Lodging is a common symptom that rootworms are at work. Since 1996, there have been seven cases of resistance to Bt crops worldwide.
Challenges to current Bt technology are an evolving problem
The western corn rootworms that are growing up in Aaron Gassmann’s lab are raising a ruckus. Among the first to be confirmed to have evolved resistance to Bt corn in the field, they are the children of the corn that no one ever wanted. Evidence that they exist threatens to cut short the Bt benefits that many farmers rely on. Scientists across the Corn Belt are urging farmers to review rootworm control strategies and take proper precautions before heading to the field this spring.
The most destructive insect pest in U.S. corn production has always been an ornery adversary. Throughout the years, rootworms have defeated a number of conventional insecticides and, in some regions, the cultural practice of crop rotation. Transgenic crops producing Bacillus thuringiensis (Bt) toxins came as welcome relief when they arrived in the corn field in 2003, yielding better grower safety, cost savings, improved crop quality and better protection of beneficial insects.
USDA’s National Agricultural Statistics Service estimates that insect-tolerant crops containing Bt traits were planted on 73% of the cotton acres and 65% of the corn acres in 2011. That trend is expected to accelerate with the commercial launch of refuge-in-a-bag corn hybrids in 2012.
"This dependence imposes tremendous selection on pest populations for resistance," says Gassmann, an entomologist at Iowa State University. "Since 1996, there have been seven confirmed cases of resistance to Bt crops worldwide. The technology is still working on the vast majority of acres, but what we’re seeing in the field signals a need for sound insect resistance management and integrated pest management."
So far, the rootworm resistance issue appears to be limited to some regional hot spots and a single Bt event (Cry3Bb1). Gassmann’s confirmation in northeast Iowa was followed by reports of significant corn rootworm pruning and stalk lodging in other states. Monsanto Company reports that state inquiries were isolated to 437 fields in 11 Corn Belt states—totaling less than 0.2% of the acres planted with the company’s rootworm traited corn hybrids.
University of Illinois Extension entomologist Mike Gray says western corn rootworm is a story of resilience, flexibility and adaptation to many different management strategies. "However, there are some patterns of behavior we can use to keep them confused—if we can change our own patterns of usage behavior," he says.
Western corn rootworm surveys in Illinois actually showed extremely low densities of adult populations in both corn and soybean fields during 2011. "In fact, in many counties, we could not find western corn rootworm adults," Gray says. The surveys were made in late July and early August in fields selected at random (five corn fields and five soybean fields per county).
Gray says successive wet springs and saturated soils at the time of larval hatch in late May and early June hampered numbers. Extensive use of Bt hybrids and spraying of tank mixes (pyrethroid/fungicide) to both corn and soybean fields also contributed to the low numbers.
Overuse a concern. Put simply, resistance is a natural biological response to repeated use of the same control technology. That’s exactly what Gassmann found while investigating farmer complaints that Bt hybrids were experiencing significant larval feeding.
"In all cases, fields experiencing severe rootworm feeding had a history of continuous corn production and the use of the same Bt trait for three to six years," Gassmann says.
The Cry3Bb1 trait is sold commercially as YieldGard VT Triple and Genuity VT Triple Pro and is one of the two insect trait components in hybrids that are branded as SmartStax.
In Gassmann’s research, the average level of rootworm feeding injury found among lodged plants in problem fields was 1.8 nodes. The Iowa State University root injury scale ranges from 0 nodes (no feeding injury) to 3 nodes (heavy feeding injury). Trait tests determined that all roots sampled from problem fields were from plants that produced Cry3Bb1.
Gassmann also found that survival of western corn rootworm on Cry3Bb1 corn in laboratory bioassays was up to three times higher for insects from problem fields than from control fields where injury was not reported.
- March 2012