Record high numbers of corn borer moths have been captured in light traps across the southern South Dakota counties in the past two weeks and it's essential that farmers check corn for corn borers, a South Dakota State University specialist said.
SDSU Extension Entomologist Mike Catangui says light trasp at the Southeast Research Farm near Beresford registered a catch of 424 first-brood moths on the night of June 16.
"This large a number has never been seen before in that location since trapping was started in 1996," Catangui said. "About 200 first brood moths per night were the previous highest counts in the same light trap in 1997 and 2000."
Field inspections of corn plants near Beresford on June 28 by Catangui have indicated that most of the eggs deposited by the moths have already hatched into larvae. Up to four live larvae were found per plant. The moths were still laying eggs and some eggs have not hatched yet.
The corn borer larvae found in the whorl were already in their second instar stage of development. They will start burrowing into the stalks when they reach third instar within two weeks. Once inside the stalk, any attempt to spray for corn borers will be ineffective.
Catangui said spraying with an insecticide is recommended if one out of four (25 percent) of the corn plants show signs of corn borer infestations such as "shot holes" on the leaves, larvae in the whorl, and egg masses on the underside of leaves. This economic threshold assumes four live larvae per plant, $1.80 per bushel corn market value, and $8.00 per acre insecticide-plus-application cost.
Insecticides labeled for use against the European corn borer larvae include Asana XL (7.8-9.6 fluid ounces per acre), Capture 2EC (2.1-6.4 fluid ounces per acre), Furadan 4F (1.5-2.0 pints per acre), Lorsban 4E (1-2 pints per acre), Mustang (2.9-4.3 fluid ounces per acre), Penncap-M (2-4 pints per acre), Pounce 1.5G (6.7-13.3 pounds per acre), Sevin XLR Plus (1.5-2.0 quarts per acre), Tracer (1-3 fluid ounces per acre), and Warrior (2.56-3.84 fluid ounces per acre). Always read and follow label directions.
Data on the current flights of the corn borer moth can be found on the Internet at http://www.abs.sdstate.edu/plantsci/ext/ent .