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Tracking The Resistance Mystery

While Indiana and Illinois researchers puzzle over the development of Western corn rootworm hatches in first-year cornfields where soybean/corn rotations are practiced (see "Population explosion," January issue, page 40), Nebraska researchers are tracking the apparent resistance of adult rootworm beetles to repeated doses of methyl parathion on nonrotated irrigated corn ground. In 1994, corn growers in parts of York and Phelps counties noticed that beetles were surviving as many as three applications of the aerially sprayed insecticide at increased rates. Previously, the rootworm had been efficiently controlled with one application.

"These are the classic signs of an insect resistance problem," says Lance Meinke, University of Nebraska-Lincoln entomologist. The good news is that it appears the resistance hasn't spread. "As far as we know, it hasn't moved. Last summer we bioassayed beetles from across the Corn Belt and only found resistant beetles in several areas of Nebraska," he adds.

Rootworm studies. After reports of trouble in 1994, Nebraska researchers initiated field surveys and found that beetles were much more tolerant to carbaryl and methyl parathion in some areas than they were in others. They launched a formal study in 1995 to gauge the susceptibility of rootworm beetles to methyl parathion, carbaryl and bifenthrin - the three insecticide classes used for beetle control.

Of their 16 collection sites the beetle populations of the two problem areas in Phelps and York counties proved to be 10 to 17 times more resistant to methyl parathion than populations elsewhere in the state. They were 8.3 to 9.3 times more resistant to carbaryl, and all populations were susceptible to bifenthrin.

Last summer, greater numbers of the more resistant beetles began showing up in the York population. "Some of the York populations were harder to kill this year," says Meinke. "We're not sure if this is an anomaly or a progression of the resistance development."

Efficacy of soil insecticides. A crucial question is whether rootworm larvae, the damaging immature stage of the insect, will develop resistance, as the adult beetles did. Most commercial soil insecticides are organophosphates, the same class of chemicals as methyl parathion, to which beetles are developing resistance. To test the efficacy of organophosphates against rootworm, the researchers in 1996 and 1997 established trials in fields with known resistance problems to test nine commercially available soil insecticides.

In mid- to late July, when larvae had finished their damaging feeding stage, plants were inspected. Of the nine products used in Phelps County trials, Counter, Lorsban and Aztec applied at planting time performed the best, says Robert Wright, University of Nebraska entomologist, cautioning that more extensive trials are needed. York County results were inconclusive because of low larval populations in 1997.

Beetle resistance. The beetle insecticide resistance problem may have its origin in a similar corn rootworm scare 40 years ago. At that time, rootworm larvae in south-central Nebraska developed resistance to organochlorine insecticides and eventually spread throughout the Corn Belt. During that scare, Phelps County corn growers controlled adult rootworm beetles almost exclusively through aerial applications of insecticide. For the past decade, many growers have continuously used the same methyl parathion product to which beetles are now resistant.

This exerts intense genetic selection pressure on the beetle population, Meinke warns. The few beetles resistant to the insecticide have an evolutionary advantage: More of their offspring survive, and soon, resistant types become predominant in the population.

Meinke cautions growers in affected areas to alter their management practices to guard against damage due to resistance problems. Rotating corn with other crops is an effective defense. He urges farmers to experiment with other beetle control insecticides or to switch to a larval control program using soil insecticides. "The more insecticides used, the more pressure that adds to the rootworm population to develop resistance," he says.

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