Pests of the Week

Scout Soybeans for Gall Midge and Corn for Western Bean Cutworm Eggs

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Soybean gall midge larvae infesting soybean stems (top photo) have been reported in four states so far. Meanwhile, western bean cutworm moths are laying eggs (bottom photo) this month across the Corn Belt. (Photos by Justin McMechan, UNL, and Scott Williams, DTN)

ROCKVILLE, Md. (DTN) -- The Fourth of July is long gone, but there are still some colorful displays underway in many corn and soybean fields of the Midwest.

The bright white or orange larvae of the soybean gall midge are tunneling through soybean stems in Iowa, Nebraska, Minnesota and South Dakota. And across the Corn Belt, the eggs of the western bean cutworm moth are doing their annual transition from white to tan to purple, before tiny caterpillars burst forth, hungry for corn pollen.

SOYBEAN GALL MIDGE

Adult gall midge flies were first spotted in the Midwest this year in mid-June. Now, farmers and scientists in Iowa, Minnesota, Nebraska and South Dakota are reporting that the pest's tiny larvae are worming their way through soybean stems, where they can weaken and eventually kill the plant.

Scientists are urging growers in these states to scout for the pest, which took many in the industry by surprise last year. "Soybean fields that are post-V3 and adjacent to fields that had issues last year are most at risk," said University of Nebraska Extension entomologist Justin McMechan. The midge larvae range in color from white to bright orange and feed inside the stem, near the base of the plant. Usually, infested soybean stems will turn black at the soil line and snap easily, exposing the feeding larvae. But some growers and scientists in South Dakota and northeast Nebraska are also observing a white-brown swelling on infested stems, called a gall, said McMechan.

Gall midge infestations are usually concentrated along field borders. Last year, scientists and farmers documented yield losses of up to 90% to 100% in those field borders, with final field-wide yield losses averaging between 17% and 30%, McMechan said.

Unfortunately, the soybean gall midge is hard to treat. Once larvae are hidden inside the stem, there is no way to reach them and the adult flies have had a prolonged emergence period this year, which can outlast insecticide applications. "We're actually now seeing the second generation of adults emerging from this year's soybean fields," McMechan explained. "So far, we have not had more than two days in east-central Nebraska with no adults emerging since June 14. It's frustrating."

Only soybeans that were planted in early June into fields adjacent to fields infested last year are likely to benefit from an insecticide application at this point in the summer, he noted. Scientists and companies are actively testing different insecticides against the midge this year, as well as evaluating host resistance, McMechan said.

"With this really long emergence period, we're going to have difficultly managing it with insecticides," he noted. "I'm hoping that what we're seeing this year is not normal and that perhaps the unusual start to the season has expanded its emergence."

See more information on scouting and managing the pest from the University of Nebraska here: https://cropwatch.unl.edu/…

See a video on how to scout for it here: https://twitter.com/…

WESTERN BEAN CUTWORM

The western bean cutworm (WBC) moths are underway across the north-central Midwest, and moths are laying their eggs in cornfields, warns DTN Entomologist Scott Williams. "Sandier soils are more vulnerable," he noted. "WBC likes these soil types because it is easier to burrow into the ground when it's time to pupate. The deeper it digs down, the easier it is to avoid cold winter weather and emerge the following season."

The WBC caterpillars will feed on corn pollen, silks and finally burrow into the ear, where their feeding can cause yield loss and quality issues from mold and fungal diseases. Once inside the ear, they are protected from insecticide applications, so growers have only a narrow window to target eggs and young caterpillars.

Farmers should scout cornfields that are approaching tasseling, Williams said. Since the hatched caterpillars feed on corn pollen, adult moths target pre-tassel fields for their larval nurseries.

The WBC moth lays tidy clusters of tiny white eggs on upright corn leaves near the whorl, often tucked away from sight.

As the eggs mature, their color changes, Williams noted. For the first day or two, they are bright white. By the fourth day, they have dulled to a light tan color. Finally, between days four and 11, they darken into a deep purple hue, before the tiny larvae pop out. "Once the eggs hatch, the caterpillars feed on the egg shells for a day and then move up to the tassel," Williams said. Ultimately, they will burrow into the developing corn ear.

"Check 20 plants in five areas of the field for egg masses to determine the percentage of plants infested," Williams said. "If the number of infested plants exceeds five plants out of 100 for higher value seed corn, or eight out of 100 for lower value crops, a foliar spray is needed," he said. "Once the caterpillars are in the ears, any intervention will be ineffective. But scouting by pulling back the husks can help growers anticipate damage and plan accordingly."

Remember that the western bean cutworm is not reliably controlled by any Bt trait except Vip3A (Viptera hybrids). It is no longer listed as a target of the Cry1F (Herculex) trait, and growers using hybrids with that trait are urged to scout and treat their cornfields as they would non-Bt hybrids, Michigan State University Extension entomologist Chris DiFonzo said in a university fact sheet.

See more on scouting and managing the WBC here: https://entomology.unl.edu/…

See more from DTN's Scott Williams here: https://www.dtn.com/…

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

Follow her on Twitter @Emily_Unglesbee

(PS/BAS)

Emily Unglesbee