If we consider acreage, sweet corn is our most widely grown vegetable in Kentucky. Locally-grown sweet corn is very popular, and in some ways, the first local sweet corn of the year signals the start of summer. While many people look forward to sweet corn, no one likes to find corn earworm in their ear. Corn earworm is the most difficult and common of our sweet corn insect pests. Corn earworm is the caterpillar that feeds on the kernels at the tip of the ear. The husks leaves hide the earworm and its damage until they are peeled away. More often than not, only a single corn earworm is found on a damaged ear as the larger caterpillars are cannibalistic on smaller ones. While backyard gardeners may not mind cutting off the tips of damaged ears, earworms can ruin the reputation of a commercial producer.
Corn earworm can overwinter locally in the soil, and winter weather conditions can impact their survival. The moths (Figure 1) will also migrate in large numbers into the Midwest from southern regions, but these migrations usually occur later in the summer as compared to local populations. During this past winter more ground froze as a result of prolonged cold temperatures, which would mean that the local populations likely had low survival. So as a result, early and mid-season corn may be at lower risk of corn earworm. But late-planted sweet corn that tassels in mid August or later will be exposed to the larger populations migrating from the South.
Corn earworm is attracted to volatiles produced by the fresh silks. When the silks emerge, the female moths are attracted to the silks for egg laying (Figure 2). As the silks dry, they become less attractive to the moths. It is important for producers to monitor and know when the field begins to silk. I recommend beginning sprays for earworms when 90% of the silks have emerged. Growers will need to reapply sprays on a 3- to 7-day interval based on corn earworm pheromone trap catches.
There are Bt sweet corn hybrids that are available that provide varying levels of earworm protection. In 2000, ‘Attribute’ sweet corn became available as the first type of Bt sweet corn. These lines, while not “bullet proof” to corn earworm, greatly reduce the need for earworm insecticide sprays. In 2011, ‘Performance’ sweet corn became available with multiple Bt genes for even higher levels of earworm protection, as well as tolerance to gylphosate herbicide. This year, ‘Attribute II’ hybrids have been released with stacked Bt genes for earworm protection. Both ‘Attribute’ and ‘Attribute II’ hybrids have the LibertyLink trait, so the EPA approved them for use with Liberty herbicide (glyfosinate).
While we do have several different types of Bt technology that provide high levels of control, producers still need to monitor for earworm and manage them as needed; these are not stand alone tactics when populations are extremely high. Producers using Bt sweet corn must also abide by resistance management plans and destroy stalks within 30 days of the final harvest.
When spraying for corn earworm management, timing and coverage are important. Producers should target the middle third of the plant as the ear is the only part needing protection. A boom sprayer with drop nozzles can properly place insecticides in this target zone. Two nozzles on each side of the row directing spray from above and below the ear can provide excellent coverage. There have been some resistance concerns with pyrethroid insecticides (IRAC group 3 mode of action) particularly during the heat of the summer when this class may not be as active due to high temperature. There are other modes of action that are available including IRAC groups 1A, 5, and 28. See the UK publication, Vegetable Production Guide for Commercial Growers (ID-36) for a listing of recommended corn earworm insecticides.
By Ric Bessin, Extension Entomologist