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.
The good news is the rootworm rascals were found to still be susceptible to the Cry34/35Ab1 trait, which is used in SmartStax along with the Cry3Bb1 trait.
The lack of cross-resistance is impor-tant, Gassmann notes. "These two toxins are acting independently," he says. Seed companies are increasingly moving away from single-trait technologies and stacks of Bt toxins.
"Pyramiding of multiple Bt toxins that target the same pest could delay the evolution of resistance to either toxin," Gassmann says.
Still, he says, growers need to understand that any technology can break with overuse. "We’ve known for a long time that western corn rootworm can increase survivorship within a few generations in lab conditions," Gassmann says. "Laboratory data indicates that all commercial [Bt] proteins are vulnerable to pest resistance."
Low dose. One of the factors that may have contributed to the resistance is that Bt corn producing Cry3Bb1 is not considered to be a high-dose event against corn rootworm.
High-dose events are generally thought to delay resistance by making the inheritance of resistance more recessive. Gassmann says the ability of heterozygous-resistant western corn rootworm to survive on Bt may have diminished the ability of refuges to delay resistance.
In the case of European corn borer, Bt traits in the marketplace are considered high-dose—delivering 25 times the toxin needed to kill the insect target. European corn borer is also highly mobile, mating in grassy edges. Western corn rootworm is more sedentary and travels little.
"Since the day these products were commercialized, we’ve had survivors," Gassmann says.
Although not all entomologists agree, Gassmann believes integrating refuges into the bag is a smart move. Not only does it force growers to observe the refuge, but movement studies suggest western corn rootworms may have difficulty finding each other within the structured refuge.
Research will continue, but meanwhile growers should be very cautious in their hybrid choice in light of the developments in Iowa and northwestern Illinois, Gray says. "Many producers have utilized a single-tactic approach for too many years, and now unfortunate consequences are beginning to emerge."
If you encountered rootworm protection issues in Bt corn in 2011, Gray recommends implementing the following alternatives for 2012:
- Rotate to soybeans or another non-host crop.
- Select a non-Bt hybrid and apply a soil insecticide at planting.
- Use a different type of Bt hybrid that expresses a different corn rootworm Cry protein than one that performed poorly in your fields during the last growing season.
- Use a pyramided Bt hybrid that expresses multiple Cry proteins targeted against corn rootworm.
- Implement a long-term integrated approach to corn rootworm management on your operation.
Dig to Determine
Sometimes you just have to start digging to find out what’s really going on in your corn fields. Dave Shenaut, Monsanto technology development representative, thinks that every corn grower should be digging in his fields around pollination time.
"I realize it’s hot and uncomfortable out there during that stage, but you’ll be amazed how much you can learn about your crop," Shenaut says.
Keep in mind that the rootworm must feed to ingest the Bt toxin. So you should see some evidence of scarring in all fields.
Non-Bt fields are most susceptible to larval damage. Continuous Bt corn fields with previous damage are good candidates for scouting too. Sample corn plants in different areas of the field to estimate infestation levels.
A power washer might be needed to wash off roots for a good clean look.