Mid-July should be one of the easiest times to keep sweet corn ears free from caterpillar pests because none of the three key pest species are now abundant in Ohio, and many acres of grain corn are now silking and offering a good habitat to the few corn pests that are active. Once the large fields of grain corn are past the fresh-silk stage, then any plantings of late sweet corn will become very attractive to corn pests. We have had detection of some corn earworm moths but at low to moderate density. At present, we are between generations of European corn borer, but the moths of the new generation are likely to begin emerging in the next week or two.
However, the western bean cutworm is a 4th caterpillar species that is becoming a key pest of sweet corn, particularly in July and August, particularly in northwest Ohio. This is a key pest to look for over the next week. This pest is a caterpillar feeds on kernels of ears in both sweet corn and field corn. Feeding damage is usually at the tip end of the ear, but can be in the middle or butt end of the ear. There are often several western bean cutworm larvae in one ear, which makes it different than the corn earworm, that also feeds on kernels at the tip of the ear, but which typically is found as a single larva per ear. The newer BT sweet corn hybrids in the Attribute II series (from Syngenta) provide genetic control of the western bean cutworm, but BT sweet corn hybrids in the Performance series (from Seminis) and the older Attribute series (from Syngenta) do not control this pest. Pheromone traps detected the first activity of this moth this year during the week of 5 – 11 July in Sandusky, Champaign, and Clark Counties. Once the moths are detected, sweet corn fields should be scouted to monitor eggs and young larvae. Scouting should concentrate on plantings in the emerging-tassel stage. Look at 20 consecutive plants in each of 5 random locations per field. Examine the flag leaf (the leaf below the tassel), where eggs are usually laid. Eggs are laid in masses. Eggs are white when fresh, then they darken to purple when ready to hatch. Hatch will occur within 24-48 hours once eggs turn purple. Our tentative threshold for sweet corn is to consider treatment if eggs or larvae are found on more than 1% of plants for fresh-market or on more than 4% of plants for the processing market. Insecticide applications must occur after egg hatch, or after tassel emergence, but before larvae enter the ear. Pictures and additional details on western bean cutworm can be found in our OSU fact sheet: http://ohioline.osu.edu/factsheet/ENT-40 . Trap reports on western bean cutworm from several Ohio locations can be found using this link: https://docs.google.com/spreadsheets/d/10gh3rHahdxLKkXQapGyEPxWsjHYRmgsezOoFHnwtyEo/edit#gid=441280294
Corn earworm moths are present at low to moderate density as detected by pheromone traps at six Ohio locations; catch ranged from 0 to 11 moths per trap in the past week. This is slightly down from a few weeks ago when trap catch was 12-15 at some sites. The corn earworm moths will be laying their eggs on silks of sweet corn. Sweet corn can be protected from corn earworm infestation by insecticide sprays during silking. When the number of CEW moths caught in traps is moderate (1 to 13 moths per day, or 7 to 90 moths per week), then sprays should be applied every 4 days if the daily maximum temperatures is below 80 degrees F, or every 3 days if the daily maximum temperatures is above 80 degrees F. More information about trap-based spray schedules is available using this link: http://u.osu.edu/pestmanagement/crops/swcorn/ . Trap reports on corn earworm from several Ohio locations can be found using this link: https://docs.google.com/spreadsheets/d/10gh3rHahdxLKkXQapGyEPxWsjHYRmgsezOoFHnwtyEo/edit#gid=0
True armyworm moths are still more abundant than usual this year and remain a threat to young late-planted sweet corn. Young plantings should be scouted. Our threshold rule is to treat by spraying insecticide if 35% of plants are infested during seedling or early whorl stages. Trap counts for armyworm moths in Columbus can be found for pheromone traps : https://docs.google.com/spreadsheets/d/10gh3rHahdxLKkXQapGyEPxWsjHYRmgsezOoFHnwtyEo/edit#gid=1122468773 , and daily counts for armyworm moths in a blacklight trap are shown here:
-by Celeste Welty, Extension Entomologist