Mar 10, 2021
Pyrethroids’ effectiveness against corn earworm eyed

For the past decade, pyrethroids have seemed to be losing their effectiveness for controlling corn earworm in sweet corn. They work well in some years and not in others.

New research from Ohio State University (OSU) says the amount of insect pressure – the number of moths arriving from the southern U.S. – is the key factor affecting the year-to-year variability in control.

That’s the message of Celeste Welty, Extension entomologist at OSU. Welty spoke on the topic at the recent Great Lakes Fruit, Vegetable & Farm Market EXPO that was held virtually.

Corn earworm moths migrate up from the southern U.S. every year. Weather affects the migration and makes the arrival time variable.

The big surge in the numbers of arriving moths usually comes in mid-August. This is when field corn is usually done silking and its silks are not attractive to the moths. “Local field corn isn’t attractive to moths so they really hone in on late season, sweet corn,” Welty said.

Celeste Welty
Celeste Welty, Extension entomologist at Ohio State University, said “local field corn isn’t attractive to moths so they really hone in on late season, sweet corn.” Photo: Ohio State University

Researchers at OSU in a long-term study compared the degree of corn earworm control in late-season sweet corn with pyrethroids considering use rates, the time of moth arrival and moth numbers. Warrior was used as the pyrethroid. The 10 years of results showed some clear trends.

Warrior provided excellent control (99% undamaged ears) in two of the 10 years. Warrior provided good control (94% to 96% undamaged ears) in three of the years and fair control (75% to 80% undamaged ears) in two of the years. Warrior provided poor control (18% to 60% undamaged ears) in three of the years. Researchers determined this variability of control was related to the numbers of arriving moths.

Warrior was not able to provide good or excellent control when the corn earworm population was high or very high during silking. A high or very high population correlated to more than 35 moths being caught per pheromone trap per day.

Warrior did provide excellent control in years when the corn earworm population was low, correlated with less than 10 moths per trap per day. This excellent control was provided despite the known occurrence of pyrethroid resistance in Midwestern corn earworm populations.

The timing of corn earworm moth arrival didn’t have a significant impact on the amount of control by Warrior. Control was excellent if moth numbers were low throughout the silking period. Control was also excellent if the population was low during the first two weeks of silking, even if the population rose to high levels during later silking.

Use rate did matter. In 2009, 94% of the ears treated with the full rate of Warrior showed no kernel damage while only 75% of the ears treated with a reduced rate had no damage. The damaged ears at the low use rate also had more damaged kernels per ear. Trials in all the remaining years only included the maximum rate.

Silking is the critical time for corn earworm control in sweet corn. The corn earworm moths lay their eggs on the silk. The moths prefer fresh silk that’s newly emerged but will lay eggs on wilted or brown silk if fresh silk is unavailable. The eggs hatch in 48 hours and the small larvae tunnel down the silk into the ear.

August is the challenging month. Most field corn is done silking and late sweet corn is just starting. If late planted field corn is silking, that can dilute the effects of corn earworm moths.

Growers should start a spray schedule when fresh silk begins to show – if moths are active. “This really shows the value of trapping,” Welty said. “Make it part of your plan. Watch the traps during the three-week silking period.”

Make the first spray when 25-50% of the plants show silk – if the moth catch is high. Make the first spray when 50-75% of the plants show silk – if the moth catch is low. “Those early silks are what you want to protect,” Welty said. With low pressure, you can wait until more silks emerge.

Repeat the sprays every two to three days when the pest pressure is high. Treat every four to six days when the pest pressure is low.

“Pyrethroids have residual activity but fresh silks grow rapidly – up to 1.5 inches per day,” Welty said. “Growers must spray frequently to keep all of the new silk protected.”

“Pyrethroids are much less expensive,” Welty said. “In a year with few moths in the trap, pyrethroids have been working fine.” Use the maximum rate and consider the risk.

“If you have to make sure you absolutely have no damage you may not want to take the risk – even in a low trap year,” Welty said.

If corn earworm numbers spike, there are new active ingredients.

Coragen (chlorantraniliprole) and the pre-mixes Besiege and Elevest, and Radiant, a spinetoram, and Blackhawk, a spinosad, are all effective. Older products like Lannate (methomyl) can also still be helpful. Having both a supply of a pyrethroid and a newer active ingredient on hand is a good plan.

Sweet corn, field corn, tomato and pepper are all host plants for corn earworm.

Fall armyworm and European corn borer are also sweet corn pests growers don’t want to forget about, although corn earworm is the big threat.

European corn borer is a local pest. “It’s always around,” Welty said. There is one generation in June and another generation in August.

Fall armyworm is a migratory pest from the southern U.S. and is a problem in July and in August. “Fall armyworm can be a very small problem or a really big problem,” Welty said. “It’s a really good idea to trap for it if you want a comprehensive pest management program.”

— Dean Peterson, VGN correspondent

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