The corn earworm, Helicoverpa zea, attacks a wide variety of vegetable crops, but can be devastating in sweet corn production. While the larvae cause relatively minimal damage to an individual ear, often consuming only a few kernels at the tip, this damage (along with frass and larvae) makes the ear unmarketable in most markets, and relatively few damaged ears in a field can make the entire field unharvestable for most large commercial markets. Management of this problem requires frequent insecticide applications during ear development, with daily application required when pest populations are moderate to high (which occurs throughout southern Georgia from early summer through fall production). Frequent applications are required to control hatching larvae before they enter the silk channel and are protected from additional insecticide applications. I am not aware of any insecticide that will control larvae once inside of the husk. The need for frequent application implies the necessity for relatively cheap insecticides to keep control costs economical. Thus, pyrethroid insecticides have dominated this use for decades. Unfortunately, recent resistance to this group of insecticides in the corn earworm is an obvious threat to the status quo.

Dr. Phillip Roberts has monitored corn earworm response to pyrethroid insecticides with the adult vial technique for decades. His results have shown a relatively steady climb in resistance for several years, reaching levels of concern in the last few years. We conducted a larval bioassay last year, which also showed survival of corn earworm larvae at rates that should have been highly effective.

We have been conducting similar bioassays this year. The basic approach is collection of larvae from corn ears in the field (most have been non-treated research fields to avoid obvious selection for resistant individuals), placement on soybean leaves treated with a pyrethroid (Warrior II), and monitoring mortality at 72 hours. We use the high labelled rate of the insecticide mixed as if spraying at 100 gpa as our 1X rate, and test at 0.5, 1 and 2X rates.

Thus far in 2024, we have tested 4 populations and results have been similar for all four (see attached graph). Survival even at the highest exposure (2X rate) has been about 30 percent. This strongly suggests that the pyrethroid insecticides are not likely to provide adequate control of corn earworm in sweet corn. Alternative products that are efficacious include Lannate, Coragen/Vantacor, Radiant, Blackhawk and pre-mix products that contain one of these active ingredients. Most of these products do have more restrictive use patterns which require greater attention to rotations and total use, and all are more expensive, making the economics of sweet corn production much more challenging.

Posted in: