Beware black cutworms

Tracking migrating insects. Some important corn pests such as black cutworms, armyworms and corn earworms survive the winter in more southerly regions, migrating northward each spring with the aid of weather systems.

Each year, members of the University of Illinois Extension field staff work with the Illinois Natural History Survey's Kelly Estes to gauge local and state-wide risk of insect damage to corn and soybean. This is done through the use of pheromone traps.

Black cutworm (BCW) moths are attracted to the sex pheromone lure and are trapped in glue when they investigate the trap expecting to find a mate (Figure). In 2014, the BCW trap at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) was installed on March 31.

Weather complications. Unfortunately, portions of this trap were lost in wind events on both April 4 and 28. During several stretches in April cool night time temperatures likely grounded many moths that would have otherwise been out searching for a mate.

Projecting into the future after a significant flight. Although a significant flight occurred on April 15, lost traps may mean that additional significant black cutworm flights may not have been detected.

Significant flights indicate that high populations of BCWs have arrived in an area and have begun seeking mates. After mating, BCW females lay their eggs on weeds. If these weeds happen to be in a corn field when the eggs hatch, larvae begin to feed on corn, initially causing small pin-sized holes.

Just like corn plants that need to accumulate degree days to reach certain points in their life cycle, the same concept applies to BCWs. It takes approximately 300 degree days for larvae to reach a size (4th instar) where they can begin to cut corn plants (Figure). If a corn plant is cut below the growing point, it will not survive. Corn is most at risk for economic damage between the first and fourth leaf stage of growth.

According to historical weather data at the NWIARDC, we would expect to reach 300 degree days, and black cutworms to begin cutting around May 18.

Black cutworm management. Fields most at risk for black cutworm injury include those heavily infested with winter annual weeds. Scouting corn fields starting near May 18 can help producers to identify potential problem fields before economic injury has taken place.

University of Illinois Entomologists stress that decisions about whether to spray insecticides should not be based projected cutting dates alone. Certain Bt traits do provide protection, but under heavy infestations, may not provide adequate control. Scouting is recommended before considering an insecticide treatment.

For more information.

  • A fact sheet about BCW is available on the University of Illinois IPM website.
  • If you are interested in BCW trap counts for other areas of the state region, please visit the North Central IPM website.