• Change in response is sufficiently high to cause economic 

root injury

Investigations to confirm resistance
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Confirmed Resistance

● Confirmed Resistance is defined as 

 Heritable significant change in response to Bt proteins in 

bioassay leading to increased survival to adult

and

 Change in response is sufficiently high to cause economic 

root injury

and

 Population persists in the field causing reduced product 

performance

● Affected area determined by geographic extent of nearby 

Unexpected Damage cases
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Responses to Confirmed Resistance

Inform stakeholders in affected area, including 

growers, extension, crop consultants etc

Follow best IPM practice recommendations to 

manage corn rootworms in the affected area

Continue to monitor in the area for changes in 

resistance levels

Transition from proactive IRM to 

IPM in the presence of resistance
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Process Overview

Performance 

Inquiry

• Recommend CRW 

best management 

practices

• Investigate extent 

of lodging, root 

injury

• Damage is greater 

than expected?

yes

Unexpected 

Damage

• Recommend 

additional CRW 

management tools

• Quantitative ELISA 

on plant tissue, 

Bioassay rootworm 

collection

• Population is less 

susceptible than 

normal?

yes

Suspected 

Resistance

• Recommend 

alternative CRW 

management tools

• Characterize 

rootworm survival 

and damage to 

SmartStax 

• Reduced 

susceptibility is 

heritable and 

damaging?

yes

Confirmed 

Resistance

• Implement IPM 

program for the 

resistant 

population

• On-going research 

to understand 

resistance genetics 

and biology, 

monitoring

• Monitor for 

changes in 

resistance levels

Investigation Complete

On-going communication with the grower about performance of 

SmartStax and rootworm 

no

nono