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Pay close attention to potential cutworm feeding when scouting this spring

Bruce Potter, IPM specialist

Figure 1. Black cutworm moth season captures to
 May 10, 2024. Shading represents the maximum
 two-night captures for trap(s) in the county.
Migrating black cutworm (BCW) moths continue to arrive in MN, based on the work of a network of pheromone traps run by cooperators which has been tracking their arrival. The earliest significant capture occurred in Brown Count April 8. Since then, trap locations across southern Minnesota have had significant captures (Figure 1). Some of these, particularly in the Minnesota River Valley, continue to be unusually high. This week, even Polk County, the northern most trap in the cooperative trapping network, had a significant capture. The trap network indicates, but does not guarantee, the potential for greater than usual black cutworm activity, and over a wide area.

The larvae from moth arrivals before April 17 will be at, or near 2nd instar. These larvae will still be too small to cut corn, instead feeding on leaves of corn or small weeds. Pay attention when you see pieces of corn or weed leaves that are missing or on the ground.

Once the growing larvae have molted three times and reach the 4th instar stage, they are large enough to cut off small corn. When the cutting occurs at or above the growing point the injury will have little effect on corn. However, BCW larvae have a nasty habit of feeding below ground at or below the growing point. Leaf feeding from these larvae should be increasingly visible and some of these larvae should be able to cut small corn by the last week of May. The larvae will be active until mid-June. Large larvae can kill five leaf corn by cutting below or tunneling into the growing point.

On the other hand, the growing point of dicot crops, such as soybeans and sugarbeets are above ground when they emerge. Small sugarbeets can be killed by larvae smaller than the 4th instar. Some of the largest BCW moth captures were in areas of southern Minnesota where that crop is grown. Soybean yields have a greater ability to compensate for reduced stand than corn or sugarbeets. However, pockets of yield reducing stand loss can occur when cutworm density is very high.

Begin scouting corn as it emerges

Temperature based predictions to help time scouting for 2024 moth captures can be found in Table 1. Of course, any larvae from later arriving moths will be earlier in development, and later in crop injury. Keep in mind, these degree day projections are only guidelines and the temperatures the eggs and larvae are exposed to vary with solar radiation, crop residue and soil moisture.

It is important to find black cutworm infestations before stand loss occurs but most of us don’t have time to wander aimlessly through fields hoping to find a cutworm. The following checklist could help you determine which fields are at greater BCW risk:
Figure 2. Where weed and insect management meet.
Some healthy common lambsquarters were attractive
 to egg-laying BCW moths well before you noticed them.
  1. The field was unworked soybean stubble when significant flights occurred (Table 1) and/or has history of early season weeds such as lambsquarters (Figure 2) or has an emerged cover crop.
  2. The field is in or near a county where a significant moth flight was detected, especially where rainfall events were similar. Because the trapping network is not capable of detecting all localized immigrations, it is always important to be alert to cutworm injury.
  3. Corn fields planted to a hybrid without an above ground Bt trait. These traits can be overwhelmed when they are attacked by large numbers of large BCW larvae; a scenario that occurs when large larvae move from dense weeds to corn.
  4. Corn with less than five leaves is at greatest risk. More rarely, somewhat larger corn may be killed by larvae tunneling into the growing point.
  5. Corn rootworm insecticides and seed treatments provide varying levels of BCW control.
Table 1.  Significant moth captures, biofix dates, post-flight degree day accumulations, and estimated dates of corn leaf feeding, cutting and projected end of cutting by county and based on historical average temperatures.
County 2-night
Approx. post-flight
as of May 10
current max
BCW stage
Estimated start
of corn leaf
start of corn
Projected end
of cutting3
Brown 8 Apr 8 205 2nd instar Apr 23 May 19 Jun 11
Steele2 8 Apr 14 186 2nd instar Apr 30 May 22 Jun 13
Renville2 10 Apr 16 136 1st instar May 6 May 26 Jun 15
Redwood2 9 Apr 17 132 1st instar May 6 May 25 Jun 15
Nicollet1 11 Apr 17 134 1st instar May 6 May 25 Jun 15
Renville1 12 Apr 17 126 1st instar May 7 May 27 Jun 16
Swift1 8 Apr 17 118 1st instar May 7 May 27 Jun 17
Renville1 26 Apr 26 92 1st instar May 10 May 29 Jun 18
Renville2 18 Apr 26 92 1st instar May 10 May 29 Jun 18
Houston1 11 Apr 27 101 1st instar May 9 May 29 Jun 18
Jackson1 10 Apr 27 91 1st instar May 10 May 29 Jun 18
Nicollet1 24 Apr 27 85 egg May 11 May 29 Jun 18
Pipestone1 9 Apr 29 72 egg May 12 May 30 Jun 20
Redwood1 8 Apr 29 81 egg May 11 May 29 Jun 18
Martin2 13 May 7 39 egg May 14 Jun1 Jun 20
Nicollet1 11 May 7 41 egg May 14 Jun 1 Jun 20
Renville1 24 May 7 39 egg May 14 Jun 2 Jun 21

1Based on 90 degree-days (base 50F) after significant flight (leaf feeding begins)
2Based on 312 degree-days (base 50F) from significant flight. 4th-6th instar larvae are large enough to cut corn. Small plants, e.g. sugarbeets, can be cut earlier.
3Based on >641 degree-days (base 50F) after significatn flight pupation.
Source of degree day projections: Midwest Regional Climate Center U2U

For more information

You can find additional scouting information in black cutworm on corn. Previous and future weekly reports can be found at

This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.

Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.

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