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Corn+Soybean Digest

Corn Earworms Becoming a Threat to Soybeans in the South

Non-Bt corn varieties are often visited by corn earworms. However, control can be difficult and most field corn growers don't spend the money to take them out. But what about when they invade soybeans?

Ronnie Levy, Louisiana State University soybean and grain specialist, says the movement from corn to soybeans is the biggest challenge.

“We often see bean fields that need to be treated because of their proximity to corn fields,” says Levy. “Earworms can really damage small pods and reduce production.”

He points out that there are many insecticides on the market to control corn earworms, also known as earworms in soybeans, bollworms for cotton and headworms for sorghum.

For example, Virginia Extension Entomologist Ames Herbert tests for earworm resistance most years to determine if they are tolerant of pyrethroids. His adult vial test has adult moths placed in vials treated with cypermethrin for 24 hours. If they survive they are considered resistant.

THERE WAS A 40% survivorship in late July 2009 — “very high for us,” says Herbert. “Survivor rates like that could indicate the need to consider non-pyrethroid options, especially if this level of survivorship continues or increases, and if corn earworm populations are high in soybeans.”

“If growers spray for earworms or other insects in soybeans, they should look hard at rotating to other insecticides with different modes of action when additional treatments are needed,” adds Levy. “That could prevent any resistance problems from occurring.”

He says the many pyrethroids and other chemicals that should be effective in controlling corn earworms include: Mustang Max, Hero and Brigade from FMC; Baythroid XL from Bayer CropScience; Asana XL from DuPont; Karate Z/Warrior/lambra-cyhalothin from Syngenta Crop Protection; Tracer from Dow-AgroScienices; and Ambush from Amvac Chemical. Orthene, an organophosphate, is from Valent and also very effective.

Virginia Tech University offers a method of determining the economic threshold of corn earworms in your soybean field. Go to, then enter specific data to determine the threshold.

For example, if you are scouting with a sweep net, have 21-in. soybeans priced at $9 and an insecticide application cost of $7.50/acre, the threshold would be about 1.6 worms per 15-sweep sample.

Again, earworms in corn usually aren't treated, unless you're growing sweet corn or some specialty food corn, says Ed Bynum, Texas AgriLife Extension entomologist.

THE WORMS GET into the whorl and it's difficult to get the insecticide to them. When they feed on kernels in the ear, treatment is almost impossible, he says. Levy notes that earworms in field corn could lead to increased aflatoxin.

“Damaged kernels become more susceptible to diseases,” he says, noting that growers in locations where aflatoxin has been a problem should look into the need for earworm control plans.

The addition of more Bt corn hybrids in the years to come will help southern growers in their corn earworm and other insect control efforts. But unless there are fewer needs for a 50-50 or 80-20 refuge regions for Bt vs. conventional corn, and unless there are Bt soybeans in the pipeline, the bugs may always be a threat.

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