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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.

Distribution

The black cutworm – Agrostis ipsilon Hüfnagel (Lepidoptera: Noctuidae) – is widely distributed in the temperate regions of the world. Although a native of North America, it can’t survive winters in Minnesota or other latitudes with freezing winter temperatures. In these areas, annual infestations are produced by moths migrating from southern overwintering areas each spring.

Host range

The larva -caterpillar - is the life stage that damages crops. In addition to corn, the larvae feed on a wide range of broadleaf and grass crops, weeds and other plants. Black cutworm adults feed on plant nectar.

Description and life cycle

The black cutworm goes through a complete metamorphosis with egg, larval, pupal and adult stages.  Depending on when the moths arrive in Minnesota and temperatures,  black cutworms will go through one or more generations until late-summer conditions trigger a southward migration.

Adult

Black cutworm moth
Figure 1: Black cutworm moth. Note color pattern and dagger marking. Photo: Mark Dreiling, Bugwood.org.

The adult black cutworm is a moderate-sized moth with a wingspan of about 1.5 inches (Figure 1). The forewing is dark brown to black with a lighter distal - away from the body when spread for flight - edge.

Identifying features include a small black dagger or dart-shaped mark that extends outward from a faint kidney - reniform - spot near the light to dark boundary of the forewing.

The hindwings do not have obvious markings and are pale gray with darker gray near the veins and edges.

These markings on the wings are most visible on newly emerged or recently arrived specimens with intact wing scales. Moths become difficult to identify when they lose their wing scales and markings with age or when otherwise damaged. 

Egg

Eggs produced by spring migrant moths are often laid before crops are planted.

The female moth can lay 1,000 eggs or more, singly or in small groups of up to 30 on grasses, weeds and crop debris. Females seek low-lying and weedy areas to lay eggs.

While not winter-hardy, the black cutworm eggs can tolerate colder temperatures more than other life stages. Eggs hatch in five to 10 days depending on temperature.

Larva

Black cutworm larvae are gray to nearly black, with a light dorsal band and a ventral surface that is lighter in color (Figure 2). The distinct head is dark brown. The larvae have three pairs of true legs and five sets of fleshy prolegs (4 abdominal and one anal).

Overall, the larva has a greasy appearance; earning the common name “greasy cutworm” in some parts of the world.

Under magnification, the skin of larger larvae has a granular appearance. Black cutworm larvae can be distinguished from the more common dingy cutworm and several other corn-attacking species by the unequally sized small, dark, wart-like  bumps - called tubercles- on the upper edges of each body segment.

On the black cutworm, the front tubercle is obviously smaller than the rear. On most other cutworm species, these tubercles are nearly equal in size (Figure 3).

As they grow, cutworm larvae molt and pass through several larval stages or instars. There can be from six to nine larval instars, but seven instars is most common. Diet influences the number of larval instars, with a less suitable diet leading to prolonged development and more instars. A full-grown larva is about 2 inches long.

Larval development from egg hatch to pupa takes approximately 28 to 35 days, depending on temperature.

 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
black cutworm closeup of tubercles
Figure 3: Magnified view of rough skin texture and unequal-sized front and back tubercles on top edges of black cutworm body segments. Photo: Marlin Rice.

Pupa

Black cutworm pupa
Figure 4: Black cutworm pupa. Photo: Merle Shepard, Gerald R. Carner and P.A.C. Ooi, Insects and their Natural Enemies. Associated with Vegetables and Soybean in Southeast Asia, Bugwood.org.

The mature larva burrows into the soil and forms an earthen cell where it  pupates. The naked pupa is approximately 3/4 inches in length. It is initially  orange-brown in color, becoming dark brown it develops (Figure 4). The pupal stage lasts 12-15 days.

The temperatures encountered by the pupae are suspected to determine whether the resulting moths remain in the area, migrate north in the spring or south in the fall.

The entire life cycle from egg to adult takes 35 to 60 days, depending on temperature and food quality. Multiple generations are produced until weather conditions trigger southward  migration.

Crop damage

Plant tissues are damaged by the chewing mouthparts of the feeding larva. Larvae are active, moving and feeding mainly at night. 

Small larvae feed on leaves, creating irregular holes and can cut small weed seedlings.

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

While feeding near or below the soil surface, 4th 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. This meristem remains below the soil surface until the 5th leaf and later corn stages. In contrast, the dingy cutworm, a species commonly confused with black cutworm, cuts plants at or above the soil line.

Dry soil conditions can encourage cutting below ground, at or below the growing point. When corn emergence is delayed by late planting or cold soils, it's vulnerable to waiting cutworms that can cut it off before it emerges.

When corn plants are too large to cut (after 5th leaf stage), late instar larvae tunnel into the stem. This may kill the plant by cutting water and nutrient flow or by damaging the growing point.

Most biomass is consumed during feeding of the last two larval instars.

Natural enemies

A range of fly, wasp and nematode parasites have been isolated from black cutworm larvae. Cutworms can also be infected by viral and bacterial pathogens. Bird, mammal and insect predators (notably ground beetles) also impact cutworm larval populations. Birds, bats and motor vehicles prey on adults.

Long-range migration and field risk

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Management

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. Scouting and rescue insecticide applications are the best defense against yield loss from black cutworm.

Predicting black cutworm development and damage

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

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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. In dry, windy weather, cut leaves or plants rapidly wilt, dry and may blow away to leave no sign except missing stand (Figures 11 & 12).

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. This can be frustrating, so why bother? Knowing the size of the cutworm larvae will help determine the potential for future damage (Figure 23).

Early (second) instar
Figure 11: Early (second) instar black cutworm and its leaf feeding on one-leaf corn. Photo: Bruce Potter.
A fourth instar
Figure 12: A fourth instar black cutworm and corn plant cut at soil. Photo: Bruce Potter
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Controlling black cutworm 

Bt hybrids 

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, corn might still be damaged under heavy cutworm pressure.

An at-plant insecticide is probably not that helpful in preventing additional cutworm stand loss when added on these hybrids. Remember, the Cry34/35 Ab1 (Herculex RW protein) is not the same as the Cry1F above-ground protein.

In addition to timing of tillage, spring weed and cover crop growth, the Bt protein in the planted hybrid can help prioritize fields for scouting. The Handy B corn trait table shows which Bt proteins control black cutworm and several other insect species.

Seed treatments

High rates of neonicotinoid 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, however. Seed-applied diamide insecticides (e. g. chlorantaniliprole) can also affect black cutworm larvae.

Large numbers of late-instar cutworms moving from weeds to take a bite of corn can overwhelm seed-applied insecticides and Bt in corn tissues.

Concerns about Bt-resistant corn rootworm populations have led some farmers and ag advisors to add a soil insecticide to Bt-RW corn for added root protection. That decision, however, should be considered an entirely separate issue than cutworm management.

Soil-applied insecticides

Soil-applied insecticides at planting can provide 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 and can be targeted to fields with actual infestations.

T-band applications for granular insecticides, if so labeled, are sometimes more effective on cutworm than in-furrow applications. However, a t-banded insecticide has its own performance risks and the placement is not necessarily more effective on corn rootworm. Incorporate the insecticide bands as indicated on the label. Windy planting conditions reduce the accuracy of band placement and later blowing of loose, dry soils can also reduce efficacy of non-incorporated bands. Always read the pesticide labels and use the appropriate rates.

Foliar insecticide applications

Fortunately, cutworms are controlled well with rescue insecticide applications and many post-plant insecticide products provide effective control of black cutworms. Several compounds within the pyrethroid, organophosphate, carbamate, and diamide groups are labeled for post plant/post-emerge cutworm control. Spot treatments can be effective when combined with careful scouting.

Black cutworms tend to remain lower in the soil when the top few inches of the soil profile are dry, meaning that insecticide applications can be less effective. A rotary hoe or row cultivation before application (or after application if below-ground feeding continues) can help improve the efficacy of some insecticides by incorporating insecticides and encouraging cutworm movement.

Good coverage of row area and plants is important. Do not skimp on water and match spray volume and pressure to nozzles designed for insecticide application. Although spraying in the late afternoon or evening will place the insecticides in the field closer to larval activity, it can also reduce control if a temperature inversion prevents the spray from settling on the field.

Make sure cutworms are still present and actively feeding if you decide to treat. Check just before spraying to ensure stand loss is still progressing. 

CAUTIONS

It is important to read pesticide labels. Be cautious of potential interactions between some organophosphate insecticides (e.g., Thimet, Counter, Lorsban) and some ALS herbicides. These interactions can cause severe or  temporary crop injury.  Some PPO and HPPD herbicides may also interact with insecticides.

Cultural control

Maintain good early-season weed control, including the timely termination of cover crops,  can reduce the attractiveness of fields to egg-laying females.

Some above-ground  Bt traits effective on European corn borer also  have some effect on black cutworms and can be considered in high-risk situations.

Tillage after egg laying has little impact on either egg or larval survival, unless the field is kept black for a couple weeks after egg hatch. This is long enough to starve the larvae but unfortunately, is a yield-adverse planting strategy.

Comparisons and photos of other cutworms

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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What about other crops?

The growing points of broadleaf crops are above ground. Plants will be killed if cut below the cotyledons; even climbing cutworm species can be a threat. Since yield loss from cutworms is related to stand loss, crops that are less able to compensate for stand loss are at greater risk.

While black cutworm larvae will cut soybeans, they are less likely to create a yield limiting problem in this crop. Soybeans are seeded at a much higher plant density and can compensate (up to a point) for reduced stand much better than corn.

Sugarbeets are at risk because of yield and quality sensitivity to beet stand. In addition, they are planted early and often with an oat cover which may encourage black cutworm egg laying. Cutworms will move to beet seedlings as oats and weeds are killed by herbicides.

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

Reviewed in 2021

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