2020 UMN Cooperative Black Cutworm Trapping Network Report #8
2020 University of Minnesota Cooperative Black Cutworm Trapping Network
For more information: https://z.umn.edu/bcw-reporting
June 1, 2020
Because of the early planting progress, many cooperators had stopped trapping before the May 16-22 trapping week. However, the traps still operating moths in those areas that received heavier rains saw an influx of (Figure 1, Table 1). Most of this spring’s significant trap captures (Figure 2) can be correlated with rain brought by systems originating in the south.
Other than late-planted sweet corn and soybeans, these latest captures will be too late to affect most rapidly growing crops.
Larvae from early flights should be 4th instar stage and large enough to cut small corn at this time, those from later significant flights May 6- 20 able to cut small corn after June 7 (Table 2).
This week’s heat and the abundant moisture (in many areas) will allow rapid crop growth. The largest corn I‘ve seen is late four-leaf and soybeans pushing the 1st trifoliate.
In addition to black cutworms, light and pheromone traps have been picking up chronic low – moderate levels of armyworms, another migratory insect that we track.
Date County with capture
May 16 Dodge (6), Olmsted I, Nobles (3), Renville, Steele (2)
May 17 Sibley (3), Olmsted I, Dodge (6), Steele (7)
May 18 Brown (13), Meeker (3), Olmsted I (4), Olmsted II (3), Redwood (9), Sibley (2), Wabasha, Waseca I (8)
May 19 Freeborn (8), Kandiyohi (2), Meeker, Olmsted II, Renville (4), Wabasha, Waseca I (8)
May 20 Freeborn, Olmsted II, Nobles (2), Wabasha, Waseca I (2)
May 21 Freeborn, Jackson (3), Nobles (2)
May 22 Blue Earth (7/3nights), Meeker
Table 1. Positive captures in pheromone traps reported April 25- May 1. Numbers in parentheses are multiple captures for that trap and date.
Table 2. Estimated black cutworm development for significant moth captures as of May 22. Current and forecast degree-days are generated from the U2U Corn Growing Degree-Day application at the Midwest Regional Climate Center https://mrcc.illinois.edu/U2U/gdd/ using current year and NWS forecast temperatures.
Scouting for black cutworm
Scouting for cutworms is easily combined with stand evaluations and scouting weeds for herbicide selection and application timing.
The first sign of black cutworm damage is leaf feeding on emerged corn or weeds. Sometimes, larvae will cut weeds before they move to corn. Any partially cut plants will wilt.
Be wary when lambsquarters and ragweed patches begin to disappear without the aid of an herbicide. Herbicide applications may cause cutworms to switch from feeding on weeds to corn.
The leaf feeding and missing or cut plants caused by cutworms are not hard to see but it is useful to find a few of the larvae that caused the damage to determine size and species.
Cutworms are nocturnal. During the day, they hide under soil clods, crop residue, and loose soil, typically at the boundary between dry and moist soil. Cutworms will likely be deeper when soils are dry. Carefully look under pieces of residue and soil clods close to cut or injured plants. If you don’t find a cutworm near the base of an injured plant, look near a couple plants on either side in the row. Using an opossum-like defense strategy, most cutworm species roll into a semicircle and remain motionless when disturbed. Unfortunately for crop scouts and other predators, most cutworm species, including black cutworms, are more or less soil colored.
Finding cutworms in high residue, cloddy or muddy conditions is especially difficult. With leaf feeding you are looking for very small larvae. If unsuccessful, move to another area with injured plants. Looking at this optimistically, you only need to find a few to make your treatment decision. In moist, high organic matter soils, don’t mistake harmless headless, legless cranefly larvae for cutworms. In high residue situations, damage from slugs and millipedes can resemble cutworm leaf feeding. All cutworm species have a distinct head capsule and 3 pairs of true legs near the front with fleshy abdominal prolegs at the back.
Do stand counts in areas of the field with damage and note the percentage of plants with leaf feeding and those cut. To help with your decision, you can flag areas of row within the field and return the next day to determine if damage is ongoing.
Cutworm infestations in small corn (3 leaf or less) require more aggressive management than large corn. Larger cutworms can attack larger plants. Don't give up on scouting too early. Late-instar black cutworms can kill up to 6 collar corn by burrowing into the growing point.
BT hybrids, seed applied, and at-plant insecticides
Bt hybrids containing the Cry1F protein (Herculex /HX1) or Vip3a protein (Viptera), alone or in stacks, are labeled as controlling black cutworm. While they reduce risk, they might still be damaged under heavy cutworm pressure. An at-plant insecticide is probably not that helpful for cutworms when added on these hybrids. Remember, the Cry34/35 Ab1 (Herculex RW protein) is not the same as the Cry1F above-ground protein.
High rates of neonicitinoid seed treatments (e.g. Poncho, Cruiser, Gaucho) are very effective on many seed and seedling insects and they can provide some protection against black cutworm. They may not always provide satisfactory cutworm control. Some recently labeled seed applied diamide insecticides (e.g. Lumivia) also provide some control.
Large numbers of late-instar cutworms moving from weeds to take a bite of corn can overwhelm insecticides and Bt in corn tissues.
Soil applied at-plant insecticides can provide some control of cutworm larvae. However, they are not recommended as insurance applications for two reasons. At planting, it is difficult to predict which individual fields will have economically damaging cutworm infestations. Secondly, post-emerge insecticide rescue treatments work very well.
Fortunately, cutworms are controlled well with rescue insecticide applications and many post plant insecticide products provide effective control of black cutworms. Spot treatments can be effective when combined with careful scouting. Make sure you still have cutworms present if you make a decision to treat. In springs when the top several inches of soil are dry, black cutworms tend to remain lower in the soil profile and insecticides are less effective. In dry conditions, a rotary hoe or row cultivation can help improve insecticide efficacy by incorporating insecticides and encouraging cutworm movement.
Be cautious of potential interactions between organophosphate insecticides (Counter 20G is one example) and some corn herbicides. Scouting and rescue insecticides applications are the best defense against yield loss from black cutworms.
The following parameters can help you focus your scouting efforts on the highest risk fields:
- Later plantings and in the case of corn fields that are still five leaves.
- Corn fields without a Bt trait effective against black cutworm
- Fields with a history of winter annual and early-emerging annual weeds
- Minimum tilled fields or fields with full width tillage that were not tilled and planted when significant flights arrived in a nearby county.
We would like to thank this year’s trap cooperators (Table 3). I apologize if I missed someone that assisted or were delegated to run a traps! We appreciated their efforts and would hope that you grow or otherwise work with crops in MN do to!
This is the last issue of this newsletter for 2020 unless there are reports of damage.
Please let us know if you see cutworm damage in your crops.
Bruce Potter and Travis Vollmer
This project is supported, in part, by the farm families of Minnesota and their corn check-off investment.