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Corn+Soybean Digest

Digging for the Problem: Scout for Corn Rootworm

WEST LAFAYETTE, Ind. -- To prevent yield loss at harvest time, producers should begin scouting for corn rootworms to determine if an insecticide needs to be applied, says a Purdue University entomologist.

Rootworms feed on nodal and brace roots in the soil, says integrated pest management supervisor John Obermeyer. The damage may lead to lodging during summer storms and high winds.

"Yields will be cut dramatically if lodging occurs just before or during pollination," Obermeyer says. "Soil pathogens infect damaged plants through the corn rootworm feeding wounds. This may lead to stalk rot during dry down."

Obermeyer suggests checking a seven-inch cube of root mass and surrounding soil. He says to dig up the plant's root system with a shovel and place it on a dark surface, such as a black plastic garbage bag. Then, break up the clods and examine the plant and soil by hand.

"Producers need to look for one-fourth- to one-half-inch long, slender, creamy-white larvae with brownish-black heads and tails," Obermeyer says. "They should tear away leaf collars to see if rootworms are in close proximity to nodal roots and if there is root scarring and pruning. Rootworms also may have their tails sticking out of the roots."

The process should be repeated several times in different areas of the field, Obermeyer says. If averages of two or more larvae per plant are found, apply an insecticide, he says.

Obermeyer says insecticides applied after planting must be directed at the base of the plant. It is also important to incorporate the insecticide by cultivating the soil near the plants, he says.

"Throwing soil up around plants will promote the establishment of brace roots," he says. "A good brace root system will help prevent plant lodging and reduce losses due to rootworm feeding."

For no-till fields, Obermeyer says irrigation or rainfall will be the only way the soil insecticide works its way down into the plant's roots. Insecticides applied at the time of planting may be diminished by sunlight and rainfall, making them less useful, he says.

"The sun's ultraviolet rays will degrade insecticide exposed on the soil surface," Obermeyer says. "T-band applications may leave a considerable percentage of product exposed if there wasn't any incorporation."

Insecticides are pushed deeper into the soils if there are high levels of moisture. Since insecticides vary in solubility, too much moisture may move chemicals further into the soil and off target, he says.

Related Information: Managing Corn Rootworm

Click here to download a information about Corn Rootworm in pdf format

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