Bruce Potter, IPM specialist
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.
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| Figure 1. Black cutworm moth season captures to May 10, 2024. Shading represents the maximum two-night captures for trap(s) in the county. |
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:
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:
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| Figure 2. Where weed and insect management meet. Some healthy common lambsquarters were attractive to egg-laying BCW moths well before you noticed them. |
- 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.
- 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.
- 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.
- 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.
- 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 capture |
Biofix date |
Approx. post-flight degree-days as of May 10 |
Estimated current max BCW stage |
Estimated start of corn leaf feeding1 |
Estimated start of corn cutting2 |
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 https://mygeohub.org/groups/u2u/purdu_gdd
For more information
You can find additional scouting information in black cutworm on corn. Previous and future weekly reports can be found at https://z.umn.edu/bcw-reporting.
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.
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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