Some cornfields in Iowa are now silking. And even though the state's farmers had a longer planting period than usual this year because of the wet spring, most cornfields will be well into the reproductive stages shortly. Tristan Mueller, program manager for the Iowa Soybean Association's On-Farm Network, says now's the time to be watching your fields for adult corn rootworm beetles.
"Beginning with your continuous cornfields, scout now to make counts of the adult beetles, to get an idea of the population of beetles present in your fields this summer," Mueller advises. "You need to scout and get an estimate of the beetle population even if you planted a corn hybrid that has Bt resistance to corn rootworm.
"Last year, we documented high levels of root damage and silk feeding in several corn-on-corn fields planted with hybrids that had Bt rootworm traits," says Mueller. He says root damage may be high, even in Bt RW corn hybrids, if egg-laying was heavy in that field the previous year. Taking beetle counts now will tell you whether silk clipping could be heavy enough to reduce yields.
Stalk lodging was caused by corn rootworm damage in this 2012 cornfield that was planted to a Bt RW corn hybrid.
Over the next week or so, Iowa State University crop scouts will be checking fields where ISA On-Farm Network corn rootworm replicated strip trials are located, so it's important for participating farmers to get their applied data submitted for these fields.
Possible rootworm resistance to Bt traits in corn hybrids is raising questions from farmers
Mueller says there have been several questions from farmers about rootworm resistance to Bt traits expressed in corn hybrids. The corn hybrids have the traits that are supposed to limit rootworm damage, but some farmers are seeing root damage in these fields. "If you planted a Bt RW corn hybrid, counting beetles now and monitoring for root damage later will tell you whether there's a possibility that the corn rootworm population in your field is resistant to the trait," Mueller says.
"Control by the Bt trait requires that larvae eat some of the root. If the number of beetles in the field was high last year, there could have been a massive amount of eggs deposited in the field, resulting in a lot of visible root feeding this year. Even with a lot of root feeding, very few larvae will develop into adult beetles if the trait is working. If scouting shows high numbers of adults in the field and you suspect resistance to the Bt RW trait is a possible cause, this can be verified through Iowa State University."
Should you spray insecticide on fields to control adult corn rootworm beetles?
There are two reasons you might want to consider controlling adult beetles in a cornfield. First is to protect this year's crop by preventing excessive feeding on silks. The second reason, for continuous cornfields only, is to prevent beetles from laying eggs that could harm next year's crop. The eggs overwinter, hatch next year and the rootworm larvae chew on corn roots.
The economic threshold for controlling adult beetles depends on the cost of applying insecticides, but information from the University of Nebraska, as presented in this Monsanto guide suggests it may take more than five beetles per plant for silk clipping to lead to significant yield loss.
If you're planning to plant corn next year on a field that's in corn now, Purdue University suggests that if the scouted count is more than one beetle per plant, spraying an insecticide this summer may be justified in order to reduce the potential for rootworm damage in next year's corn crop.
If you do your own scouting, using this new app can make recording numbers quick and simple
Other information on corn rootworm management can be found in the February 2013 issue of ISA's On-Farm Update and in this Iowa State University rootworm management recommendation by Aaron Gassmann, an ISU entomologist.
Mueller reminds everyone that if you're doing your own scouting, using the new ScoutPro app for smart phone or tablet app can make recording numbers and locations in the field quick and simple. "ISA still has a number of copies of ScoutPro available free to ISA members," Mueller says. Email him to get your free app. firstname.lastname@example.org