Consensus recommendation:

Managing Western Corn Rootworm Resistance to Bt on the Fringe

Submitted to the EPA-FIFRA Scientific Advisory Panel

Scientific Uncertainty Associated with CRW Resistance Monitoring for Bt Corn PIPs

2 December, 2013

This document is supported by the following field crops entomologists:

Chris DiFonzo, (corresponding author)

Michigan State University

288 Farm Lane, East Lansing, MI 

difonzo@msu.edu

Tracey Baute, Ontario Ministries of 

Agriculture and Food & of Rural Affairs

Ridgetown, Ontario, Canada

Ron Hammond, The Ohio State University

OARDC, Wooster, Ohio

Christian Krupke, Purdue University

West Lafayette, Indiana

Andy Michel, The Ohio State University 

OARDC, Wooster, Ohio

Art Schaafsma, University of Guelph

Ridgetown, Ontario, Canada

Elson Shields, Cornell University

Ithaca, New York

Jocelyn Smith, University of Guelph

Ridgetown, Ontario, Canada

John Tooker, Pennsylvania State University

State College, Pennsylvania

The first Bt corn targeted for rootworm control was commercially marketed in the U.S. in 

2003. The Bt toxin in this corn, Cry3Bb1, was originally sold as Yieldgard Rootworm, 

then combined with corn borer Bt toxins and/or herbicide tolerance genes in YieldGard 

Plus, VT Triple, and Genuity VT TriplePro. Beginning in 2010, Cry3Bb1 was also

available as part of a two-toxin pyramid for rootworm control (with Cry34/35Ab1)

incorporated into the SmartStax platform. The introduction of pyramids resulted in a 

reduction in refuge percentage from 20% to 5%, and a change of refuge placement from 

structured to in-the-bag. Adoption of Bt traits by producers has been high. When all of 

the currently available trait packages are considered, the Cry3Bb1 toxin is expressed in 

the majority of Bt hybrids targeting rootworms in North America.
Performance problems (i.e. fields exhibiting excessive root damage and lodging) with 

Cry3Bb1 hybrids were first reported in southern Minnesota and eastern Iowa in 2009. 

Registrants classified fields with unexpected levels of root damage and high beetle 

numbers as product performance inquiries. The number of problem fields in Iowa and 

Minnesota has increased since that time, and additional reports have since come from 

Colorado, Illinois, Kansas, Missouri, Nebraska, South Dakota, and Wisconsin. Using a 

combination of field and laboratory studies over multiple field seasons, Gassmann et al. 

showed that western corn rootworm larvae from problem fields in Iowa survived better 

on Cry3Bb1 corn than larvae from fields without unexpected damage. Corn 

entomologists in the Midwest now agree that this, and several other ongoing research 

efforts, demonstrate evidence of field-evolved resistance to Cry3Bb1 by western corn 

rootworm. Further, analysis of field populations in Iowa now suggests resistance to 

another toxin, mCry3A, and cross-resistance between this toxin and Cry3Bb1.

Suspected Resistance in the Focus Area

Agricultural production in the focus area for rootworm resistance is characterized by 

intensive corn cultivation coupled with low crop and landscape diversity. Rootworm 

resistance in this focus area appears to have arisen independently in single fields 

(point-sources). In hindsight, the original problem fields all shared the common 

characteristics of planting to multiple years of continuous corn, with the same (Cry3Bb1) 

Bt trait. Continuous corn fields typically have high rootworm pressure, which maximizes 

the opportunity for resistance evolution because selection pressure is applied to a large 

rootworm population over a large area, year-after-year. Any alleles for Bt resistance -

even if rare – are likely present in rootworm populations across the Corn belt. The 

pattern of problem fields to date suggests that resistance to Cry3Bb1 evolved 

independently, in multiple locations across multiple states (as opposed to spreading 

from a single point source), often in areas of intensive production of continuous corn 

expressing the same Bt toxin. Local spread of beetles from point sources could then 

contribute to widespread ‘neighborhood’ failures.

The registrant of Cry3Bb1, Monsanto, responded to reports of performance issues with 

a set of recommendations to manage Bt resistance, starting with crop rotation to a nonhost (typically soybeans). However, the limited crop diversity in the central Corn Belt, a 

direct need for grain for livestock, and highly favorable economics for corn production, 

make crop rotation an unpalatable option for many producers. Furthermore, resistance 

in the focus area is so widespread that crop rotation would have to be practiced on a

massive, coordinated scale to halt the spread of resistance to new areas. As an 

alternative to rotation, Monsanto recommends using a different Bt trait targeting 

rootworms, alone or as part of a pyramid, or applying a granular or liquid insecticide in 

combination with a conventional or Bt hybrid. In practice, this usually has resulted in 

growers planting the SmartStax pyramid (Cry3Bb1 + Cry 34/35Ab1), often with a soil 

insecticide. Use of an insecticide over-top a Bt hybrid is not recommended by corn 

entomologists in the Midwest; insecticides alone should provide satisfactory control in 

most cases, without the associated economic costs of the Bt trait. Insecticides may also 

mask a resistance problem. Except for crop rotation, most options listed above have 
one primary shortcoming – they continue to exert selection pressure. Resistance 

arises when selection pressure (i.e. Bt toxin) is consistently applied to a pest 

population. There is little utility in continuing to apply the same pressure when 

resistance is present, or at risk of occurring.

Suspected Resistance in the Fringe Area

Production practices differ in important ways across the corn-growing regions of North 

America. In contrast to the central and western areas of the Corn Belt, agricultural 

production in the eastern Corn Belt is characterized by higher crop and landscape 

diversity, more crop rotation, lower levels of irrigated corn, and smaller field and farm 

sizes. Thus far, rootworm resistance to Bt corn in this ‘Fringe Area’ (Indiana, Michigan, 

New York, Pennsylvania, Ohio, Ontario) is rare; only a handful of fields with unexpected 

damage have been reported to two of us, in central Michigan (DiFonzo) and in New 

York State (Shields). However, it is prudent to assume that the genetic basis for 

Cry3Bb1 resistance is present in the corn rootworm population across the Fringe region,

as in the Focus area. It follows that resistance is likely to arise in fields planted to

continuous corn with the same Bt rootworm trait. Predictably, the fields in 2013 with 

unexpected damage in Michigan and New York were continuous corn fields in areas of 

heavy livestock production, planted to either Cry3Bb1 or mCry3A hybrids for multiple 

seasons.

The current status of Bt-resistant rootworm in the Fringe is somewhat analogous to the 

movement of an invasive species from an established infested zone into a new area. 

Current examples in the Great Lakes region include emerald ash borer, which 

established and spread from Detroit, and the gypsy moth, which is still moving from east 

to west into Wisconsin and Minnesota. For both of these insects, ‘Slow-the-Spread’ 

programs involving education, monitoring, and remedial action were put in place to 

delay pest population movement. Beyond the range of the established infestation, the 

remedial action often involves eradicating isolated hot spots. In the case of rootworm 

resistance to Cry3Bb1, our goal is to use similar principles to slow any spread of 

resistance from the focus areas, and in particular to reduce the development of new 

resistant populations within the Fringe. The goal is to preserve the usefulness of 

Cry3Bb1 and other Bt traits for as long as possible.

Recent history suggests that suspected resistance to Cry3Bb1 Bt hybrids (characterized 

by high levels of rootworm feeding upon roots and/or lodging of plants), and to other Bt 

toxins like mCry3A, will be reported in additional areas of the corn belt in the near 

future. Therefore, the corn entomologists on the Fringe have taken the following actions:

* Discover isolated point sources early. Growers, extension personnel and agribusiness 

representatives servicing the fringe area are being informed of the potential for Bt 

resistance and urged to report suspect fields to companies and the authors of this 

document. Specific at-risk areas or producers (for example, dairy producers who

typically plant high levels of continuous corn) are particularly targeted for education 

focusing on detecting, reporting, and managing point sources of rootworm resistance. In 
fact, such targeted education efforts by two university entomologists directly resulted in 

the reporting of the suspect fields in Michigan and New York. 

Further, random sampling of adult beetles for resistance monitoring in the Fringe does 

not seem particularly efficient, given what we know about where and how resistance 

arises. We thus recommend that industry target high risk fields (continuous corn), farms 

(livestock and dairy production), and areas (high rates of Bt adoption, based on seed 

sales data) for their monitoring efforts. Such direct efforts are more proactive and may

detect shifts in susceptibility sooner than a random program.

* Respond and document isolated point sources. When a suspected case of resistance 

is found, every attempt is made by us to document field history, plant damage, and 

beetle population levels, and to confirm Bt expression. If possible, a sample of adult 

beetles is taken for eventual egg collection and resistance testing. For example, in

2013, eggs were sent to the University of Guelph-Ridgetown for testing in 2014. 

Adjacent corn fields are also sampled with permission. The following season, this point 

source and surrounding fields should continue to be monitored for beetle emergence 

and damage.

*Eliminate point sources. For fields with suspected resistance, our recommendation –

the sole recommendation – is rotation the next season to a non-host crop. Rotation

must be coupled with early and complete control of volunteer corn with a grass 

herbicide. Rootworm eggs that hatch in a problem field the following year exhibit 100% 

mortality if no hosts are present, making rotation the single-most effective measure to 

reduce the spread of resistance. The goal is to eliminate all offspring of resistant 

beetles, effectively wiping out the point-source of genetic material for resistance to 

spread. Note that in some areas in the Fringe (Indiana and southwest Michigan),

western corn rootworm females may lay eggs in non-host crops, resulting in some 

rootworm pressure in first-year corn. This behavioral modification is known as the 

soybean variant. The variant does not change our recommendation. Rotation to a nonhost crop is still the single-most powerful and robust method of reducing populations 

over a large area – no rootworm larvae will survive and develop without corn in the field. 

Bearing this behavior in mind, and to enhance resistance management further, we also

recommend that adjacent at-risk fields be rotated. If this cannot be done, then adjacent

fields should at least be monitored the following year, and rotated as soon as practical

as a precautionary step.

When there is absolutely no option to rotate a suspect or adjacent at-risk field out of 

corn the following season, then the field should not be planted to a hybrid expressing 

the failing toxin, or to a toxin with demonstrated cross resistance (for Cry3Bb1, this is 

mCry3A) - alone, with an insecticide, or in a pyramid. Instead, the field should be 

planted with a rootworm hybrid expressing a different Bt altogether, or to a conventional 

hybrid treated with soil insecticide. Although rootworm adults will still emerge under this 

scenario, there will be no additional selection of the population by the failing toxin. The 

suspect field should then be rotated as soon as possible. No other practice trumps the 

sole recommendation for rotation to deal with a ‘hot-spot’ on the Fringe.
The sole recommendation of rotation is more restrictive than the multiple mitigation 

strategies recommended in the focus area. However, our collective knowledge of 

rootworm biology and behavior suggests that this approach will be extremely powerful. 

Rotation also offers the advantage of eliminating exposure of the problem population to 

Bt toxins or other insecticides, which have the potential to worsen the situation. The 

assumption that eradication of point sources (i.e. targeted rotation of problem fields) will 

reduce resistance spread has not been tested in the field. But given the speed with 

which Cry3Bb1 resistance evolved and its widening distribution, common sense dictates 

taking a proactive approach in the east, i.e., targeting the sampling for resistance to

high risk areas and changing cropping practices where problems are detected. Most 

producers in the Fringe are ideally equipped and geographically situated to rotate to 

non-corn crops (typically soybeans, sometimes wheat or alfalfa) with minimal disruption 

to cropping practices; this represents an important competitive advantage compared 

with many of the areas where resistance has been reported to date.

Further Information:

 Gassman et al. 2011. Field-evolved resistance to Bt maize by western corn rootworm. PLoS ONE

6(7): e22629. doi:10.1371/journal.pone.0022629

 Gassmann. 2012. Field-evolved resistance to Bt maize by western corn rootworm: predictions from 

the laboratory and effects in the field. Journal of Invertebrate Pathology 110:287-293

 Gassman et al. 2013. Resistance of western corn rootworm to Bt corn: data from the laboratory and 

field. ESA National Meeting, Nov. 10, 2013, Austin TX.