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With Bt corn hybrids, rootworm-feeding scale may not still apply

Root ratings are the best way to measure feeding damage from hungry northern and western corn rootworms, but are they the best way to evaluate the performance of rootworm-resistant Bt corn?

"The root ratings system introduced back in 1971 and revised in 2000 was basically developed to compare soil insecticides," says Kevin Steffey, University of Illinois Extension crop sciences and entomology specialist. He adds that the system had been the gold standard for almost four decades, providing valuable insight when evaluating the performance of a host of soil insecticides, from carbamates and organophosphates through the newer pyrethroid products.

But does the same rating scale for rootworm damage in plots treated with soil insecticides apply in equal measure to rootworm damage to corn with transgenic rootworm traits?

The current 0-3 root rating scale developed by Iowa State University is based on nodal feeding. "A node is simply a circle of roots," Iowa State University entomologist says Marlin Rice.

A root rating of one means that one node or its equivalent has been completely eaten. A root rating of two means two nodes have been eaten, and so forth. Feeding that falls under one full node is conveyed as a percentage. For example, a root rating of 1.5 means that one and a half nodes, or the equivalent of one and a half nodes, have been eaten.

According to Larry Bledsoe, Purdue University entomologist, root mass and even node size can vary from hybrid to hybrid. As a result, a root rating of 0.5 for one Bt corn hybrid may look more like a 0.75 on another Bt corn hybrid.

"It's completely different from soil insecticide trials, where we're planting the same hybrid and sometimes even the same lot numbers," explains Bledsoe.

In addition to root mass and node size, root architecture also varies from hybrid to hybrid. "In some hybrids, the distance between nodes is longer and roots tend to branch out as they grow," says Bledsoe. "In other hybrids -- or where there's drought -- the internodal distance is shorter and roots tend to be packed together."

Root regeneration may also vary from one hybrid to the next.

"Some regenerate roots slowly, so when they are pruned, these hybrids suffer the whole season and you see the response in lost yield," notes Bledsoe. "Other hybrids regenerate roots very rapidly."

So how can growers compare one rootworm trait to another? Syngenta Seeds entomologist Von Kaster suggests results from multiple rootworm control trials are more telling than results from just one or two.

"When you look at rootworm results from multiple trials at multiple sites, you’re getting a broad view of insect control over a range of hybrids and genetics, as well as weather, soil and cultural conditions," he says. "For example, results from 2004 through 2007 Syngenta and university trials representing 57 locations show that hybrids with the Agrisure RW trait generated root ratings averaging just 0.22."

Entomologists agree that it's important to look at a variety of results when evaluating any corn rootworm control method.

"We have two seasons worth of efficacy data for Agrisure RW and a few years on top of that for YieldGard and Herculex, which have been around longer," Bledsoe says. "Our results and other data from the eastern Corn Belt demonstrate that all three traits provide good control of rootworms. They further demonstrate that all of the transgenics are definitely an improvement over the granular and liquid soil insecticides that we've been using for the last 40 years."

Root ratings are the best way to measure feeding damage from hungry northern and western corn rootworms, but are they the best way to evaluate the performance of rootworm-resistant Bt corn?

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