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Corn+Soybean Digest

Corn Rootworms Are Fighting Back

Insecticide resistance. Those two words can frighten growers. The insecticides they use to ensure a healthy, profitable crop stop working.

It happened in the late 1950s with western corn rootworms, and, in central Nebraska, it's happening again with the same pests.

Fighting back this time, though, are the adult beetles, not the larvae.

University of Nebraska entomologists have confirmed that rootworm beetles in Phelps and York counties are showing resistance to some organophosphate and carbamate insecticides. Most growers in those counties switched to aerial applications in the '50s, when rootworm larvae developed resistance to soil-applied organochlorine insecticides.

Rumors of resistance started in '94, and a formal study was launched in '95. In the past three years, much of the Corn Belt has been tested for resistant beetles, but it appears the problem is isolated in the two Nebraska counties and not spreading.

"If it's moving, it's moving slowly," says Lance Meinke, University of Nebraska entomologist.

Meinke and colleagues Blair Siegfried and Robert Wright have been studying the phenomenon in the lab and with on-farm trials.

They specifically looked at methyl parathion, carbaryl and bifenthrin -- insecticides commonly used for beetle control.

In the two counties, beetles were 10 to 17 times more resistant to methyl parathion and 8.3 to 9.3 times more resistant to carbaryl than beetles in other parts of the state. All populations were relatively susceptible to bifenthrin.

But the crucial question remains -- will the rootworm larvae, the insect's damaging stage, develop the same resistance as the beetles?

Most commercial soil insecticides targeted at stopping the larvae are organophosphates, the same class of chemicals as methyl parathion. Meinke and Wright tested soil insecticides in Phelps and York counties in 1996 and '97.

The on-farm trials were conducted in fields that were known to have resistant beetle populations. The researchers applied nine commercially available soil insecticides at planting and examined corn roots in mid- to late July for damage.

Not all insecticides performed the same. Most provided good control, but one showed potential resistance problems.

"One of the key points we're trying to make is that there's not an across-the-board relationship with organophosphates," says Meinke. "The insecticides vary enough within that class that you almost have to look at each one individually to see if it's going to have a resistance problem or not."

At this point, Meinke doubts that growers need to be overly concerned about resistant larvae.

"I don't think current larval susceptibility levels will significantly affect the level of control provided by most products in the field, at least not yet," he says.

Meinke and his colleagues recommend crop rotation as the No. 1 line of defense against corn rootworms. If growers in the affected area want to use a soil insecticide when corn follows corn, they should evaluate several products to see what works on their farm, say the scientists.

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