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 Time Approaching to Evaluate Corn Roots for Injury by Corn Rootworms Minimize
Location: BlogsWisconsin Crop ManagerInsects and Mites    
Posted by: WCM Staff 7/7/2005 12:57 PM
Over the next couple of weeks of July, we enter the crop season window during which to evaluate corn root injury by corn rootworms. We are planning to dig roots, soak and then wash the soil away with a high pressure hose stream, and quantify root injury in our UW Madison Entomology research trials, insecticide efficacy program, and on-farm studies between the dates of July 18th and July 25th.

Time Approaching to Evaluate Corn Roots for Injury by Corn Rootworms

 

Eileen Cullen, Extension Entomologist UW Entomology Department

 
Over the next couple of weeks of July, we enter the crop season window during which to evaluate corn root injury by corn rootworms. Corn rootworm larvae (3 instars) complete feeding on corn roots by mid-July as a general rule, depending on temperatures and heat unit accumulation. After third instar, corn rootworms pupate in the soil and will emerge as adult beetles throughoutJuly and into August. Last week (June 30th) AgriView newspaper, gleaning information from the WI DATCP statewide Pest Survey team, reported that an estimated 50 percent larval hatch had occurred as far north as Wausau. So, we are on course for a typical rootworm year in which we expect peak larval feeding to have occurred by mid-July.
 
We are planning to dig roots, soak and then wash the soil away with a high pressure hose stream, and quantify root injury in our UW Madison Entomology research trials, insecticide efficacy program, and on-farm studies between the dates of July 18th and July 25th. In relatively cooler years, with delayed corn rootworm egg hatch and fewer heat unit accumulations we would push this back a bit into later July. Remember, the later into July and past the first week of August you go to evaluate root injury, the more difficult and time consuming the process becomes due to corn root regeneration. Some hybrids will regenerate roots more than others, but many hybrids do regenerate root mass growth from main roots previously pruned back by rootworm larvae. Trying to differentiate between regrowth and the point of original rootworm injury can be a hassle, and the whole evaluation process is smoother if you evaluate roots as soon after larval feeding peak as possible.
 
Reasons for evaluating corn root injury most often include measuring efficacy of corn rootworm larval control tactics in on-farm strip trials or more intensive replicated small plot research designs (soil insecticide, seed applied insecticide, Bt corn rootworm hybrids, untreated check). Additionally, you may be called to the field for corn root evaluation in first-year corn fields following soybean in areas of southeast and southern Wisconsin where the Variant Western corn rootworm has been confirmed or is suspected. Finally, corn growing in a “goose-necked” shape, lodged or completely fallen, in either continuous corn or first-year corn fields should be evaluated to make sure the cause of lodged corn is indeed rootworm larval injury. This is particularly important after storms accompanied by winds. Corn can lodge due to other environmental stresses, so it’s important to accurately diagnose the problem before attributing it to rootworm injury. Fortunately, this diagnosis is getting easier (more intuitive and quantitative) due to recent publication of the 0 to 3 Node-Injury Scale by researchers at IowaStateUniversity (see related article this issue titled Research Brief: 0 to 3 Node-Injury Scale Published for Corn Rootworm Rating).
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