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.
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.
Characteristics and life cycle
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 back, with the outside third markedly lighter.
Markings on the forewing are, for the most part, indistinct. There’s a distinct small black dagger-shaped mark that extends outward from a faint, kidney-shaped (reniform) spot at the forewing’s light to dark boundary.
The scales of the hindwings are pale gray to darker gray near the veins and edge.
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. Eggs hatch in five to 10 days depending on temperature.
Females seek out low-lying and weedy areas to lay eggs. While not winter-hardy, the eggs can tolerate colder temperatures more than other life stages.
Black cutworm larvae are gray to nearly black, with a light dorsal band and a ventral surface lighter in color (Figure 2). The distinct head is dark brown. The larvae have three pairs of true legs and five sets of fleshy, abdominal prolegs.
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 (Figure 3).
Black cutworm larvae can be distinguished from the more common dingy cutworm and several other species attacking corn by the unequal sized and dark bumps (tubercles) on the upper edges of each body segment.
On the black cutworm, the front tubercle is obviously smaller than the rear. On the dingy cutworm these tubercles are nearly equal in size.
As they grow, cutworm larvae molt and pass through several larval stages or instars. There are six to nine larval instars with seven instars most common. Diet influences the number of larval instars, with poorer diet leading to prolonged development and more instars. Full-grown larvae are about 2 inches long.
Larval development from egg hatch to pupa takes approximately 28 to 35 days depending on temperature.
The mature larva burrows into the soil and creates an earthen cell to pupate. The naked pupae are orange-brown, becoming dark brown as they age and are approximately 3/4 inches in length (Figure 4).
The pupal stage lasts 12 to 15 days. It’s believed the environmental conditions encountered by the pupae determine if the resulting moths remain in the area, migrate south in the spring or north in the fall.
The entire life cycle from egg to adult takes 35 to 60 days. Multiple generations are produced until weather conditions trigger migration.
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.
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.
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:
A large number of moths produced in the overwintering areas.
The proper weather systems, at the right time, to aid moth migration into the state.
Attractive and suitable sites for egg laying that will be planted are planted to susceptible crops (e.g., late emerging corn).
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.
Two or more generations of the black cutworm occur in Minnesota. Typically, only the first generation larvae, produced by migrant moths, are damaging to corn.
The migration habits of the black cutworm have been documented on several continents. North American black cutworm moths use prevailing winds help them move north in the spring and south in the fall.
In central United States, black cutworm moths migrate northward from over-wintering areas near the Gulf of Mexico, Texas and northern Mexico when appropriate weather systems occur.
How cutworms travel
Black cutworm moths can move short distances north on their own, but they take advantage of a much more efficient transport method to move long distances quickly. In the spring, moths can make it from South Texas to Minnesota within two days.
How do they do it? The moths hitch a ride on nocturnal low-level jet streams. These efficient transport systems are a common feature of the Great Plains in spring and summer. They’re also used by other migrant Lepidoptera, aphids, leafhoppers and even rust spores.
In North America, these low-level jets are powered by high elevation in the west and warm moist air in the Gulf of Mexico. Cool, dry, low pressure in the western plains interacts with moist high-pressure systems in the eastern plains to create strong southerly flows that are especially strong at night (Figure 6).
Each winter, black cutworms are presumed to overwinter only as far north as topsoil remains unfrozen. Emigrating moths fly upward from the overwintering areas at dusk. If weather systems cooperate, they’re whisked off by surface winds and rising air in advance of thunderstorms into the lower-level jet stream.
These winds are strongest at night, moving at 30 to 80 miles per hour, and can occur from about 330 to 3,000 feet in altitude. The flight is mostly passive with moths carried along until they decide to “drop out,” encounter cold air or rain out in thunderstorms. These migrating moths arrive in the north in excellent shape.
The ideal weather pattern for spring migration into Minnesota involves a high pressure center to our east with a strong low pressure center approaching from the west. This pattern produces strong, persistent southerly winds that can bring black cutworm moths northward.
Conditions that support migration to Minnesota
Two ingredients are necessary for black cutworm moths to arrive in Minnesota.
First, the air parcels reaching Minnesota must have passed through the overwintering areas when migrating adults are present. Second, the track of the low pressure center is critical. If the low tracks too far south, migration is cut off south of Minnesota. If the low tracks through Minnesota or northern Iowa we have the potential for moths to drop out or precipitate out in Minnesota.
These weather systems may stall with the frontal boundary cutting across Minnesota. In that case, if you’re south and east of the front, watch out!
Several lows may ripple across the moist air pumping northward and compound the moth deposition in Minnesota. Moths often drop out on the edges of heavy rainfall.
Radar studies in the 1980s found most evening migrating insects move at an altitude of 1700 feet or so. You can use wind trajectories to estimate where a significant immigration event (eight or more moths over two consecutive nights) might have originated. Migration south in the late summer and fall is assisted by southerly flows associated with cold fronts.
The overwintering cutworm species lay eggs based on soil type and previous year’s vegetation.
Black cutworm moths arriving in Minnesota seek out areas with crop debris, sheltered areas and low spots in the field to lay eggs. Early-season weed growth is very attractive to the moths. Areas with dense populations of winter annual (e.g. shepherdspurse, Capsella bursa-pastoris L.) and early-spring emerging (e.g., lambsquarters, Chenopodium album L.) broadleaf weeds in fields are often infested.
Similarly, overwintering cover crops might attract egg-laying moths. Black cutworm damage associated with winter rye has been observed in Minnesota and Iowa (Figure 7).
Tillage and crop rotation
Egg-laying black cutworm moths are less attracted to fields after spring tillage. Unworked fields, or fields with reduced tillage where more crop debris is on the surface, attract more egg-laying moths.
The higher risk of black cutworm damage associated with crop residues and tillage can be seen in tillage plots at the Southern Research and Outreach Center in Waseca during 1985 (Figure 8) and 1986 and the Southwest Research and Outreach Center near Lamberton during 2001 (Table 1).
Table 1: Black cutworm damage to corn as affected by soybean tillage (crop residue and weed growth at two locations)
|Tillage system||Corn plants cut: Waseca||Corn plants cut: Lamberton|
|Fall moldboard plow/spring field cultivation||5.0%||--|
|Fall chisel plow/spring field cultivation||10.1%||1.4%|
|Spring field cultivation||--||3.0%|
Table values are for Waseca in 1986 and Lamberton in 2001. Source: K. Ostlie and B. Potter.
You can predict black cutworm development and damage using pheromone traps and degree-days.
Like most other moths, black cutworms are attracted to light. Black light traps capture both male and female moths throughout the flight but captures aren’t predictive of moth density. In addition to lights, male moths are attracted to a chemical sex lure (sex pheromone) released by females.
Pheromone traps use a synthetic version of this sex pheromone and for a short period after they arrive, unmated migrant males are attracted to the traps (Figures 9 and 10). You can use these captures to estimate moth population density and predict the potential for crop damage.
Since insects are cold blooded, their activities, including how quickly they grow, depends on the temperature of their environment. The effect of temperature on growth is known as temperature dependent development.
An organism grows and develops faster if it’s exposed to cumulative heat. Similar to predicting corn growth with degree day accumulations (a.k.a. growing degree, heat units, growing degree days), we can use degree-days to predict what stage the cutworm eggs, larvae or pupae will be at.
How to calculate degree-days
There are several ways to calculate degree-days for insect development but for crops and black cutworm, the simple model works fine.
First, you need to know the maximum and minimum daily temperatures. Secondly, you also need to know the minimum temperature (lower development threshold or base temperature) at which cutworm growth occurs. Conveniently, we can use a 50 degrees Fahrenheit lower developmental threshold for both corn and black cutworms.
Technically, larval development can be limited or cease under several conditions:
Temperature for part of the day exceeds the developmental threshold development
Temperatures reach the upper temperature threshold (e.g. 86 degrees for corn)
Individual life stages can have different threshold temperatures and temperature dependent development rates
Temperatures where the eggs and larvae are located are slightly different than air temperatures
Some black cutworms go through fewer or extra larval stages (instars).
Fortunately, for our purposes, these subtleties can be ignored and we can use the following equation:
A daily degree day accumulation = ((Maximum temperature + minimum temperature) / 2) - developmental threshold temperature
For an example of calculating degree-day accumulations: The daily high was 70 and the daily low was 48. The degree-day accumulation would be:
((70+48) / 2) – 50 = 9
Daily degree-day accumulations are summed over the time period of interest.
When to start degree-day accumulations
To know when to start the degree-day accumulations we need a “biofix.” That biofix is a significant moth capture (Eight or more moths over a consecutive two-night period) and is where the black cutworm pheromone trapping network comes in.
The black cutworm life cycle, from egg to moth, takes 1.5 months or more. The simple degree-day model for development predicts that larvae are large enough to cut plants after 300 hundred degree-days have accumulated from a moth flight.
Only fourth instar cutworm larvae or larger can cut corn plants. We can use degree-days to predict when larvae will be large enough to cause visible damage, begin to cut corn and cease feeding.
Scouting corn crops for black cutworms should start before 300 degree-days accumulate after a significant catch. This will, of course, happen sooner if warm and later if cool, but is about three weeks in a typical Minnesota spring.
Table 2: Temperature dependent development and feeding damage of the black cutworm
|Cumulative degree-days (base 50 degrees Fahrenheit)||Stage||Activity|
|0 (biofix)||Significant moth capture||Egg-laying|
|91-311||1st to 3rd instar||Leaf feeding|
|312-364||4th instar||Cutting begins|
|431-640||6th to 7th instar||Cutting slows|
Scouting for cutworms is easily combined with stand evaluations and scouting weeds for herbicide selection and application timing.
Be wary when lambsquarters and ragweed patches begin to disappear without the aid of an herbicide and 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 cutworm larvae will help determine the potential for future damage (Figures 11 and 12).
Knowing which species is present is important to understand the extent of the threat. Black cutworms are more damaging to corn than some other species. Dingy cutworms are a more common species in Minnesota that feeds at or above the soil surface. As a result, it doesn’t cut corn below the growing point.
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 a possum-like defense strategy, most cutworm species roll into a semicircle and remain motionless when disturbed. Unfortunately for us 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. Move to another area with injured plants if unsuccessful.
Looking at this optimistically, you only need to find a few to make your treatment decision. Do not confuse headless, legless cranefly larvae with cutworms. All cutworm species have a distinct head capsule and three 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.
Cutworms infestations in small corn (three leaf or less) require more aggressive management than large corn. Don't give up on scouting too early. Late-instar black cutworms can kill up to six-collar corn by burrowing into the growing point.
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|
There are more detailed dynamic black cutworm thresholds available. They use stand, crop stage, projected damage and crop price. However, caution is advised when dynamic thresholds generate lower thresholds below those described above.
Yield loss, actual or measurable, doesn’t begin with the first missing corn plant. When grain prices are higher and you have a good emerged stand, you could easily be treating cutworm populations that wouldn’t reduce stand enough to actually hurt yields.
The rescue insecticide calculator (Table 5) is adapted from a University of Illinois publication and is an example of a dynamic threshold that’s used in several management guides.
Modern corn yields and prices could indicate treatment at a very low percentage cut plants using this worksheet, perhaps leading to over-reactive treatment decisions. However, the yield loss factors are still useful when combined with yield loss by stand reduction charts.
Table shows yield loss factors and equations (see below) to calculate the profitability of a rescue insecticide treatment for black cutworm. Source: University of Illinois.
Table 5: Yield loss factor for calculating corn yield loss: When moisture isn’t limiting
|Average cutworm instar||1 corn leaf||2 corn leaves||3 corn leaves||4 corn leaves||5 corn leaves|
Table 6: Yield loss factor for calculating corn yield loss: When moisture is limiting
|Average cutworm instar||1 corn leaf||2 corn leaves||3 corn leaves||4 corn leaves||5 corn leaves|
Projected bushels per acre yield loss = _______ yield loss factor x _______% plants cut (decimal) x _______expected yield (bushels per acre)
Projected money loss per acre = _______bushels per acre loss x $_______(price per bushel)
Preventable loss per acre = $_______projected loss per acre a x _______% control*
*95 percent control with adequate moisture, 80 percent control with limited moisture
$ return (+/-) for insecticide treatment = $_______ preventable loss/a - $_______ control cost per acre
Bt hybrids, at-plant insecticides, and seed treatment
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 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 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.
Large numbers of late-instar cutworms moving from weeds to take a bite of corn can overwhelm seed treatments and Bt in corn tissues.
Some folks have been adding a soil insecticide to Bt-RW corn in areas with Bt-resistant rootworm populations. That is an entirely separate issue than cutworm management.
Soil applied at-plant insecticides can provide control of cutworm larvae. However, they aren’t recommended as insurance applications for two reasons. At planting, it’s difficult to predict which individual fields will have economically damaging cutworm infestations. Second, post-emerge insecticide rescue treatments work very well.
T-band applications for granular insecticides, if so labeled, are sometimes more effective on cutworm than in-furrow applications. However, the banded insecticides aren’t necessarily more effective on corn rootworm.
Always read the pesticide labels and use the appropriate rates. Incorporate the insecticide bands as indicated on the label. Windy planting conditions reduces the accuracy of banded applications when not incorporated. Later blowing of loose dry soils can also reduce efficacy of non-incorporated bands.
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 best defense against yield loss from black cutworms.
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.
Table 6 lists some of the species that might be found in Minnesota corn fields. Most species can overwinter in Minnesota as eggs or larvae. Black and variegated cutworms cannot winter here and migrate into the state each spring.
While we can project cutting dates for the black cutworm, 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’re 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 6). 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.
Table 6: Some cutworm species found in Minnesota corn
|Species||Eggs laid in||Number of generations||Overwinters as||Likely habitat|
|Black||Spring-summer||3||Adults migrate||Late-tilled fields, early weeds|
|Darksided||Summer||1||Eggs||After weedy crop|
|Dingy||Summer-fall||1||Larvae||After sod, alfalfa, weedy fields|
|Redbacked||Fall||1||Eggs||After weedy crop|
|Variegated||Spring-summer||2||Adults migrate||In and after alfalfa, weeds|
Why it’s important to identify the species
Species identification is important to determine damage potential. Small larvae of all species feed on weeds and leaves and cannot cut corn. Dingy, redbacked and variegated cutworms are primarily leaf feeders feeding at or above the soil surface. Consequently, they don’t usually cut corn below the soil line and growing point and the plant recovers.
However, unlike the climbing cutworms, the larvae of some cutworm species (e.g., glassy, sandhill, dark-sided, claybacked and black) tend to feed below ground at or below the growing point. This potential for their 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 leaves can be killed. Fortunately, damaging black cutworm populations are infrequently encountered.
With a bit of practice, the black and dingy species are easily distinguished 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, but equal on the dingy cutworm.
Other Minnesota insects that cause damage that might be confused with cutworm include the hop vine borer and common stalk borer.
Broadleaf crops have their growing points above ground at emergence. This means a cut plant can be killed and 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 seldom 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 oats cover which may encourage black cutworm egg-laying. Cutworms 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.
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