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Corn+Soybean Digest

Slam The Door On Corn Rootworms

Richard Cortese routinely applies a granular insecticide at planting to slow down corn rootworms. But lately he's been counting on an aerial application at silking to close the door on rootworm feeding.

He applies Slam, a Sevin-based insecticide bait marketed by BASF. Blended with a concentration of cucurbitacin, a natural extract from wild Buffalo gourd bait, it lures adult rootworm beetles into a feeding frenzy. The females are killed before they can lay eggs that would overwinter and hatch into larvae to feed on corn roots the following spring.

With a 6 oz per acre application rate, Cortese saw a 90%-plus reduction in adult beetles after a 1996 application. That translated into less rootworm feeding and higher yields last year, making the $8-10/acre application cost well worth the expense.

Cortese farms corn and wheat in central Texas near Temple. His rootworm control results coincide with extensive rootworm research by USDA, Texas A&M University and a private pest management consultant.

"Aerial application of Slam provides a 90-96% reduction in adult rootworms within two weeks after application," says James Coppedge, a USDA-ARS entomologist at College Station.

His studies on 2,500 central Texas corn acres are in cooperation with Jesse Cocke, a private consultant and former Texas A&M entomologist, and the Bell County Extension Service.

Much of the research funding comes from Texas corn farmers through a half-cent- per-bushel checkoff. Results should be beneficial to growers in the Corn Belt and elsewhere where northern, western and Mexican corn rootworms hamper production.

If left uncontrolled, rootworms can drastically reduce production and cause corn to lodge, increasing harvesting losses. Cortese, who normally plants corn in late February in the mild-winter climate, has regularly applied granular insecticide at planting for early rootworm control.

"We put down a granular insecticide (Force) at a rate of 6-8 oz per 1,000 linear feet of corn, at a cost of about $8-10/acre," he reports.

The granules provide good control through the first three root nodes of growth. But by mid- to late spring, when overwintered eggs begin to hatch into root- feeding worms, only 20-30% of the insecticide's killing power remains, says Cortese.

"We're seeing benefits in coming back with Slam," he says. "We spray it at peak adult beetle emergence from the soil at silking (late May to early June), before they lay their eggs or have a chance to do any severe silk pruning damage."

Corn treated with Slam in '96 provided good control of larvae on last year's corn crop grown on the same fields.

"Corn roots were healthier and larger, with much less rootworm feeding damage," says Cocke.

Cocke, who helps Cortese monitor his rootworm program, says corn following the previous year's Slam application has about a 2.5 root rating three months after planting.

"At that rating, there is barely any rootworm damage," says Cocke. "That 2.5 rating compares to a poor root rating of 4.5 to 5, where two to three brace root nodes are destroyed on untreated fields."

Cortese, as well as the researchers, indicates that the bait doesn't harm beneficial insects needed to control the damaging ones.

"It has no impact on the number of beneficial insects in treated corn, nor is there an increase in the (spider) mite population," says Cocke.

In the research, Slam was applied in 1996 on a three-mile by four-mile study area, which included the 2,500 corn acres and 900 acres of grain sorghum, also a popular crop in the region. Only about 21 grams per acre of Sevin were actually applied in the 6 oz/acre Slam formulation. It was sprayed from nozzles with no screens or cores to provide a coverage area of one droplet per square inch of leaf.

About 700 screen-wire emergence cages were used in treated and untreated fields last year to monitor rootworm populations that emerged from the soil. Beetle plant counts were also made and fields with one beetle per two plants were treated.

"Slam has a full label for corn, but we had to obtain a temporary label for use on grain sorghum," says Cocke. "From what we've seen, there is nearly full adult rootworm control. And it should work just as effectively on all rootworm types nationwide."

One possible negative of using the bait is the potential for a very narrow window of application.

"If it's applied properly at the right time, we can receive good control," says Cortese. "But if it's applied outside a week to 10-day window (at silking), then some damage can already have been done."

Nonetheless, he believes the product could become a complete part of his overall corn program.

"We always have to measure our input costs," he says. "We have to determine whether our rootworm problems can be fully controlled enough with the granular application at planting to prevent corn from laying down.

"At least Slam gives us an alternative. It appears to be a good midseason control program that will provide additional control for the next year."

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