Corn earworm larvae can cause great damage to corn crops. Photo credit: USDA
Corn earworm is one of the most damaging pests to sweet corn crops, and is especially prevalent in the Midwest, according to Richard Weinzierl, Extension Specialist and Professor of Entomology at University of Illinois, Urbana-Champaign. The pest, which typically makes its first appearance between June and August, was less active for many growers in 2015 because of lighter than average moth flights, Weinzierl says.
“Corn earworm does not overwinter in the Midwest, instead it migrates in on weather fronts,” Weinzierl explains. “Typically, those fronts originate from Texas and Louisiana and the moths ride them and end up deposited here. This year, we just didn’t pick up those flights, so it was lighter than usual for that reason.”
Pheromone Traps
Despite the smaller-than-usual numbers, the pest still presented challenges to sweet corn growers because of how difficult it is to control once present in the field.
So what measures can you employ to tackle corn earworm? One of the first steps Weinzierl suggests is to buy a cone-shaped pheromone trap, which he says should be installed a few days before the first ears start to silk.
“Corn earworm is a problem only after the onset of silking, not before that. When the very first planting is about to silk, get the earworm trap out, change lures in it every two weeks, and check it every day,” Weinzierl explains.
He suggests using wire Hartstack traps, which can be found through various Midwestern suppliers. Because the traps only catch male moths lured by pheromones imitating the sex attractant from female moths, females will still be actively laying eggs on silks. This means the traps are strictly for monitoring, not control.
He also explains that if your traps are catching more than three to 10 moths per trap per night when silking begins, any sprays should be applied within two days after first silk.
Scouting And Monitoring
Trying to spot corn earworm eggs on silks can be a practice in futility.
Photo by Richard Weinzierl
Because corn earworm larvae feed only within the silk channel and on the kernels themselves, scouting for damage on leaves or stalks is not an option. Although eggs are laid on silks, Weinzierl says looking for them as a scouting method is almost always ineffective. “We never recommend that approach to sweet corn growers, because it’s too hard to see everything,” he says.
“The eggs are glued individually to the silks at the tip of the ear, and there are hundreds. You’ve got to be able to look through it from the base of the ear to the tip to see the tiny, white eggs. While they are visible, if you walk through the field looking for eggs and say ‘No, I didn’t see any,’ that is simply not enough. The only other choice is to try to spray every field of sweet corn every three days from the time it starts to silk until the time you start picking, and that’s not a good option.”
Another way for growers to at least be aware of the potential arrival of corn earworm is to monitor moth flight patterns.
“Growers should monitor the moth flight beginning and continuing throughout the whole summer as soon as the first planting starts to silk. As each successive planting reaches silk, it becomes vulnerable. Moths lay their eggs on the silk, the larvae hatch in a couple days and tunnel into the ear. And once you find the larvae in the ear, it’s too late.”
Bt Sweet Corn Varieties
One of the only preplant decisions available to growers seeking to control corn earworm is the decision to plant Bt (Bacillus thuringiensis) sweet corn varieties, Weinzierl says.
He mentions the Attribute II series (Syngenta), which is most effective against corn earworm, and the Performance series (Seminis), which also is improved over earlier Bt technologies.
Nevertheless, he says growers need to remain vigilant to keep this pest in check. “You still have to monitor and sometimes spray,” he adds.
Chemical Controls
The chemical controls that have traditionally been used to manage corn earworm were in the pyrethroid class of insecticides, but Weinzierl points out that in recent years pyrethroid resistance has evolved in southern source areas, so additional control measures are necessary when moths from those areas reach Midwest sweet corn fields.
“The pyrethroids have become pretty inconsistent so growers usually have to use something along with them or instead of them, and the options that most people would chose are Coragen (DuPont Crop Protection), Radiant (Dow AgroSciences), and Belt (Bayer CropScience),” he says.
As for when to apply insecticides, Weinzierl says the best time is just after silking begins if moths are present.
“You need to have an insecticide on within two to three days after the first silks show. That’s how long it takes eggs to hatch and larvae to go into the ear. The spray interval from that time on is typically two to three days if it’s really hot and moth flights are heavy,” he says.
He goes on to explain that sprays should continue until the silks are brown, which is typically right before harvest.
For organic growers, Weinzierl suggests Entrust (Dow AgroSciences), and also mentions biological controls as an option when pest pressure is not heavy. He notes some growers have released parasitic wasps to control corn earworm, but when growers are seeing up to 250 to 300 moths in a trap per night, the parasitic insects are simply not able to keep up.
At the end of the day, there is little that can be done once corn earworm is present in the field, Weinzierl concludes. “Buy a trap, keep a fresh lure in it, and check it every day.”