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Black cutworms are in northwest Minnesota

Pheromone traps.

Some corn and soybean pests such as black cutworms (BCW), armyworms and corn earworms are unable to survive the Minnesota winter, migrating northward each spring with the aid of weather systems.

Each year University of Minnesota Extension IPM specialist Bruce Potter coordinates and reports on the monitoring of BCW moths migrating into Minnesota through a network of pheromone traps.  These traps do two things, attract and trap moths.  Small pieces of rubbery lure material are impregnated with synthetic sex hormones (pheromones) like those that are given off by female BCW moths.  Male moths are attracted to the lure and are trapped in glue when they investigate the trap expecting to find a mate (Figure).

Figure. Black cutworm moth captured in a pheromone trap (photo: Angie Peltier).

Those that are monitoring traps will identify and count the BCW moths, identifying the point in the BCW migration when there are enough moths back in the area for mating and egg laying to occur.  Catching eight BCW moths over a two day period is called a significant flight or biofix.  Like other cold-blooded animals, it is the temperature of the environment that dictates how quickly different developmental stages in the BCW life cycle will occur after egg laying.

I have participated in this trapping network since arriving nearly three years ago. Working from home due to the COVID-19 pandemic has had one positive side-effect. While I would typically lose the trap at some point each year due to the strong open country winds at the Northwest Research and Outreach Center, this year I set the trap up just outside of Crookston in my yard which is surrounded by shelter belt trees and so was protected from high winds. 

Projecting into the future after a significant flight.

Just like corn plants that need to accumulate growing degree days to reach certain points in their life cycle, the same concept applies to BCWs.  The reason this matters is because there are only certain developmental milestones that are important for causing stand loss in corn.  BCW that have reached the fourth larval stage (instar) are large enough to cut corn seedlings and plants cut below the growing point will not survive.  If a larva burrows into a plant’s growing point, the plant may also die.  Corn is most at risk until the 6th leaf collar (V6) growth stage.  Until corn has developed its seventh leaf collar, larvae can continue to be a cutting risk through the remainder of the larval stages until they pupate.

A significant flight of eight BCWs were caught in my backyard over the nights of June 7 and 8.  According to historical local weather data and BCW degree days Bruce Potter projects that egg hatch would occur on June 13.  The third larval stage (instar) that is nearly large enough to cut corn (and should get one scouting) is projected occur on June 26 and BCWs are projected to pupate on July 15.

Black cutworm management.  

UMN IPM specialists stress that decisions about whether to spray insecticides should not be based projected cutting dates alone.  Certain Bt traits do provide some protection in corn, but under heavy infestations, may not provide adequate control.  Unless crops have outgrown their period of vulnerability, scouting corn fields starting around June 26 can help producers to identify potential problem fields before economic injury has taken place.  Weekly scouting until the fifth leaf (V5) growth stage in soybean and the sixth leaf collar (V6) stage in corn is recommended before considering an insecticide treatment.

For more information.

Ostlie, K. and Potter, B. Corn Pest Management. Black Cutworm.
Koch, R. and Wold-Burkness, S. Soybean Pest Management. Black Cutworm.
DeFonzo, C. Editor. 2020. Handy Bt Trait Table.
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