CRW resistance is now a reality

Corn rootworm (CRW) resistance is no longer “what if.” With the release of research findings this summer by Aaron Gassmann, assistant professor, Department of Entomology, Iowa State University, resistance to Cry3Bb1, a leading CRW trait, is now a reality.

Problems were identified as far back as the 2009 cropping year. That year, University of Minnesota researchers identified possible trait failure.

In both cases, the researchers gathered beetles and conducted laboratory experiments to evaluate susceptibility to Cry3Bb1. While the Minnesota researchers had limited success getting sufficient beetles in the southeastern part of the state, beetles were gathered near Luverne, Minn., and evaluated.

“We found significant ability to survive,” said Ken Ostlie, professor and Extension entomologist, University of Minnesota. “The analogy is the tip of the iceberg. We are finding something, but does it represent a shifting population? Is it a wider scale problem emerging in the future, or is it something unique about the sites (soils, cultural practices, etc.) that lend themselves to performance issues?”

Although Gassmann’s announcement refers to a limited number of sites, they may provide an early warning of what could happen on a larger scale if proper stewardship isn’t in place. Proper stewardship includes planting the required refuge and following integrated pest management (IPM) principles. Refuges are important because they will delay the onset of resistance. More recently, Environmental Protection Agency (EPA) approval of reduced refuge/pyramid trait platforms was based on the idea that such platforms would be as or more effective in controlling CRW resistance development. As more problem spots in fields are identified, and each year if more are identified as CRW resistant, that theory may be tested.

Now that his findings have been released, Gassmann expects to get more calls from growers experiencing CRW resistance. “It will be interesting to see what happens as awareness grows,” he says. “It is reasonable to expect more vigilance by people looking in their fields, and there may be more problems reported.”

First reports in 2009

When problem fields were reported in the 2009 cropping season, Graham Head, strategy lead, Monsanto Global IRM, suggested that such problems had been seen consistently at a low frequency since these technologies were introduced and had not been found to be protein related, but rather were associated with certain biological or climatic conditions such as heavy insect pressure or higher moisture levels. They were he said, “...basically management issues.”

Like Gassmann, Ostlie has continued to get reports of problem fields, though unlike Iowa, the number reported in Minnesota was down in 2010 from 2009. Many of the fields that did report problems were repeat fields and included fields where the grower had stuck with the same trait and had the same problem under widely different environmental conditions. This was a sign that something other than management issues was to blame.

“This suggests that it isn’t a hybrid/environment interaction, but a beetle population component,” said Ostlie. “The plants are doing what they are supposed to do, but we are not getting the results we would hope.

Are the traits working?

One of the areas where Ostlie saw problem fields was in the southeastern part of the state. There, growers who followed IPM suggestions and changed traits or went to layered traits with pyramid platforms like SmartStax as well as growers who rotated to avoid the problem altogether, saw at least temporary relief from CRW damage.

“The dilemma is knowing what the problem is if you see a pocket of lodging in the field,” noted Ostlie. “If the grower isn’t looking at roots to see if performance is downgraded or counting the number of beetles, they are in a sense operating blind. Growers with corn following corn and using the same traits should definitely dig up roots and check beetle numbers to see if next year could lead to poor performance.”

Ostlie pointed out that when traits are working, lots of fields will have reduced beetle numbers. Even if the beetles are resistant, they may not be in enough abundance to cause performance problems. Head noted that a single corn plant can have a population of 1,000 Western CRW, adding that even 99 percent control with Cry3Bb1 technologies will allow some to survive.

When Gassmann’s research was released this summer, Head said Monsanto was collaborating with Gassmann to better understand the initial data and determine if and how they impact the company’s IPM recommendations to growers. “It appears he (Gassmann) has demonstrated a difference in survival in the lab, but it is too early to tell what the implications are for growers in the field,” said Head.  He noted that “Ongoing studies with Dr. Gassmann and others are critical to understanding the factors that lead to these performance challenges. The studies will help us further refine local agronomic recommendations and identify other crop protection products that offer growers the best options to manage corn pests and improve overall corn production.”

Get serious about IPM

Gassmann doesn’t think it is too early to encourage growers to get serious about their IPM efforts and explore more integrated approaches. Ryan Kurtz, insect resistance management specialist, Syngenta, suggested getting all the tools out on the table.

“Minimizing the threat of insect resistance requires a grower to seek and use in-seed traits, seed care and crop protection technologies in combination with conscientious farm management practices, including the appropriate refuge acres and crop rotation with soybeans where possible,” he said.

As Gassmann noted in his research paper, “A common pattern observed among problem fields in this study was the consecutive planting of the same type of Bt corn on the same acres over several seasons. Even with resistance management plans in place, sole reliance on Bt crops for management of agricultural pests will likely hasten the evolution of resistance in some cases, thereby diminishing the benefits that these crops provide.”

Unfortunately from a multi-tool, IPM standpoint, the reliance on Bt crops is getting more not less with the recent approval of layering Bt events with pyramid platforms that also permit reduced refuges. Questions have been raised as to whether these reduced refuges even with layering will speed resistance development.

Which position is correct?

Head points to theoretical and empirical studies and reviews by expert groups as supporting the EPA decision. However, in a recent letter to the editor in Nature Biotechnology, Andrei Alyokhin, School of Biology and Ecology, University of Maine, stated that “all presently available studies except a single, unpublished, industry developed model, either recommend a larger refuge size, assume the absence of cross-resistance (between traits) or both.”

We may find out all too soon which position is correct. Ostlie pointed out that there are research findings that suggest the current refuge system isn’t operating as well as hoped, even at the 20 percent level. “If you put in reduced refuge with Refuge In a Bag, there are definite concerns among some entomologists that they will dramatically shorten the life of these traits,” he said.

Gassmann explained that while pyramiding with two toxins targeting a specific pest should reduce the risk of resistance, reducing the refuge size increases the risk of resistance developing. “The question is which factor will have the greatest impact on resistance,” he said. “In general, pyramiding is still the best route, however, because reducing the refuge size increases the risk of resistance, it becomes a question of how much risk we are willing to accept.”

 

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No matter what you have living things always try to reproduce and will become restistance. The only way they won't become restistant and that's if they were dead. Remember dead weeds and bugs don't reproduce. This means we can't have any around and EPA won't allow us to have that becauce we just made it extinct!!!