By KEVIN WALKER
Michigan Correspondent
 
AMES, Iowa — A group of researchers at Iowa State University has published a study that shows corn rootworm resistance in an important variety of genetically modified corn.

The corn that was developed contains a single Bacillus thuringiensis (Bt) protein called Cry3Bb1, which is supposed to kill the corn rootworm. Cry3Bb1 is used in Monsanto’s YieldGard, VT Triple and Genuity VT Triple PRO corn products. There are other Bt corn products that are unaffected by this development.

The research, led by scientist Aaron Gassman, was published in the July 29 issue of PLoS ONE, an online journal. Gassman and his co-authors found out about serious injury to corn in some fields in Iowa and ended up undertaking a study to find the cause.

They designed research involving four problem fields and four control fields. They fed larval offspring of corn rootworm taken from the fields, corn leaf tissue from different varieties, including Cry3Bb1.

They found that larvae descended from corn rootworms taken from the problem fields were surviving the ingestion of Cry3Bb1 – that is, they were resistant to the Bt corn. This was true in particular when the farmer had planted this variety of corn at least three years in a row.

“Survival of Western corn rootworm (WCR) in Cry3Bb1 maize in laboratory bioassays was significantly higher for insects from problem fields where farmers reported severe root injury to Cry3Bb1 maize, than from control fields where such injury was not reported,” the study said.

“Furthermore, there was a significant correlation between the number of years Cry3Bb1 maize had been grown in a field and survival of Western corn rootworm on Cry3Bb1 maize. These data indicate that the Western corn rootworm is evolving resistance to Cry3Bb1 maize in some populations in Iowa, USA.”
The authors wrote this is the first case of the WCR, or any species of beetle, evolving resistance to a Bt toxin in the field. Insects collected from problem fields did not display resistance to another Bt protein, Cry34/35Ab1. They also wrote there’s no evidence of cross-resistance between the Bt toxins in question.

Monsanto, which invented the trait, did not provide anyone to interview for this story, but offered a statement on its website reacting to the ISU study, which it helped to fund. It states the products containing the event in question still work on 99 percent of corn rootworms, but that the company still takes Gassman’s findings seriously.

It adds it needs to understand the findings better before it decides if it will change its integrated pest management (IPM) recommendations to growers. “It appears (Gassman) has demonstrated a difference in survival in the lab, but it is too early to tell whether there are implications for growers in the field,” the statement said.

It goes on to say since the products with the trait were launched in 2003, there have been persistent but low incidences of “performance issues” with those products, but that those haven’t increased in number over time. More studies, possibly different agronomic recommendations, plus new products will help to defeat the corn rootworm, the statement adds.

Kraig Roozeboom, a crop production and cropping systems specialist at Kansas State University, was one of a number of experts reacting to news of the study.

“There are differences, of course, but I think this is similar to many of the herbicide resistant weeds that we are dealing with on a broad scale now,” he said. “They first showed up ‘in a few fields,’ but are becoming a widespread issue.”

John Obermeyer, an entomologist and IPM specialist at Purdue University, said Monsanto’s reaction to the development is “as expected.” He said it’s a truism to say that resistance is showing up only in a few fields.

“Our take on it is obviously it’s a concern that it’s showing up in fields in Iowa and now, in Illinois,” Obermeyer said. “(Monsanto) is going to call it a tolerance rather than a resistance, but Gassman has pretty well proven that it is a resistance. We fully anticipate that the beetles will spread and that the resistance will spread.”

He said he and his colleagues are most anxious to see what happens after farmers switch to other corn varieties. “We hope then that the rootworm population will be suppressed,” he said.