Posted 21 July 2008. PMN Crop News.
Corn Producers Now Should Scout for Leaf Aphids
South Dakota State University. www3.sdstate.edu
Brookings, South Dakota (July 17, 2008)--Corn leaf aphids are landing in fields in South Dakota, and corn growers should scout their fields.
"Right now, the goal is to go out and scout fields. They need to be scouted when the corn is at V-10 growth stage, or just before tasseling," said Jon Kieckhefer, an Extension agronomy educator based in Brookings County. "Look for aphids in the whorls of the plants or even on the tassels as the tassels begin forming."
Aphids can reproduce very rapidly on corn, and in large numbers can cause some serious yield losses. Kieckhefer said that as few as 45 to 50 aphids per plant at V-10 or just before tasseling may significantly impact yield.
Kieckhefer said producers finding those numbers, at that point in the plant's growth, should consider spraying to try to limit aphids.
Kieckhefer said recent research in Europe indicates that corn leaf aphids can be numerous on genetically engineered (Bt) corn compared with conventional corn.
Bt-corn hybrids, depending on the gene engineered into the plant, may be resistant to European corn borers, corn rootworm larvae, western bean cutworm larvae, or black cutworm larvae. Adult corn leaf aphids tend to choose plants uninfested with caterpillars.
"Aphids can identify plants that have caterpillars feeding on them, including the European corn borer, because the plants release a chemical that is identical to the chemical the aphids use as an alarm signal," said Kieckhefer. "When the aphids detect that in corn, they tend to avoid those plants that have caterpillars feeding on them."
SDSU Extension Entomologist Mike Catangui said that corn leaf aphids are an emerging insect pest of both Bt-corn and conventional corn in South Dakota.
"Corn leaf aphids have syringe-like mouthparts and feed on the sap of corn; when numerous, they can cause wilting and can severely interfere with pollination, resulting in ‘barren' corn ears at harvest," Catangui said. "Injuries to corn occur during or immediately before pollination, so timely scouting and treatment are crucial."
A procedure for calculating economic thresholds to include current or predicted corn market value, cost of controlling the insect, and yield potential is available at this link: www.ipm.iastate.edu.
Catangui cites an example of the possible economics of spraying cornfields with corn leaf aphid infestation.
"At $7 per bushel corn, 150 bushel per acre yield potential, $10 per acre control cost, and normal moisture conditions, spraying would be cost-effective even if only 3 percent of the corn plants are covered with aphids in the tassel," he said.
Insecticides labeled for use against corn leaf aphids in South Dakota are Asana, Capture, Cobalt, Delta Gold, Hero, Lorsban 4E, Mustang Max EC, Penncap-M, Proaxis, and Warrior. Producers are reminded to read and follow label directions before using pesticides of any variety.