Pest alert: Black cutworm

The UMN Extension IPM program sets traps for black cutworm Agrostis ipsilon Hufnagel (Lepidoptera: Noctuidae) around the state each year, which are monitored by Extension educator Bruce Potter and a team of volunteers. In 2019, high numbers have been observed across the state but particularly in Southern MN. Coupled with late planting dates, cutworms may pose a challenge to growers this year. 

While cutworm is typically considered a corn pest, they can cause damage to vegetable crops as well. While most reports have cited cutworm feeding on plants in the cucurbit family (cucumbers, pumpkins, and melons), they also feed on beets, carrots, cucumber, leafy greens, peas, potato, pumpkin, snap beans, and sweet corn. Black cutworm moths fly north to Minnesota from early May through June, and lay their eggs in and around fields. As the larvae develop over approximately a one month period (with six to nine larval instars), they damage plants by feeding on the roots and stems, and young plants are especially vulnerable. 

Photo: Adam Sisson, Iowa State University, Bugwood.org

A few tips for managing cutworm include: 

1 Transplanting: If you're transplanting, wait until plants are large enough to sustain some feeding damage. Rather than transplanting at the 2-3 leaf stage, it can help reduce the effects of feeding if you can wait until the 5-6 leaf stage. At this stage, the root systems and stems are larger and the plants are less likely to die from feeding damage.

2. Weed management: In general, good weed management can help to reduce populations. Cutworm moths often lay their eggs on weeds before spring planting, and by reducing weed populations, the field will be less attractive for egg laying. Winter annuals and early season spring weeds are common sites for egg laying, especially lambsquarter. Moths can also lay eggs in crop residues left in fields.   

3. Diatomaceous earth can be used to keep cutworm out of your fields. This can be a bit expensive, but you could place some at the base of each transplant, or just put it in rows with the most susceptible crops. 

4. High tunnel prevention: We saw cutworm larvae very early this year (early May) in a high tunnel in Long Prairie MN. If you have a high tunnel, keep an eye out next year for early damage. The warmer environment in high tunnels allows larvae to develop more quickly in high tunnels than in field conditions. If plants are grown on plastic, the plastic may provide additional protection to the larvae, making control more difficult. Given the high value of high tunnel crops, this is an environment where diatomaceous earth applications might make sense, and where weed management is absolutely critical. 

5. Chemical control: Economic thresholds recommended by the University of Wisconsin include: 

·         Snap bean= 2 larvae/ row foot

·         Potatoes= 4 larvae/row foot

·         Sweet Corn= >5% of plants damaged

·         Leafy greens= <3% of the stand affected

For organic systems, a soil drench with pyrethrum + neem (both organic approved insecticides) would provide some, but not perfect control. For a complete list of insecticides that can be used on black cutworm in each crop, see the 2019 Midwest Vegetable Mangement Guide for Commercial Growers. 

For more info on black cutworm, see the Black Cutworm Reporting Network page.  (This is written in the context of corn growers, but all of the info about life cycles is relevant to fruit and vegetable growers).  

Author: Natalie Hoidal, Extension Educator, Fruit and Vegetable Production Sytems, hoida016@umn.edu. Thanks to Eric Burkness of the UMN IPM program for reviewing this article.