Corn earworm tends to be a problem every year on sweet corn in Kansas. The earworm moth lays eggs on developing silks at night. When the egg hatches, the larva crawls down the silk and into the ear.
Feeding starts at the tip of the ear and works down. Though several earworms may hatch and attack a single ear, only one is usually present at harvest due to the cannibalistic nature of the insect. Control is challenging as silks continue to grow over a period of time. This means that even if silks are treated, new silk will appear that hasn’t been protected. Applications every 2 to 3 days are needed for insecticides to be effective, especially in late June to early July when peak flight of these moths usually appear.
There is a three-week period from silking to harvest, but there is only a two-week period from when the silks appear to when they begin to dry. Since moths prefer juicy silks and shun those that have started to dry, insecticides are only needed the first two weeks of silking.
Homeowners can use cyfluthrin (Baythroid; Bayer Vegetable and Garden Insect Killer) or spinosad (SpinTor; Captain Jack’s Dead Bug Brew; Conserve; Borer, Bagworm, Tent Caterpillar & Leafminer Spray).
Spinosad is an organic product. Commercial growers have additional choices including zeta-cypermethrin (Mustang Max), bifenthrin+zeta-cypermethrin (Hero), spinetoram (Radiant) and flubendiamide (Belt).
Though more time consuming, mineral or other light horticultural oils may also be used. The oil is placed inside the silk end of the ear with a medicine dropper (½ to 3/4 of a dropper) after the silks brown.
This will coat the earworms already present and likely suffocate them, though some damage to the tip of the ear will likely have occurred.
Applying the oil before the silk has browned may interfere with pollination, leading to incompletely filled ears. (Ward Upham)