2024 University of Minnesota Cooperative Black Cutworm Trapping Network
Report #4 April 27 – May 3, 2024
For more information: https://z.umn.edu/bcw-reporting
May 9, 2024
Greetings:
Black cutworm (BCW) moths continue to take advantage of spring weather systems to move into MinnesotTraps in Houston, Jackson, Redwood, and Nicollet Counties had significant captures. Most of the larger captures occurred April 27 to 28. For example, a trap in western Nicollet County had a 23-moth capture on April 27 (Figure 1, Table 1,2).
Counties with significant captures (8 or more moths /two nights) over the trapping season so far can be seen across the southern part of the state. A cluster of Minnesota River Valley counties has seen unusually high (for Minnesota) captures (Figure 2). Migrating moths do not drop uniformly across the landscape. Although most trap sites have caught at least one moth, the mosaic of captures are a reminder to not place too much confidence in any one trap.
Begin scouting corn as it emerges. Degree day predictions to help time scouting for 2024 moth captures can be found in Table 2.
The larvae from moths arriving before April 17 should be hatched or hatching now. 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 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.
On the other hand, the growing point of dicot crops, such as soybeans and sugarbeets are above ground when they emerge. Small sugar beets can be cut by larvae smaller than the 4th instar and some of the largest BCW moth captures were in areas of southern Minnesota where that crop is grown.
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:
- The field was unworked soybean stubble when significant flights occurred (Table 2) and/or has a history of early season weeds such as lambsquarters (Figure 3) 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 tunnelling into the growing point.
- Corn rootworm insecticides and seed treatments provide varying levels of BCW control.
You can find additional scouting information in black cutworm on corn.
True armyworm (TAW)
Armyworm moth migration detections in pheromone traps continue to be lower than black cutworm. However, this is starting to change. Blue Earth Co. captured 2 TAW moths, Stearns and Polk Co. captured single moths and the McLeod Co. trap captured 9 moths this trapping period. More recently, a LeSuer Co. trap captured 4 moths on May 4 and a Cottonwood Co. trap had a 2-night capture of 10 moths on May 8.
Why is there a difference between when and where BCW and TAW show up? The two species may use different weather systems to aid northward movement because the two species do not winter in the same areas and/or the moths are ready to migrate at different times. Populations of the two species may have different overwinter and early spring success.
The growth of winter cereals and grasses is quite lush this spring where I’ve already started to see lodging. This is a great egg-laying habitat for any TAW moths that arrive. TAW moths lay eggs and their larvae feed on grasses and grass crops. However, starving larvae occasionally feed on soybeans and other broadleaves when a rye cover is terminated. Stay tuned.
The TAW’s tropical cousin the fall armyworm (FAW) also migrates into Minnesota but much later in the year. FAW has found its way to Africa by 2016 and now threatens crops there. In contrast to TAW, some FAW larvae are able to develop well on soybeans and other broadleaves when a grass host is nearby.
This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.
Until next week,
Bruce Potter