When it comes to Bt resistance in Western corn rootworm (CRW), we are fast moving from stage three to four in the five stages of change. According to psychologists, we start with no intention to change our behavior. We move to awareness that a problem exists, but no action is needed. The third stage involves intent to take action. The fourth is overt change, requiring commitment of time and energy. The fifth involves working to prevent relapse and consolidate gains. How quickly the industry moves to stage four and what changes are made will determine if stage five is ever reached.

Dealing With Corn Rootworm Bt Resistance"We have a tip of the iceberg situation, knowing some problem fields exist and that they are probably more expansive than we've realized," said Ken Ostlie, professor and Extension entomologist, University of Minnesota. "There has been a lot of discussion between the EPA, research entomologists and the seed industry. Discussions include what's going on with pyramiding traits and monitoring and defining the problem in the field so we catch important events."

At this point, the argument is still largely focused on "Is the problem resistance?" and "If it is resistance, what can be done about it?" To date, the problem fields Ostlie refers to are in Minnesota, Iowa, Illinois and Nebraska where Western CRW resistance to the Cry3Bb1 trait has been identified. Although confirmed sites are still limited, 16 out of 28 Nebraska crop consultants surveyed this winter reported unexpected rootworm injury to Bt hybrids in 2011. Total acres were still minor, and all took place in continuous corn. Of the 16 reporting injury, 14 cited Cry3Bb1 trait fields. Though confirmed reports are still limited, Ostlie warned against complacency.

"It's not a huge issue in terms of the number of fields or acres that have been affected, but if you look at the geographic spread, there is a real threat out there," he said.


That Bt resistant Western CRW populations would develop in Nebraska or elsewhere is no surprise. Resistant populations are simply a fact of nature. How quickly a resistant population expands to be a problem depends on how much exposure the pest has to the product and how effectively the trait controls the target pest. A minimal exposure and a highly effective dose will likely hold off resistance indefinitely. With broad exposure, but a highly effective dose, resistance will evolve slowly. However, broad exposure and a less effective dose will encourage resistance to develop quickly.

Unfortunately, Bt trait control of CRW is the latter. Unlike the highly effective dose trait that controls European corn borer (ECB), traits for CRW offer less than optimal control. "Because of the modest dose, a small change in survival can translate to a big difference in larval feeding on the root systems," explained Ostlie.

Of course, it didn't have to be this way for CRW control. When first considered for approval, mathematical models were introduced that supported the need for a 50 percent refuge. This advice was echoed when dual modes of action (pyramid) traits with refuge in the bag (RIB) were introduced. Only the large refuges would prevent resistance from developing to plants containing a single trait, suggested Bruce Tabashnik and Fred Gould, entomologists from the University of Arizona and North Carolina State University, respectively. Plants containing two or more toxins should have a 20 percent refuge, they added.

"Requiring a 50 percent refuge for these technologies would ask farmers to sacrifice a large portion of these benefits and use insecticides as a substitute for rootworm control," said Danielle Stuart, spokesperson, Monsanto. "This approach would likely decrease farmer compliance with refuges and actually lead to greater resistance risk. Annual industry surveys have consistently shown that farmers are less supportive of and less likely to comply with larger refuges that add logistical and insecticide costs, slow down planting, and reduce yield and profits."

In fact, a sizeable number of growers have refused to follow even the 20 percent refuge that was adopted. A report on refuge compliance by the Center for Science in the Public Interest (CSPI) found that 90 percent compliance in 2003 to 2005 had fallen to around 75 percent by 2008. Industry surveys have consistently suggested compliance at higher levels. The Agricultural Biotechnology Stewardship Technical Committee (ABSTC) claimed an 85 percent compliance rate in 2010 in their report to the EPA.


With confirmed reports of resistance, the seed industry adopted a more rigorous approach this past year. Actual 2011 sales data for more than 3,000 growers was reviewed by the ABSTC. The on-farm assessment showed 41 percent of inspected farmers failed to fully comply with refuge requirements in 2011. If accurate across the board, it indicates either a dramatic decline or a more realistic assessment.

Although the National Corn Growers Association celebrated the fact that a "majority" of growers are following the mandate, the CSPI report suggested that 90 percent needed to follow it if resistance was to be avoided. Even though the industry has relied on encouragement, enforcement teeth have been limited. If a customer is found to be non-compliant in two consecutive years, "...that grower may be denied access to purchase Bt products in the third year from Monsanto," said Stuart.

This failure was recognized as one of the arguments against RIB approval. It also argued for pyramid traits, whether in an RIB situation or not. However, Chris Tingle, solution development manager, Syngenta, pointed out that two traits in a field with resistance to one of them is the same as single trait protection.

Ostlie agreed that while the EPA is encouraging companies in the direction of pyramids and companies are moving that way in hopes of a higher level of protection and extending the life of traits, the effort may be counterproductive. "The irony is that all the pyramids have the Herculex RW Cry 34/35 trait in common," he said. "One concern is a dramatic increase in selection pressure for resistance to that trait as growers adopt pyramid trait hybrids as a CRW control strategy."

"We can go back and look at herbicide resistance," said Tingle. "The answer is the same. We need multiple modes of action, and we need to steer away from a single management tactic. Low dose rates are a reality across the industry. Mother Nature will evolve and overcome, just as she has with glyphosate. The question is how do we steward this in the short term to delay or hopefully prevent problems."


Dealing With Corn Rootworm Bt ResistanceFor Tingle, the sky is not falling with the problem, but is rather a "shot across the bow," and one that he feels can be ameliorated, if not resolved, by a total package approach. "Look at historical pressures in the area, correlate emergence in one year with suspected pressure for the following year," suggested Tingle. "The trait is the important solution, but growers should recognize the importance of seed treatment, and where they feel the need, they should consider a soil-applied insecticide such as Force."

When to add other tools to the crop kit, if you can and if you should, are all viable questions. "Certainly an insecticide overlay would be of value where there is a problem with resistance, but people may be tempted to go overboard on insecticide use," said Ostlie. "There may be granular application equipment shortages and even product shortages. Avoiding insecticides is the very reason transgenics captured the marketplace."

Rotating crops is always a good idea; however, when and where economics favor continuous corn, extensive rotation simply isn't a realistic option, noted Ostlie. Instead, he emphasized in-field evaluation of the crop and the pest on a field-by-field basis with field specific solutions.

"The grower can't plant and walk away," he said. "He needs to know what is going on in the field and look for potential problems. If you're getting a significant number of beetles, that could be a tip-off of a problem."

Ostlie admitted tasseling isn't the most pleasant time to scout a cornfield. Another problem is that traits tend to draw out CRW emergence, making it more difficult to decide when is the best time to be looking at fields. RIB seed mixes with the refuge dispersed throughout the field adds more complexity, with the potential for more prolonged emergence. That said, Ostlie sees it as necessary in a constantly evolving, but never disappearing, fact of life for corn producers.

"When resistance is found or suspected to one trait, switching traits or rotating to another control mechanism won't eliminate the problem," noted Ostlie. "Selection pressure continues. We just mask it. The number of resistant larvae may be reduced, and the problem won't rise to the level of causing damage. However, the presence of resistance is still there for future years."