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Corn+Soybean Digest

Some Rootworms Can Resist Insecticides, Study Shows

Field studies conducted this year by Purdue University and University of Illinois entomologists indicate that common rootworm insecticides are effective in most cases. However, no control method is fail-safe.

"The sciences have allowed us to take on the rootworm in four different manners," says John Obermeyer, Purdue Extension entomologist. "Farmers can choose from seed-applied insecticide treatments, where the seed comes with the insecticide already wrapped around it; liquid insecticides that typically are used with water as a carrier; granular insecticides that have been around for years; and now transgenic, or genetically modified, plants that have built-in resistance to rootworm feeding."

Rootworms attack corn plants from late May through July. Damage usually occurs between the plant's V4, or early whorl stage, and the R2 stage, after tassels appear. The pest feeds on and tunnels through corn roots, severely limiting the plant's ability to develop. Yield loss can be substantial.

Entomologists monitoring corn plots in high-risk areas of northern and west-central Indiana and northern and eastern Illinois found that insecticide effectiveness varied from region to region, field to field and even from this year and 2002.

Using a root-damage scale of 1-6, with 1 being little to no damage and 6 indicating severe root pruning, YieldGard RW-brand transgenic corn posted the best single plot rating – 1.05 at Lafayette, Ind. YieldGard's average root rating was 1.46 for the nine test plots in the transgenic plot trial. The Bt corn study was conducted separately from the other insecticide trials.

The poorest ratings for each insecticide category, with product and plot location, included:

  • Seed-applied – 4.20, Cruiser 1.4, Monmouth, IL.
  • Liquid soil-applied – 3.75, Regent, Lafayette, IN.
  • Granular soil-applied – 4.30, Empower, Wanatah, IN.
  • Transgenic – 2.05, YieldGard RW, Monmouth, IL.
In addition to Cruiser 1.4, Regent and Empower, Poncho 1250 and ProShield insecticides had at least one plot in the four insecticide categories with a root-damage rating of 3.50 or greater, indicating plants likely were predisposed to significant yield loss.

Along with YieldGard, six other branded products had no plots with damage ratings at 3.50 or greater. Those products were: Aztec 2.1, Capture, CounterCR, Force, Fortress 2.5 and Lorsban.

Overall, each insecticide delivery system received passing marks, Obermeyer says. The four systems also present their own advantages and disadvantages, he says.

“We have found that the seed-applied insecticides, which producers like because they are easy to use, don’t provide the efficacy that one would need in a high-risk rootworm area,” Obermeyer says. “However, if a farmer is in an area that is not under regular rootworm attack, then seed-applied insecticides seem to work great.

“The same is generally true of the liquid insecticides. Granulars are still the old standby and work quite well, and the new transgenics are setting the benchmark for rootworm control. But there are other considerations with the transgenics, from laying out a 20% refuge to whether or not the grain is marketable in a farmer's area.”

Regardless how a farmer chooses to protect a corn crop, there is no perfect control method, Obermeyer says.

“There is no 100% control, even with the transgenic Bt rootworm corn,” he says. “There is still some feeding taking place. The idea is to keep the feeding to a minimum so that there is no direct impact on yield.”

Because of excessive rainfall in key phases of the crop season, rootworm pressure was light this year. That could hold true again in 2004, but growers shouldn't bet the farm on it, Obermeyer says.

“The good news is that we did not have as many rootworm beetles in the 2003 season, thus not as much egg laying taking place,” he says. “But the bad news is that there are still a bunch out there and the ability of the rootworm to cause damage in 2004 corn is still there.”

To learn more about the insecticide field test, read "Rootworm Soil Insecticides: Choices, Considerations and Efficacy Results,” by Obermeyer and fellow Purdue entomologist Larry Bledsoe. The article appears in the Nov. 21 issue of the Purdue Pest & Crop Newsletter. The newsletter can be downloaded online at

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