2020 UMN Cooperative Black Cutworm Trapping Network Report #7

2020 UMN Cooperative Black Cutworm Trapping Network Report #6

For more information: https://z.umn.edu/bcw-reporting

May 21, 2020

Greetings:

BCW moth countsCooperator’s pheromone traps picked up scattered black cutworm moths moving into Minnesota throughout the trapping week of May 9 -15 with another significant flight arriving May 13-14. (Figure 1, Table 1).  Traps in Martin, Meeker, Mower, Renville and Waseca Counties had significant captures. The system that brought rain last weekend also brought additional moths (not included in this week’s report). 

Date

County with capture

May 10

Blue Earth (3), Houston, Olmsted, Steele, Waseca, Waseca II

May 10

Blue Earth (3), Houston, Olmsted, Steele, Waseca, Waseca II

May 11

Mower (2), Olmsted (2), Waseca I, Waseca II, Yellow Medicine

May 12

Freeborn, Olmsted II (2), Steele, Waseca I, Waseca II

May 13

Faribault, Renville (6), Steele, Swift, Waseca I, Waseca II (2), Watonwan I, Watonwan II (4)

May 14

Blue Earth, Freeborn I, Freeborn II, Martin I (3), Martin II (7), Meeker (9), Mower (7), Olmsted I, Renville (6), Sibley (2), Steele I (4), Waseca I (12), Waseca II, Watonwan I (1), Watonwan II (4)

May 15

Chippewa, Freeborn, Houston (3), Martin, Mower, Nobles (2), Steele II (1)

Table 1.  Positive captures in pheromone traps reported April 25- May 1. Numbers in parentheses are multiple captures for that trap and date.

The cooler weather the past couple of weeks delayed crop and cutworm development. Cutworms from the earliest flights should still be too small to cut corn plants and flights May 6 and later are likely still unhatched (Table 2).

Some of you may wonder why these pheromone traps only capture black moths for a couple days after the flight. Remember, the pheromones only attract unmated males. Egg-laying female moths are still around. They are still attracted to and can be captured by black light traps.

Identifying black and other cutworms in MN

Black cutworms are not the only cutworm species than can injure crops in Minnesota. As corn (and other row crops) germinate and begin to emerge they can be attacked by several species of cutworms. Unlike many cutworm species that overwinter in Minnesota as eggs or larvae, black cutworms migrate into the state each spring (Table 3).  There is evidence that similar to many agriculturalists, some black cutworm moths migrate south to warmer climates in the fall.

We can project cutting dates for the black cutworm but corn should be scouted for other cutworm species as soon as it emerges. Because cutworms that overwinter, particularly those that winter as larvae, begin development before migrant black cutworms arrive, they are ready to feed on corn early. Often, the first corn leaf feeding observed in the spring is from overwintered dingy cutworm larvae.

Certain species prefer particular habitats (Table 3). For example, sandhill cutworms are found in sandy soils and several species tend to be problems in crops planted into sod. Dingy cutworms are often abundant when corn is planted after alfalfa or fields that were weedy the previous year.

Species identification is important to determine damage potential. Small larvae of all species feed on weeds and leaves and cannot cut corn. Dingy, redbacked, spotted, and variegated cutworms are primarily leaf feeders feeding at or above the soil surface. Consequently, these climbing cutworms usually cut corn above the soil line and growing point and the corn plant recovers.

Table 3:  Some cutworm species that can be found in Minnesota corn

Species

Eggs Laid in

# Generations

Overwinters As

Likely Habitat

Black

SPRING-SUMMER

3

ADULTS MIGRATE

LATE-TILLED FIELDS, EARLY WEEDS

BRONZED

FALL

1

EGGS/LARVAE

AFTER SOD

CLAYBACKED

FALL

1

larvae

AFTER SOD

darksided

summer

1

eggs

after weedy crop

dingy

summer-fall

1

larvae

after sod, alfalfa, weedy fields

glassy

summer-fall

1

larvae

after sod

redbacked

fall

1

eggs

after weedy crop

sandhill

summer-fall

1

larvae

sandy soils

variegated

spring-summer

2

adults migrate

in and after alfalfa, weeds

However, unlike the climbing cutworms, the larvae of some cutworm species (e.g. glassy, sandhill, darksided, claybacked, and black) tend to feed below ground at or below the growing point. This potential for its feeding to kill corn plants makes black cutworm a threat. When larger larvae tunnel into the growing point, corn as large as five or six collars can be killed.

BCW defensive C-shape

With a bit of practice, the black cutworms are easily distinguished from many other pictures by the size of paired black bumps (tubercles) on the upper edges of each segment. These tubercles are unequal in size on the black cutworm (Figure 2).

Other Minnesota insects that cause damage to larger corn and might be confused with cutworm include the hop vine borer and common stalk borer.

Particularly when scouting mucky, high organic soils, be aware of crane fly larvae.  These large fly larvae can be mistaken for cutworms but they do not have a head capsule or legs (Figure 3).

You can find  Images of several cutworm species the Black Cutworm Facts publication. A very good reference for cutworms in the northern plains is:  Floate, K.D.. (2017). Cutworm Pests of Crops on the Canadian Prairies: Identification and Management Field Guide.  

 Crane fly larvae

Economic thresholds – When to treat a problem

Cutworms reduce yield by decreasing final stand or plant population. The generic economic threshold for black cutworm in corn is 2 to 3 percent of the plants cut or wilted when the larvae are less than ¾ inch long. The threshold increases to 5% cut plants when larvae are larger. However, with high corn prices, these thresholds could be lowered to 1% wilted or cut for small larvae and 2-3% wilted or cut for large larvae.  Acceptable plant stand also determine black cutworm economics on other crops also.

Remember to take into consideration corn populations in individual fields and adjust threshold numbers accordingly. For example, if the current plant population is at or near yield limiting levels, you can afford to lose fewer plants than in a field with a higher emerged population. The role of corn plant stands in determining yields can be found in Table 4.

The reason the black cutworm economic threshold varies by larval size is based in larval feeding. Cutworms must shed their skins (molt) in order to grow. The stage between molts is called a larval instar. Cutworms will begin to cut corn at the 4th instar (~1/2 inch long). The smaller larvae tend to cut corn at or near the soil surface while larger larvae tend to feed below ground. The larvae are full grown and cease feeding between 1½ and 2 inches long. While larger larvae will cut or tunnel into larger plants, they have less time left to feed and as a result have the potential to cut fewer plants. Table 5 gives approximate sizes in length and width of the head for black cutworm larvae.

Table 6 presents a more comprehensive method for determining economic yield loss for corn that includes factors corn stage, black cutworm larval stage, and expected crop price.

Table 4. Corn yield response to plant population (Morris, Lamberton, and Waseca, 2009-2011).Source: Corn grower's guide for evaluating crop damage and replant options
Final corn stand (plants/acre) Expected yield (%)
44,000 100
41,000 100
38,000 100
35,000 100
32,000 100
29,000 99
26,000 96
23,000 92
20,000 87
17,000 81

The reason the black cutworm economic threshold varies by larval size is based in larval feeding. Cutworms must shed their skins (molt) in order to grow. The stage between molts is called a larval instar. Cutworms will begin to cut corn at the 4th instar (~1/2 inch long). The smaller larvae tend to cut corn at or near the soil surface while larger larvae tend to feed below ground. The larvae are full grown and cease feeding between 1½ and 2 inches long. While larger larvae will cut or tunnel into larger plants, they have less time left to feed and as a result have the potential to cut fewer plants. Table 5 gives approximate sizes in length and width of the head for black cutworm larvae.

Table 5.  Black cutworm body and head capsule sizes by instar stage.
INSTAR
Body length(mm) Head capsule width (MM)
1 1 - 2 0.3
2 3 - 6 0.5
3 7 – 9 0.6 – 0.8
4 12 – 25 1.1 – 1.5
5 25 – 37 1.8 – 2.4
6 30 – 35 2.5 – 3.3
7 31 – 50 3.6 - 4.3

Table 6 presents a more comprehensive method for determining economic yield loss for corn that includes factors corn stage, black cutworm larval stage, and expected crop price.

Table 6:  Yield loss factors and equations to calculate the profitability of a rescue insecticide treatment for black cutworm.  Source: http://extension.cropsciences.illinois.edu/fieldcrops/insects/black_cutworm/
Avg.    yield loss factor for calculating corn yield loss
Larva  Moisture not limiting Moisture limiting
instar Number of corn leaves Number of corn leaves
  1 2 3 4 5 1 2 3 4 5
3 2.4 1.8 0.8 0.7 0.7 1.6 1.2 0.4 0.4 0.2
5 1.2 1.4 0.6 0.3 0.3 0.8 0.9 0.3 0.2 0.2
6 0.7 0.2 0.1 0.1 0.1 0.5 0.2 0.1 0.0 0.0

PROJECTED BU/A YIELD LOSS=
Projected $ loss/a = _______bu/a loss  x $_______(price/bu)
preventable loss/a = $_______Projected loss/a x _______% Control**(95% control with adequate moisture, 80% control with limited moisture
$ return (+/-) for insecticide treatment = $_______ preventable loss/A - $_______ control cost/a
  1. Determine average instar of the black cutworm larvae and corn leaves (collars).
  2. Consider soil moisture inadequate if the top 3-4 inches are dry and rain is not forecast.

We will have one more planned issue of this black cutworm report for 2020.

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.