One of the more challenging crops to keep insect-free is sweet corn, especially with later plantings that ripen after early August. The key pest is corn earworm, which remains hidden in the tip of the ear until the ear is shucked. Early-planted sweet corn often sustains only minimal damage, even without sprays to control earworms. However, later-planted sweet corn can have every ear in the field damaged at harvest. The larger late-season populations are the result of earworm migrations from southern states, so planting date is an important tool to manage corn earworm.
Corn earworm lays eggs singly and directly on fresh silks. Late in the season, each ear can have dozens of eggs, which will hatch in 3 to 5 days. After hatching, larvae enter tips of ears following the path of the silks, often feeding on the silks along the way. This is the time we target for control because once in the ear there is little that can be done to control them. Insecticides to control corn earworm must be directed to the silks, before eggs hatch. Once larvae are inside husk leaves, they are protected from predators and sprays.
Although corn earworm may lay dozens of eggs, we typically only find one larva on a damaged ear. The earworm is cannibalistic; larger larvae will eat the smaller larvae they encounter.
Generally, sprays for corn earworm begin at tassel emergence and are repeated at 2- to 5-day intervals until silks dry completely. While insecticides often have longer residual control, the silks continue to grow out of the ear, requiring reapplication. In some parts of the south, sweet corn may be treated daily to combat very high earworm populations.
Growers using Bt sweet corn varieties to control corn earworm are advised that these varieties alone may not provide sufficient corn earworm control. Attribute I and Performance Series Bt sweet corn will need supplemental insecticide sprays to remain free of earworms later in the season. More information on Bt sweet corn is available in the 2016 Fruit and Vegetable Research Report (PR-721).
When spraying sweet corn for earworm, the key is getting thorough coverage of ears and silks. Sprays need to be directed to the middle third of the plant, the ear zone. We recommend applications by ground equipment with hollow-cone drop nozzles used on each side of the row. Higher pressure is recommended to move leaves that interfere with coverage. For a list of recommended insecticides, review Vegetable Production Guide for Commercial Growers, 2016-2017 (ID-36).
By Ric Bessin, Extension Entomologist