by Deborah Jeanne Sergeant

Tracking patterns of corn pests can help farmers more effectively combat them. Ken Wise, livestock and field crops IPM specialist for New York State IPM at Cornell University, presented “Importance of Pheromone Trapping for Black Cutworm, True Armyworm and Western Bean Cutworm” at the recent Corn Congress, presented virtually this year.

“The purpose of the Field Corn Pheromone Trapping Network is the early alert system for potential high-level flights of moths across New York,” Wise said. “It can identify areas that need to be scouted and measure the expansion of western bean cutwork across New York.”

Found in NY in 2019, western bean cutworm is one of the recent additions to the pheromone trapping program. Black cutworm and true armyworm have been collected for two years. Participants place pheromone traps next to corn fields. Male moths are attracted to the traps. As the moths enter, they are hit with insecticide. The traps are placed in April to early June for black cutworm and true armyworm and late June to late August for western bean cutworm.

Black cutworms are greasy, pale gray to black in appearance with black spots on each larval segment. They measure 1/8-inch to two inches long. They cut off plants early in the season and do not pose a threat later in the season.

Wise said farmers who spray their weeds can protect their corn plants, as by the time the second generation comes along, the corn is too large for them to feed on.

True armyworm “flies on storms, lays eggs on grasses and prefer hayfields, but will choose grassy weeds in a cornfield,” Wise said. If multiple storms occur, the pests will lay their eggs at different times.

“You can get a mixed size of larvae and it’s hard to make decisions on what to treat,” Wise said. “If they grow at the same rate, it’s easier to make those economic decisions on true armyworm.”

True armyworm mature larvae have “variations of colors and orange stripes on either sides,” Wise said. “They look yellowish-orange in color, depending upon the molt they’re in. They feed from the edge of the leaf to the mid-rib. There are a lot of different armyworms.” They have a white stripe down the middle of the back. The pest is generally nocturnal.

“When true armyworm is in extreme populations, it can march during the day and feed and go on to the next field,” Wise said. They tend to eat 80% of the forage they are feeding on.

After true armyworm has damaged a corn crop, it may look like “a bunch of stringing mid-ribs sticking out of the field,” Wise said. “It can recover, but you may lose yield. Farmers say, ‘yesterday the field was fine and today it’s gone.’ The damage was minimal yesterday; you didn’t notice. Once you’ve sprayed out the weeds, they prey on the corn.”

Like black cutworm, they pupate in the soil. Wise said Cornell monitors for true armyworm into July to see if a second generation happens.

“Occasionally, you get help from birds,” Wise said. “Some will come into a field and make a major effort in getting rid of them.”

Wise recommended selecting BT corn and a seed treatment to prevent damage from black cutworm and true armyworm. He said insecticides should be used only when economically necessitated, not as a prevention.

Unlike true armyworm and black cutworm, western bean cutworm overwinters in the soil in a pre-pupal case.

“In the spring, they pupate and emerge before the corn tassels,” Wise said. “They will lay eggs on pre-tassel corn and pretty much not anything else. If it’s tasseled, they generally do not lay their eggs on those plants. Some entomologists say when conditions are not good and they cannot find pre-tassel corn, they may use some non-pre-tassel corn.”

As with the other pests, knowing when to expect western bean cutworm can help producers know when to scout.

“We had significant issues with black cutworms and true armyworm in 2020,” Wise said. “We will continue to improve our black cutworm and true armyworm degree day models to better predict when larvae are feeding in fields.” He also said that in some areas, true armyworm was an issue later in the summer.

“We will expand the number of traps to better capture when and where black cutworm and true armyworm flights occur in 2021,” Wise said. “It’s all about helping you know when to be in the field.”