Rootworm Resistance To Bt Corn Confirmed

Rootworm Resistance To Bt Corn Confirmed

Iowa study concludes rootworm in some fields has become resistant to a certain type of Bt corn. ISU entomologists explain what's happening and what corn farmers can do about it.

Over the last few summers, rootworms have feasted on the roots of Bt corn in some fields in four Midwestern states, suggesting that part of the rootworm population in those fields has become resistant to the crop's pest-fighting powers.

Iowa State University entomologist Aaron Gassmann this past summer sounded an alarm through the biotech industry when he published findings concluding that rootworms in some fields in Iowa have evolved an ability to survive corn that has the Cry3Bb1 corn rootworm trait. Similar crop damage has been seen in parts of Illinois, Minnesota and Nebraska, but researchers are still investigating whether rootworms capable of surviving the Bt toxin were the cause.

Figure 1. Western corn rootworm was the predominant species in fields with performance issues. Photo by Marlin Rice

Speaking at the recent 2011 Integrated Crop Management Conference at Iowa State University, Ken Ostilie, a University of Minnesota entomologist, said the severity of rootworm damage to Bt fields in Minnesota has eased since the problem first surfaced in 2009. Yet reports of damage have become more widespread, and he fears resistance could be spreading undetected because the damage rootworms inflict often isn't apparent. Without strong winds, wet soil or both, plants can be damaged at the roots but remain upright, concealing the problem. Ostilie says the damage he's observed in Minnesota came to light only because storms toppled corn plants with damaged roots.

Number of fields with this problem is small--but keep an eye on the situation

Figure 2. Severe corn rootworm injury can interfere with nutrient uptake and make plants unstable. Assessing root injury is important for determining larval pressure in a field. Photo by Erin Hodgson

Iowa State University Extension entomologist Erin Hodgson and ISU research entomologist Aaron Gassmann say that while the number of fields with resistance to Bt corn that has the Cry3Bb1 trait is small at this time in Iowa, it should be noted that similar damage has been reported in these other states.

"Our research at ISU and the research of other scientists elsewhere highlights the importance of incorporating integrated pest management and insect resistance management into corn production," says Gassmann. For now the rootworm resistance in Iowa appears to be isolated. But he says that could change if farmers don't quickly take action. The rootworm larvae grow into adult beetles that can fly, meaning resistant beetles could easily spread to new areas and lay their eggs. The eggs hatch and produce new rootworm larvae to chew on corn roots. Gassmann and Hodgson offer the following explanation, information and recommendations to farmers regarding the management of Bt corn.

First confirmation of resistance to Bt corn by Western Corn Rootworm

Starting in 2009, the Department of Entomology at Iowa State University received reports of severe corn rootworm injury in Bt corn in Iowa. In all cases, western corn rootworm was the predominant rootworm species in these fields (Fig. 1).

Gassmann compared survivorship of western corn rootworm collected from these problem fields versus insects from control fields (not associated with performance issues). The results were recently published in an article by Gassmann et al. (2011), Field-evolved resistance to Bt Maize by western corn rootworm. In general, survival on Cry3Bb1 corn was significantly higher for larvae from problem fields compared to control fields. Cry3Bb1 corn is sold commercially under the names YieldGard RW and YieldGard VT Triple. The ISU research also showed no cross-resistance with the Cry34/35Ab1 protein.

This research reports the first time a beetle has evolved resistance to a Bt crop in the field. But maybe these findings are not a total surprise. The Bt proteins targeting corn rootworm are not considered high dose like they are for European corn borer. Additionally, resistant strains of western corn rootworm can be produced in a lab within three generations, which translates to three summers in an Iowa corn field.

Management considerations for farmers planting these Bt corn hybrids

The number of fields with resistance to Cry3Bb1 corn is small at this time. But it should be noted that similar damage has been reported in Illinois, Minnesota , Nebraska and South Dakota. This research highlights the importance of incorporating integrated pest management (IPM) and insect resistance management (IRM) into field crop production, say Gassmann and Hodgson.

All problem fields visited by Gassmann in 2009 had a production history of at least three years of continuous corn with the Cry3Bb1 protein, and this likely contributed to the development of resistance.

The most effective way to prevent widespread rootworm resistance is to use sound IRM and IPM for Bt corn targeting corn rootworm, says Gassmann. Crop rotation is among the most effective management strategies for controlling western corn rootworm in Iowa. Also, consider rotating Bt proteins if planting continuous corn. And always comply with refuge requirements for the seed type, which can range from 5% to 20%.

"A soil-applied insecticide can be used with a non-Bt hybrid or the insecticide could supplement root protection for Bt corn in areas with known high larval pressure," notes Gassmann. "All corn fields should be evaluated annually by assessing root injury from corn rootworm and management strategies should be adjusted if injury above 0.5 nodes is observed for roots that are protected against corn rootworm.
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