Black Cutworm Flocks To Indiana; Scouting Proves Critical

Record numbers of black cutworm moths have descended upon Indiana fields, and after a week of warm temperatures and rainfall now is a critical time to scout, a Purdue Extension entomologist says.

Pheromone trap cooperators throughout the state monitor for the insect’s arrival, which has the potential to cause significant economic damage to field crops. A round of storms originating in southern states in March brought not only rain to Indiana, but cutworm populations not previously seen. Since that time additional captures have had the trappers very busy counting.

According to John Obermeyer, the key to successfully controlling the insect is to scout early and often and to apply insecticides when larvae are small – especially because large larvae are difficult to control.

“Producers may have a false sense of security with the seed-applied insecticides or Bt corn,” he said. “As many learned the hard way last year, these technologies don’t stand up to severe infestations, which certainly are a possibility this year.”

Infestations and black cutworm development are first determined by the moth trapping network, then by monitoring heat units once critical catches are recorded. Heat units are calculated as the difference between the average daily maximum and minimum temperatures and a critical developmental threshold temperature. For black cutworm, that threshold is 50°, meaning it takes about 300 heat units from egg hatch to the time when larvae begin to “cut” or severely damage plants.

In the southern part of the state, accumulated heat units already are well above 300 and are rapidly approaching that number in the central and northern parts. With most of the state’s corn crop already in the ground, Obermeyer said scouting now is crucial.

“Scout by inspecting 20 consecutive plants in each of five areas of a field for cutworms and feeding activity,” he said. “Be sure to check areas that had an accumulation of weedy growth before or at the time of planting.”
Obermeyer also said growers need to collect black cutworm larvae to determine the average instar, or development stage.

“A foliar, rescue insecticide may be necessary if three percent or more of the plants are damaged and black cutworm’s average larval instar is 4-6,” he said.

A larval instar guide is available inside the front cover of the Purdue Extension Corn and Soybean Field Guide, available for $7 through Purdue Extension: The Education Store here.

Pheromone trap reports and updated heat accumulation graphics are available in Purdue Extension’s Pest and Crop Newsletter here.

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