If you plant corn with the Bt corn borer trait, then you likely won't see the European corn borer, although it would be possible to see it on the refuge plants. Earlier in the season while scouting a field of refuge-in-a-bag corn, a few plants – obviously the ones without protection – showed shot-hole damage common when first brood larvae eat through leaves that were still in the whorl.
The Bt traits for corn borer offer very good protection against the pest, says Danny Greene, of Greene Crop Consulting, Franklin. However, he has found a considerable amount of corn borer feeding and damage in fields of non-GMO corn this fall. He firmly believes that there was likely enough corn borer around to have justified treatment in some of those fields earlier in the year.
The problem is catching it at the right time. Corn borers are notoriously hard to control through spraying, although it can be done, because larvae quickly burrow into the stalk if not caught at just the right stage.
This fall in non-GMO corn, Greene has found actual borers inside stalks. It could be the second, or it could even be a third generation larva, he notes.
While taking stalk samples for nitrogen analysis, he has found several stalks partially hollowed out by corn borer in these non-GMO fields. Damage can result in smaller ears because the plant doesn't have as much vascular tissue left to move nutrients up from the roots, or move sugars down and feed the roots. Corn borers can also burrow into the ear shank, sometimes causing ears to drop prematurely. So far, Greene has not seen that type of damage yet this year.
The moral of the story is that corn borers are still out there. Bt corn keeps them at bay.