Worms in the Forecast

Watch for Cutworms and Armyworms in Crop Fields This May

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Black cutworms, shown above, and true armyworms may be a threat to young crop fields starting in mid-May, based on high moth counts in some Eastern Corn Belt states this spring. (Photo courtesy John Obermeyer, Purdue University)

ROCKVILLE, Md. (DTN) -- April showers bring May ... worms. You heard that right -- entomologists in the Eastern Corn Belt are warning growers to be alert for armyworm and cutworm infestations in corn, soybean and wheat fields as soon as mid-May, based on high moth counts in some Midwest states.

Purdue University in Indiana is seeing some of the most impressive numbers so far, according to updates from integrated pest management specialist John Obermeyer.

Of the 56 black cutworm moth traps scattered around the state of Indiana, eight reported "intensive captures" for the first week of April, which means that nine or more moths were caught over a two-night period. One week later, 11 of those trap locations reported intensive captures. The state's seven armyworm moth traps were also netting robust numbers for both those weeks.

Cutworm moths arrived early in the Midwest this year, said DTN Entomologist Scott Williams. DTN Smart Traps, which catch and remotely transmit insect counts and pictures, started detecting cutworm moths as early as March 10 in Ohio, he said.

Both armyworm and cutworm moths are snowbirds -- they don't much care for Midwest winters and spend the coldest months in southern states, all the way down to the Gulf, Williams noted.

They rely on spring storm systems to migrate back up into the Midwest each year. Normally, many universities have broad monitoring systems in place, between moth traps posted at university field sites and private farmer cooperators. But the pandemic has forced many universities to reel in staff travel, which has shrunk the number of operating moth traps in some Midwest states, which may make scouting even more important than usual this spring.

Cutworm moths seek out broadleaf plants, such as winter annual weeds, to lay their eggs, Williams said. Armyworm moths prefer grassier egg nurseries, such as wheat, rye cover crops or grassy weeds.

Black cutworms often migrate into emerging corn and, as soybean planting inches earlier each year, also soybean fields. They can "cut" young plants off at the base, destroying the growing point and causing serious stand issues. Armyworms target both wheat and corn fields, and in large numbers, they can rapidly defoliate plants.

For both pests, the damage they inflict depends on their size and the crop's stage of development.

Based on Smart Trap moth counts in Indiana, Illinois and Ohio, producers in the Eastern Corn Belt could start to see damaging cutworm populations in mid-May, Williams predicted. "We'll have those first little caterpillars for a week or two, and -- depending on how warm it is in some locations -- we'll see them get to sufficient size to do damage to emerging crops by the third week of May," he said.

The plentiful rain in many Midwest states means the caterpillars should have an abundant food supply and could be large enough to inflict serious damage on young plants, Williams said.

Corn plants are most vulnerable to feeding damage from cutworms and armyworms at the V2 to V4 stage, said University of Illinois Extension entomologist Nick Seiter. As they move into the V5 through V7, the plants become large and robust enough to recover from feeding.

Don't rely on Bt traits or seed treatments to completely protect your cornfields, Seiter added. While some Bt traits supply suppression of these pests, they cannot necessarily withstand a heavy infestation.

See more on Bt traits and their target pests here: https://www.texasinsects.org/…

Wheat is most vulnerable to armyworm damage if the flag leaf is destroyed before the soft dough stage, according to a Kansas State University guide.

See the guide here: https://entomology.k-state.edu/…

See more on how to scout for black cutworm here: https://extension.entm.purdue.edu/…

See more on how to scout for armyworms here: http://extension.cropsciences.illinois.edu/…

For more details on the less common phenomenon of soybean damage from cutworms, see this Iowa State article here: https://crops.extension.iastate.edu/…

Scout carefully this spring, but remember that there is not a simple relationship between moth counts and later caterpillar infestations, added Purdue's Obermeyer in a university Pest & Crop Newsletter earlier this month.

"Many variables must perfectly align for these pests to cause a stir in high-risk fields," he wrote, noting that newly hatched cutworms and armyworms are susceptible to mortality from weather, natural predators and lack of a food source.

However, don't count on last week's lower temperatures to have wiped out these pests, he added in a later newsletter.

"The black cutworm and armyworm's most vulnerable stage for freezing, the neonate (newly hatched) larvae, were likely nestled within the soil or under plants (e.g., chickweed, cereal rye, etc.) in which they were feeding," he wrote. "Temperatures in these micro-environments are much warmer."

You can read more from Obermeyer and track Purdue's moth trap catches here: https://extension.entm.purdue.edu/…

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

Follow her on Twitter @Emily_Unglesbee

(PS/BAS)

Emily Unglesbee