Comments to US EPA 

Scientific Advisory Panel 

Agricultural Biotechnology 

Stewardship Technical 

Committee

Corn Rootworm Resistance 

Monitoring for Bt Corn PlantIncorporated Protectants (PIPs)

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Who is ABSTC?

 Ag-Biotech Companies with US Bt crop registrations, or who 

are major trait providers

– Bayer Crop Science

– Dow AgroSciences

– Monsanto

– DuPont Pioneer

– Syngenta Seeds

 Develops research and educational programs that promote 

responsible use of Bt corn products 

 Works with EPA and other key stakeholders to implement 

effective IRM programs

 Committed to applying the best science available to issues that 

enable farmers, consumers, and the environment to benefit 

from the products of agricultural biotechnology
1) Annual Corn Rootworm Sampling to 

Assess Bt Susceptibility

 ABSTC member companies collectively sample corn 

rootworm populations to monitor for resistance

 Annual population sampling and screening is unlikely 

to find populations with emerging resistance

– Rootworm resistance to Bt corn traits is highly localized

– Field-specific cropping history

 Continuous corn

 Continuous use of single mode of action

– Field-specific management practices

– Generally limited corn rootworm adult dispersal out of corn 

fields

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1) Annual Corn Rootworm Sampling to 

Assess Bt Susceptibility

 Identification of resistant individuals is not 

realistic

– Lack of high dose allows susceptible insects to survive 

in field

– Survivors in any Bt corn field are on average likely to 

be less sensitive to the Bt protein than those that died

– Resistance may involve complex genetics and 

mechanisms

– Molecular markers for resistance alleles are not 

available

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1) Annual Corn Rootworm Sampling to 

Assess Bt Susceptibility

 Sentinel plots of non-Bt corn can provide 

reference collections representing four region- wide corn rootworm biotypes for comparing with 

populations of interest collected in Bt corn fields 

showing unexpected root damage

 ABSTC recommends that annual sampling and 

screening program be discontinued

 ABSTC recommends establishment of sentinel 

plot program to provide region-wide reference 

collections

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2) Corn Rootworm Sampling in 

Response to Damaged Bt Fields

 ABSTC member companies individually 

investigate reports of unexpected damage to 

their Bt corn products

 Investigations are designed to

– Confirm root injury is unexpected

– Confirm corn rootworm was responsible for the 

damage

– Confirm that the damaged corn was expressing the Bt 

protein

– Collect rootworm adults for future bioassays, if 

available

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2.i) Unexpected Damage Thresholds

 Bt corn products for corn rootworm protection differ 

in their protection of corn roots

 Bt corn root injury is related to overall pest 

population pressure and does not always indicate 

presence of resistance

 Even so, ABSTC recommends the following rules of 

thumb for unexpected damage thresholds for 

existing products

 Single MOA Products: NIS ≥ 1.0 on more than 50% of corn 

roots assessed 

 Dual MOA Products: NIS ≥ 0.75 on more than 50% of corn 

roots assessed

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2.ii) Root sampling goals

1. Characterize damage in the affected areas 

of a field

2. Define the extent of unexpected damage 

across the field

 ABSTC recommends appropriate sampling 

that addresses the goals above

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2.iii) Adult corn rootworm 

sampling

 Corn rootworms move within a field to forage 

for fresh silks and pollen

 ABSTC recommends rootworm adult 

samples be collected from wherever most 

adults are found in the field

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2.iv) Sentinel plots to assess 

resistance allele frequency in the area

 It is not realistic to attempt to measure 

resistance allele frequencies, especially not with 

sufficient resolution to measure gradients

 Sentinel plots would be difficult to implement 

within a rootworm resistance management 

program

 ABSTC recommends registrant revisits farms 

with unexpected damage the following season to 

assess rootworm management actions and 

collect rootworm adults if available.

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3) Diagnostic Assays

 ABSTC member companies individually conduct assays 

to monitor for resistance to their products

 It is not possible to measure resistance allele frequencies 

accurately or precisely 

 Mean population fitness, such as LC50 or survival and 

development rates under field-relevant exposure 

conditions, are most relevant for non-high-dose products

 On-plant and diet bioassays both have value in 

understanding rootworm response to Bt proteins

 ABSTC is working towards a standardized diet for 

bioassays across proteins as a desired goal

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4) Defining resistance

 Resistance is quantitative and there is not a 

biologically meaningful threshold of sensitivity that 

represents resistance

 Because current corn rootworm-protected Bt corn 

events are not “high dose” small changes in 

population response can be biologically meaningful 

but may not have pest-management implications

 ABSTC recommends a focus on triggers for 

additional rootworm management actions rather than 

on definitions of resistance

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5) Determining Remedial Action Areas

 ABSTC member companies do not share customer- specific data

 Measuring resistance allele frequency gradients is 

likely to be intractable

 Corn rootworm resistance is relevant if changes in 

rootworm management are warranted

 ABSTC recommends that remedial action zones be 

established based on patterns of field performance

 ABSTC recommends focusing on fields that have a 

similar crop and trait history to those where 

resistance is known to have developed

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6) Containment/mitigation of 

resistance

 Goals of remedial action programs include

– Reduce economic impact of resistance in affected fields and 

farms

– Reduce rate of spread of resistance

 Population elimination is unrealistic

 Remediation measures should focus on sound 

Integrated Pest Management principles

 ABSTC has developed harmonized “Best 

Management Practices” for corn rootworm that are 

intended to be effective at delaying and reducing the 

impact of resistance

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CRW Best Management Practices 

Guidelines (2013)

• Lead recommendation: Regardless of crop or trait history, CRW pressure, or CRW 

resistance, rotation to a non-CRW host crop (e.g. soybeans) is the preferred 

recommendation

1. Break the CRW cycle

1. Rotate to non-host crop

2. Trait rotation (continuous corn) – Rotate different dual (pyramided) or single MOA Bt trait products that provide effective below-ground protection against CRW

• Do not plant more than 3 consecutive years to the same single Bt-traited corn product

2. Manage the insect population

1. Scout and assess potential CRW pressure levels

2. Manage high CRW populations

• More frequent crop rotation in areas with high CRW pressure (suggest no more than 3 years of 

continuous corn planting)

• Use dual MOA CRW-traited products

• Adult beetle sprays

3. Protect corn plants from root injury

1. Prescribed use of soil-applied insecticides/high rate seed treatments to manage larval 

injury

Note: All guidelines refer to management of individual fields. MOA = mode-of-action

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CRW management decision tree based 

on following defined criteria:

 CRW pressure

– CRW pressure expected based on cropping 

history and previous adult population monitoring 

or management

 CRW trait resistance concern 

– Traited-field categorized as “Unexpected 

Damage” (UXD)

– Field history along with observed lodged corn and 

root damage ratings suggesting UXD

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Lead CRW Management Recommendation: Crop Rotation*

CRW Pressure LOW

Corn Rootworm Trait Resistance Concern?

Scout fields to determine CRW pressure**

CRW Pressure HIGH Crop Rotation? Yes Non-host 

crop

N

o

• CRW traits

− Dual MOA

− Single MOA

• Soil insecticide

• Seed treatment

N

o

Y

e

s

• Dual MOA CRW trait + soil 

insecticide

• Different single MOA CRW 

trait + soil insecticide

• Non-Bt + soil insecticide

• Dual MOA CRW trait

• Single MOA CRW trait (avoid 

planting same single trait 

MOA > 3 yrs)

• Non-Bt + soil insecticide

*Use appropriate rotational practices in your region. Manage the CRW rotational variant appropriately.

**Scouting CRW adults in July & August can help manage CRW adult populations and/or predict CRW pressure for the 

following year, which can assist in determining management options. Other indicators of pressure include observed 

root damage and or lodged corn during the season or at harvest.
Summary

 Corn rootworm management has presented huge challenges for 60+ years

 Bt corn continues to be the most reliable and effective rootworm management 

tool

 Proactive resistance management is critical to extending the durability of Bt 

corn

– Plant required refuges (use products with integrated refuge seed)

– Avoid multiple years of continuous corn with the same single mode of action trait

 When resistance begins to reduce the performance of Bt corn, rootworm 

populations can continue to be managed with a combination of traits and other 

management practices

 Crop rotation is generally a highly effective pest management and resistance 

management tactic

 Remediation measures should be commensurate with the level and extent of 

resistance and should be flexible to recognize the practical realities of 

agriculture

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