European corn borer moths numbered very high during monitoring near Beresford, Canton, Centerville, Hurley, and Sioux Falls, SD.

Late-planted corn may be vulnerable, says Mike Catangui, South Dakota State University extension entomologist.

The light trap at the SDSU Southeast Research Farm near Beresford, for example, caught 1,543 moths on the night of Aug. 5. Catangui says that’s higher than the peak numbers of second-brood moths during the outbreak years of 1996 and 1997.

Producers scouting cornfields near those areas for signs of second-brood corn borer infestations should look for egg masses on the leaves and ear husks, active feeding by the larvae on the leaf collars, or newly hatched larvae on the tassels and ears.

No treatment is recommended if the corn plants are already in the dent stage of development. If insecticide treatment is necessary, it has to be done before larvae tunnel into stalks.

Established research data indicate that each second-brood corn borer larva can potentially reduce yield by 4%/plant if infestation starts at silking stage, 3% at blister stage, 2% at dough stage, and no yield loss at dent stage of corn development.

Economic thresholds will depend largely on the current stage of corn development. The economic thresholds are 38% of the plants showing infestation at silking stage, 51% of the plants showing infestation at blister stage, and 76% of the plants showing infestation at dough stage of corn development.

More information and pictures of corn borer can be found at the SDSU Extension Entomology Web site at:

www.abs.sdstate.edu/plantsci/ext/ent/