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Hodgson: Corn rootworm a ‘very adaptable pest’

By Staff | Jan 19, 2014

“Pyramided hybrids will help to delay resistance longer than using corn with only one Bt toxin.” —Erin Hodgson ISU entomologist

OKOBOJI – It was a capacity crowd of curious ag producers who listened to analysts and Iowa State University Extension field specialists on Jan. 6 tell them what many already knew – enter the 2014 crop and marketing seasons cautiously.

Issues covered included western corn rootworm management and sulfur fertilization of soils.

CRW resistance

Western corn rootworm has become resistant to some Bt traits, according to Erin Hodgson, an ISU Extension entomologist.

The CRW has been in Iowa for the last 50 years, but has overcome many management tools in its time.

She said Bt corn was introduced in 2003 and was so widely used over corn acres that it soon developed a resistance to it.

She said in 2009 fields were identified in northern Iowa with severe injury to Bt corn from CRW and later insects from those fields were confirmed to have resistance to Cry3Bb1 corn.

Since then, she said, resistance was also confirmed on mCry3A corn.

“The CRW has one generation per year so they have a lot of adaptability to management,” she said. “It’s a very adaptable pest. By comparison, aphids have 15 to 20 generations per year.”

Hodgson said with current management practices, the CRW can maintain a 5 percent survival rate when Bt traits are present.

“It’s the best we can do right now,” she said.

Hodgson said resistant rootworms is associated with continuous corn production combined with continuous use of Bt traits.

She said in addition to planting non-Bt refuges, better integration of management tactics is important to delay or eliminate resistance.

Crop rotation, she said, breaks the life cycle of the CRW and is highly effective in Iowa. Soil-applied insecticides can be used on corn that does not contain Bt toxins targeting corn rootworm.

“Pyramided hybrids will help to delay resistance longer than using corn with only one Bt toxin,” she said.

To date, crop rotation is the most effective rootworm management strategy in Iowa, Hodgson said.

Other tactics include soil insecticides, foliar insecticides for adults, and seed treatments. Producers and can further preserve Bt technology by using an integrated approach that includes rotating management tactics instead of layering strategies.

Hodgson said often times insecticides protect the primary roots, but not the secondary ones.

“Depending on your planting date, moisture, soil insecticides may not be active when the larvae hatch,” she said.

Hodgson added producers should consider treatment when there are five or more beetles per plant and when pollination is not completed.

A high adult density of beetles should be concerning.

Managing CRW is important, she said, because average root injury comes with damage to 1.8 or more nodes. Yield loss can be expected at 0.25 or greater nodes.

“For every node destroyed you lose 17 percent yield,” she said. “If you see brown on your corn roots, there are CWR larvae there.”

Recognizing the CRW is imperative to managing the problem, she said, adding that they are yellow and black in color, with males having a primarily black back, and females having three distinctive black stripes down their backs.

Sulfur fertilization

John Sawyer, an ISU Extension soil fertility specialist, told producers that sulfur deficiencies have been uncovered in many parts of the state.

Sulfur rate trials were conducted in field tests between 2006 and 2010 in northeast and central Iowa, and with the sulfur applications of varying rates, corn yield increased at 60 percent of the sites.

“If you’re applying manure to your fields, you’re likely getting enough sulfur,” he said.

He said field conditions showing the most obvious response to sulfur application included soils with coarse texture, low organic matter, eroded soil, low clay soil; side-sloped landscape position; no-till; alfalfa as the prior crop; fields that had no manure application and no recent sulfur application.

He advised producers that if they needed to apply sulfur, to apply 15 pounds per acre on fine-textured soils and 25 pounds per acre on coarse-textured soils.

He advised that if sulfur fertilizer application is to be made in the spring pre-plant, at planting or early sidedress, to use a sulfur containing fertilizer, such as ammonium sulfate, calcium sulfate, potassium sulfate or ammonium thiosulfate.

Fall applications might include a sulfate or elemental sulfate fertilizer, or manure. Research has shown that if early-season corn plant growth has visually indicated a sulfur deficiency, corn can respond to broadcast sulfate forms of fertilizer, if there is sufficient rainfall to move it into the root zone.

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