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Black cutworm

Find information about black cutworm in Minnesota corn, including their characteristics, habitat, at-risk fields, signs of damage and strategies for managing infestations.

Where they live

The black cutworm – Agrostis ipsilon Hufnagel (Lepidoptera: Noctuidae) – is widely distributed in the temperate regions of the world. It can’t survive winters in Minnesota or other latitudes with freezing winter temperatures. In these areas, migrant moths produce annual infestations each spring.

Hosts

Black cutworm adults feed on plant nectar. In addition to corn, the larvae feed on a wide range of broadleaf and grass crops and weeds.

Black cutworm moth
Figure 1: Black cutworm moth. Note color pattern and dagger marking. Photo: Mark Dreiling, Bugwood.org.
 Black cutworm larvae
Figure 2: Black cutworm larvae. Note head capsule, true and prolegs and tubercles near back. Photo: John Capinera, University of Florida, Bugwood.org

Characteristics and life cycle

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Crop damage

The larva is the damaging stage and damages plant tissue by feeding with chewing mouthparts. The potential for feeding black cutworm larvae to kill plants, thereby reducing stand and potentially yield, makes large infestations of black cutworm a serious threat to corn and other crops.

Larvae are active mainly at night. Small larvae feed on leaves, creating irregular holes, and can cut small weed seedlings.

While feeding near or below the soil surface, fourth instar and larger larvae can cut off corn plants (Figure 5), sometimes dragging the cut plants below ground. Plants cut above the shoot apical meristem (growing point) usually recover.

Black cutworm damage
Figure 5: Black cutworm damage to a young corn plant. Photo: W.M. Hantsbarger, Bugwood.org.

At-risk fields

Dry soil conditions can encourage belowground cutting, at or below the growing point. Late-planted or corn delayed by cold weather conditions can be cut off by waiting cutworms before the corn emerges.

Although too large for even late instar larvae to cut off, corn plants larger than five collars can be killed by late instar larvae tunneling into the meristem. The last two larval instars consume of the most of the plant biomass.

Natural enemies

A range of Dipteran and Hymenopteran and nematode parasites have been isolated from black cutworm larvae. Viral and bacterial diseases can also infect cutworms.

In addition, bird, mammal and insect predators (ground beetles) impact cutworm larval populations. Birds, bats and motor vehicles prey on adults.

Conditions for infestations

Yield-limiting black cutworm infestations are relatively rare in Minnesota and when they do occur, require several factors to coincide:

  1. A large number of moths produced in the overwintering areas.

  2. The proper weather systems, at the right time, to aid moth migration into the state.

  3. Attractive and suitable sites for egg laying that will be planted are planted to susceptible crops (e.g., late emerging corn).

  4. Conditions favorable for black cutworm egg and larva survival.

Although infestations can be devastating, the rarity of black cutworm problems indicates that insurance management tactics for black cutworm seldom pay.

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Predicting damage

You can predict black cutworm development and damage using pheromone traps and degree-days.

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Scouting

Scouting for cutworms is easily combined with stand evaluations and scouting weeds for herbicide selection and application timing.

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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 3/4 inch long.

The threshold increases to 5 percent cut plants when larvae are larger. However, when corn prices are high, these thresholds could be lowered to 1 percent wilted or cut and small larvae and 2 to 3 percent wilted or cut for large larvae.

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’t afford to lose as many plants as in a field with a higher emerged population. The role of corn plant stands in determining yields can be found in Table 3.

Table 3: Corn yield response to plant population

Final corn stand Expected yield
44,000 plants per acre 100%
41,000 plants per acre 100%
38,000 plants per acre 100%
35,000 plants per acre 100%
32,000 plants per acre 100%
29,000 plants per acre 99%
26,000 plants per acre 98%
23,000 plants per acre 92%
20,000 plants per acre 87%
17,000 plants per acre 81%

Table values are from Morris, Lamberton, and Waseca, from 2009 to 2011. Source: Bruce Potter

The black cutworm economic threshold varies by larval size because it’s based on 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 fourth 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 1.5 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 4 gives approximate sizes in length and width of the head for black cutworm larvae.

Table 4: Black cutworm body and head capsule sizes by instar stage

Instar Body length Head capsule width
1 1-2 millimeters (mm) 0.3 mm
2 3-6 mm 0.5 mm
3 7-9 mm 0.6-0.8 mm
4 12-25 mm 1.1-1.5 mm
5 25-37 mm 1.8-2.4 mm
6 30-35 mm 2.5-3.3 mm
7 31-50 mm 3.6-4.3 mm
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Bt hybrids, at-plant insecticides, and seed treatment

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Cultural control

Maintain good early-season weed control to reduce the attractiveness of fields to egg-laying females.

Tillage after eggs have been laid will have minimal effect on egg and larval survival.

Other cutworm species and affected crops

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.

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Always read and follow the pesticide label.

 

Products are mentioned for illustrative purposes only. Their inclusion doesn’t imply endorsement, nor does their absence imply disapproval.

Ken Ostlie, Extension entomologist and Bruce Potter, integrated pest management specialist, Southwest Research and Outreach Center

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