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Corn borer: How to find it, what to do

corn borer larva on cob
ALIVE AND WELL: This corn borer larva was found in a corncob, tunneling toward the ear shank. You need to scout for the first generation. Second-generation larvae are harder to treat.
Corn Pest Beat: Scout non-GMO traited corn closely.

My corn isn’t Bt corn borer-traited. I’ve never scouted for corn borer. What am I looking for? What should I do if I find it?

The Indiana certified crop advisers panel this month includes: Gene Flaningam, Flaningam Ag Consulting LLC, Vincennes; Greg Kneubuhler, G&K Concepts, Harlan; and Tom Stein, Ceres Solutions Cooperatives, manager of the Templeton and Boswell branches.

Stein: Damage is usually caused by first- or second-generation corn borers. First-generation borers bore into the stalk, and second-generation larvae bore into tassels, ears and ear shanks.

Reference average dates for peak corn borer moth flights. For the first generation, that’s around June 10 for northern Indiana, June 3 for central Indiana and May 26 for southern Indiana. Follow Purdue University’s Pest & Crop Newsletter, as it monitors corn borer moth numbers and peak flights. Peak moth flights for the second generation occur around Aug. 14 for northern Indiana, Aug. 7 for central counties and July 26 for the south.

The first generation lays egg masses on the underside of the leaf near the midrib. Young larvae feed until the third larval stage, and then migrate to the whorl. You’ll see larvae or their excrement, which looks like wet sawdust.

Examine whorl leaves of 20 consecutive plants in five different areas. Feeding looks like birdshot holes from a shotgun. Calculate percentage of plants with damage. Pull the whorl from at least one damaged plant in each area, unroll leaves and look for larvae. Calculate average number of live larvae per plant. Make treatment decisions based off field conditions and economics.

Flaningam: For first-generation scouting, wait three to five days after moth flights peak. At the early whorl stage, one borer per whorl equals 5.5% loss, two borers per whorl equals 8.2% loss, and three borers per whorl equals 10% loss. Figure preventable yield loss.

Here’s an example: 180-bushel corn times 30% infestation times 8.2% estimated loss at two borers per plant times 80% level of expected control equals 3.5 bushels of preventable loss. At $3.50 corn, that is a return of $12.40 per acre. 

Second- and third-generation corn borers are a bigger concern for late-planted corn. The second generation usually occurs from mid-July through mid-September.

Kneubuhler: The first generation tends to hatch from June to mid-July, around V6 to tasseled corn. The second generation hatches from mid-July to mid-September. There’s no concrete threshold of scouting for corn borer. One way is to scout 100 consecutive plants and determine a percentage infestation in four or five areas. Determinations need to be made for treatment based on larvae per plant, price of corn, yield loss estimate and cost of treating.

We’ve had very good success treating V5 to V6 corn with pyrethrin insecticides when post-applying herbicides to wipe out corn borer moths before they lay eggs.

Stein: For second-generation scouting, inspect leaves in the ear zone of 20 consecutive plants in five areas. Determine percentage of plants infested and average number of live larvae per plant.

If an application is warranted, granular forms are more effective because they fall into the whorl or lodge on leaf axils where most borers will be located. If a liquid insecticide is used, use raindrop nozzles for larger droplet size, and position them directly over the whorl or ear zone.

Purdue Extension has a great publication, E-17-W. It takes into consideration level of infestation, estimated yield, anticipated level of control, stage of corn development, control costs and corn price.

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