Get ready to spray for corn rootworm

Corn rootworms can be controlled with Bt corn. But with the 20 percent non-Bt refuge requirement and many corn lines still without the bug-resistance gene, growers may need extra steps to prevent disastrous damage.

Eleven years of studying Western Corn Rootworm (WCR) behavior are helping Texas A&M University entomologists help growers determine if and when additional spraying is needed. Their data show that the root- and leaf-hungry pests nearly always reach 50 percent adult worm emergence when a region’s accumulated degree-days reach between 1,341 and 1,364.

In the Texas Panhandle, where the majority of the studies took place, that normally occurs on July 24, says Jerry Michels, entomologist, Texas Agricultural Experiment Station in Amarillo.

It can range from July 13 to Aug. 14. That information can help growers determine if spraying is needed, or if at-plant spraying is needed the following year in continuous corn.

“Our model is temperature driven,” says Michels, “assuming that the threshold of WCR development in the soil is 50-degrees F. The model correlates closely with actual field counts of adult WCR throughout the year.”

In 2006, a dry, hot year, adult emergence began about July 1, the earliest of the 11-year study. Emergence reached 20 percent by July 8 and topped 50 percent by July 15.

Carl Patrick, Texas A&M Extension entomologist in Amarillo, says WCR beetles are ¼-in. long and yellow with black stripes. There is a single generation annually. Larvae feed on corn roots and adults feed on leaves and silk and lay eggs in the soil.

Eggs are laid in the upper 2 to 8 inches of soil in mid- to late-summer just after silk. Egg lay continues for several weeks. Eggs overwinter in soil and begin to develop the following spring when soil temperatures reach 52 degrees.

“In the Panhandle, eggs begin to hatch in mid-May,” says Patrick. “Larvae are about 1/8 inch long. They grow through three larval stages to almost ½ inch long.”

The cream to yellowish-colored larvae begin feeding on outer parts of roots. Feeding lasts 30 to 40 days. Damage ranges from minor to complete loss of the root system.

Plants may be stunted or die. Goose necked plants may develop, complicate harvest and increase losses. If silks are continually pruned to within ½ inch of the shuck, pollination may be affected.

“If there were one or more beetles per plant, an at-plant insecticide, seed treatment or active transgenic corn hybrid may be warranted the following season,” says Patrick, noting that the economic threshold is eight to 20 beetles per plant or five or more directly on the ear.

“Beetle control should be considered if silks are being eaten faster than they are emerging from husks during pollination.

Beetle control should also be considered to reduce preplant egg lay that will produce the next year’s larvae. “Area wide treatment of all corn fields with feeding stimulants containing low concentrations of insecticides have been successful in reducing subsequent larval populations below the economic threshold,” says Patrick. That practice is more common in the Midwest.

He says about 75 percent or more of the corn planted these days is Bt corn. But a Bt operation includes the 20 percent refuge area, which must include a strict insecticide regimen.

“Mixing non-Bt seed with Bt rootworm protected seed is not permitted,” he says. “For rootworm-resistant corn, no other field can be located between transgenic corn and the conventional corn refuge.”

Seed treatments and granular insecticides can provide adequate corn rootworm control. Clothianidin and Thiamethoxam are seed treatment chemicals. At-planting granular insecticides include: Carbofuran, Chlorpyrifos, Tebupirimfos plus cyfluthrin, Tefluthrin and Terbufos.

Suggested insecticides for controlling adult corn rootworm beetles include: Carbaryl, Malathion and Methyl parathion. To learn which insecticides may work best in an area, Patrick advises growers to contact their regional Extension entomologist.

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