INSECTPESTSOFCORN–STANDREDUCERS

Black Cutworm AgrostisipsilonHufnagel(Lepidoptera:Noctuidae)

KenOstlie,UniversityofMinnesotaExtensionEntomologistandBrucePotter,UniversityofMinnesotaExtensionEducatorIPMDISTRIBUTIONThe black cutworm is widely distributed in

the temperate regions of the world. It cannot

survive winters in Minnesota or other

latitudes with freezing winter temperatures.

In these areas, annual infestations are produced by migrant moths each spring.

HOSTRANGEBlack cutworm adults feed on plant nectar. In addition to corn, the larvae feed on a wide

range of broadleaf and grass crops and

weeds.

DESCRIPTIONANDLIFECYCLE

ADULTThe adult black cutworm is a moderate size moth with a wingspan of ca. 1½ inches

(Figure 1). The forewing is dark brown to

back with the outside ⅓ markedly lighter.

Markings on the forewing are for the most

part indistinct. There is a distinct small black

dagger shaped mark that extends outward from a faint kidney shaped (reniform) spot at

the light to dark boundary of the forewing.

The scales of the hindwings are pale gray to

darker gray near the veins and edge.

EGGSEggs 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

5-10 days depending on temperature. Each female can lay 1,000 eggs or more. Females

seek out low-lying and weedy areas to lay

eggs. While not winter hardy, the eggs can

tolerate colder temperatures than other life

stages.

LARVABlack cutworm larvae are gray to nearly black in color 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

IntegratedPestManagement

Figure1.Blackcutwormmoth.Notecolorpatternanddaggermarking.Photo:MarkDreiling,Bugwood.org

Figure2.Blackcutwormlarva.Noteheadcapsule,trueandprolegs,andtuberclesnearback.PhotoJohnCapinera,UniversityofFlorida,Bugwood.org

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 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.

Figure3.Magnifiedviewoflarvaskinwithtubercles.Photo:AdamSisson,IowaStateUniversity,Bugwood.org

As they grow, cutworm larvae molt and pass

through several larval stages or instars. There are 6 to 9 larval instars with 7 instars

most common. The number of larval instars

is influenced by diet, 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.

PUPAThe 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 12-15 days. It is believed that the

environmental conditions encountered by the pupae determine if the resulting moths

remain in the area or migrate south in the

spring or north in the fall.

Figure4.Blackcutwormpupa.Photo:MerleShepard,GeraldR.Carner,andP.A.COoi,InsectsandtheirNaturalEnemiesAssociatedwithVegetablesandSoybeaninSoutheastAsia,Bugwood.org

The entire life cycle from egg to adult takes

35–60 days. Multiple generations are

produced until migration is triggered by weather conditions.

CROPDAMAGEThe larva 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,

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.

Dry soil conditions can encourage cutting

below ground or at or below the growing

point. Corn planted late or otherwise had

emergence delayed 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. Most of the plant biomass

is consumed by the last two larval instars.

Figure5.Blackcutwormdamagetoyoungcornplant.Photo:W.M.Hantsbarger,Bugwood.org

NATURALENEMIESA range of Dipteran and Hymenopteran and

nematode parasites have been isolated from

black cutworm larvae. Cutworms can also be

infected by viral and bacterial disease. Bird, mammal and insect predators (ground

beetles) also impact cutworm larval

populations. Birds, bats and motor vehicles

prey on adults.

MANAGEMENTYield-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 to susceptible crops (e.g. late emerging corn); and 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.

BLACKCUTWORMMIGRATIONTwo 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.

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 southern Texas to Minnesota within 2 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 are used not only by the black cutworm but other migrant Lepidoptera, aphids, leafhoppers and even rust spores take advantage of this rapid transport system. 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 are 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 330–3,000 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.

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 1,700 feet or so. Wind trajectories can be

used to estimate where a significant

immigration event (8 or more moths / 2

consecutive nights) might have originated.

Migration south in the late summer and fall is assisted by southerly flows associated with

cold fronts.

WHATMAKESAFIELDHIGHRISKFORBLACKCUTWORMDAMAGE?

Fieldtopography,tillageandcroprotationThe 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. shepherds’ purse,

Capsella bursa-pastoris L.) and early-spring

Figure6.Nocturnallow-leveljetsareoftenassociatedwithprecipitationandthunderstormsastheydrawwarmmoistairfromtheGulf.Source:NationalWeatherServiceAdvancedHydrologicalPredictionService.

emerging (e.g. lambsquarters,

Chenopodium album L.) broadleaf weeds in

fields are often infested (Figure 7). Similarly, overwintering cover crops might

attract egg-laying moths. Black cutworm

damage associated with winter rye has

been observed in Minnesota corn.

Figure7.Goodearly-springhabitatforblackcutwormegg-laying.Unworkedsoybeanresiduewithlambsquartersgrowthinalow-lyingportionofthefield.Photo:BrucePotter,UniversityofMinnesota

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 attack

associated crop residues and tillage can be seen in tillage plots at the Southern

Research and Outreach Center in Waseca,

MN during 1985 (Figure 8).

Figure8.Influenceoftillageandpreviouscroponthepercentageofcornplantscutbyblackcutworm.Waseca,MN.K.Ostlie,1985.

Fall tillage that buries crop residue and

spring tillage that eliminates early spring

weed growth before the flight arrives reduces the risk and severity of black

cutworm attack. Historically, soybean

residue is more attractive than corn, but

this may be partially due to the amount of fall tillage or to species and numbers of

broadleaf weeds in the seedbank between

the two crops.

Table1: Blackcutwormdamagetocornasaffectedbysoybeantillage(cropresidueandweedgrowth).K.OstlieandB.Potter.

CORNPLANTSCUT(%)

TILLAGESYSTEM WASECA1 LAMBERTON2

FALLMBPLOW/SPRINGFLD.CULT. 5.0 -

FALLCHISELPLOW/SPRINGFLDCULT. 10.1 1.4

SPRINGFIELDCULTIVATOR - 3.0

RIDGETILL 14.7 7.9

NOTILL 10.2 1.0

FALLSTRIPTILL - 4.0

SPRINGSTRIPTILL - 9.21WASECA,MN,1986.2LAMBERTON,MN,2001

PREDICTINGBLACKCUTWORMDEVELOPMENTANDDAMAGE:USINGPHEREMONETRAPSANDDEGREE-DAYS

TrackingmothflightsLike most other moths, black cutworms are

attracted to light. Black light traps capture

both male and female moths throughout the

flight but captures are not 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 & 10). These captures

can be used to estimate moth population

density and predict the potential for crop

damage.

Degree-daysandblackcutwormgrowthanddevelopmentSince insects are cold blooded, their

activities, including how quickly they grow,

depend on the temperature of their

environment. This effect of temperature on growth is known as temperature dependent

development. An organism grows and

develops faster as it is 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.

Figure9.Pheromonetrapandblackcutwormlureinearlyspring.Photo:TravisVollmer,UniversityofMinnesota

There are several ways to calculate degree-

days for insect development, but the simple

model works fine for crops and black

cutworm. 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°F lower developmental threshold

for both corn and black cutworms.

Figure10.Significantblackcutwormmothpheromonetrapcapture.Photo:BrucePotter,UniversityofMinnesota.

The degree-day concept is not exact under

field conditions. Technically, temperatures

where the eggs and larvae are located are

slightly different than air temperatures. A

sine model for accumulating degree-days can best account for limited larval development

can occur whenever temperature for part of

the day exceeds the developmental threshold.

Development ceases at an upper temperature

threshold (e.g. 86°F for corn plants). Individual life stages can have different

threshold temperatures and temperature

dependent development rates. Finally, some

black cutworms go through fewer or extra

larval stages (instars). Fortunately, for our

purposes, these subtleties can be ignored and the following simple degree-day model is

accurate enough to time black cutworm

scouting efforts.

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°F and

the daily low was 48°F. The degree-day

accumulation would be:

[(70+48) / 2] – 50 = 9.

The daily degree-day accumulations are

summed over the time period of interest.

To know when to start the degree-day

accumulations we need a “biofix”. That biofix

is a significant moth capture (8 or more

moths over a consecutive 2-night period) and

is where the black cutworm pheromone

trapping network comes in.

The black cutworm life cycle, from egg to

moth, takes 1½ months or more. Only

cutworm larvae 4th instar or larger can cut

corn plants. The simple degree-day model for

black cutworm development predicts when

larvae will be large enough to cut plants (when more than 300 hundred degree-days

from a moth flight have been accumulated).

Degree-days can be used to predict when

larvae will be large enough to cause visible

damage, begin to cut corn and cease feeding

(Table 2).

Scouting corn crops for black cutworms

should start before 300 degree-days

accumulate after a significant catch. This is

about three weeks in a typical Minnesota

spring but will, of course, happen sooner if

warm and later if cool.

Table2:Temperaturedependentdevelopmentandfeedingdamageoftheblackcutworm.

CUMULATIVEDEGREE-DAYS(BASE50OF.)

BLACKCUTWORM

STAGE ACTIVITY

0(BIOFIX) SIGNIFICANTMOTHCAPTURE EGG-LAYING

90 EGGHATCH -

91-311 1ST–3RDINSTAR LEAFFEEDING

312-364 4THINSTAR CUTTINGBEGINS

365-430 5THINSTAR CUTTING

431-640 6TH-7THINSTAR CUTTINGSLOWS

641-989 PUPA NOFEEDING

SCOUTINGFORBLACKCUTWORMScouting for cutworms is easily combined

with stand evaluations and scouting weeds

for herbicide selection and application

timing.

The first sign of black cutworm is usually

leaf feeding on emerged corn or weeds. Sometimes, the larvae will cut weeds before

they move to corn. Look for wilted, partially

cut plants.

Be wary when lambsquarters and ragweed patches begin to disappear without the aid of

an herbicide. Post emerge herbicide

applications may cause cutworms to switch

from feeding on weeds to corn.

Figure11.Early(2nd)instarblackcutwormanditsleaffeedingon1-leafcorn.Photo:BrucePotter,UniversityofMinnesota

Leaf feeding and missing or cut plants are

not hard to see, but it is useful to find a few

of the larvae that caused the damage and

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 & 12).

Figure12.A4thinstarblackcutwormandcornplantcutatsoil.Photo:BrucePotter,UniversityofMinnesota

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. For example, dingy cutworms are a very common cutworm species in Minnesota that feed at or above the soil surface. As a result, it does not 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, increasing the difficulty of detection by crop scouts 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.

Take 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.

Cutworm infestations in small corn (3 leaf or less) require more aggressive management than large corn. In a worst-case scenario, large cutworms can cut corn and other plants before they have a chance to emerge. Do not give up on scouting too early. Late-instar

black cutworms can kill up to 6-collar corn by burrowing into the growing point.

While not recommended as insurance treatments, corn rootworm and seed

treatment insecticides labeled for cutworm

can provide some control. Make sure

cutworms are still present!

Economicthresholds–WhentotreataproblemCutworms reduce yield by decreasing final

stand or plant population. The generic economic threshold for black cutworm in

corn is 2-3% 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.

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 3.

Table3:Cornyieldresponsetoplantpopulation(Morris,Lamberton,andWaseca,2009-2011).Source:Managecornplantingdecisionstooptimizeyieldandeconomicreturn

FINALCORNSTAND(PLANTS/ACRE) EXPECTEDYIELD(%)

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 (~½ 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 4 gives approximate sizes in length and

width of the head for black cutworm larvae.

Table4:Blackcutwormbodyandheadcapsulesizesbyinstarstage.

INSTAR

BODYLENGTH(MM)

HEADCAPSULEWIDTH(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

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, does not

begin with the first missing corn plant. High

grain prices and a good planted and emerged

stand means you could easily be treating

cutworm populations that would not 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 is 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.

Table5:Yieldlossfactorsandequationstocalculatetheprofitabilityofarescueinsecticidetreatmentforblackcutworm.Source http://extension.cropsciences.illinois.edu/fieldcrops/insects/black_cutworm/

YIELDLOSSFACTORFORCALCULATINGCORNYIELDLOSS

AVG.CUTWORMINSTAR

MOISTURENOTLIMITING MOISTURELIMITING

NUMBEROFCORNLEAVES NUMBEROFCORNLEAVES

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

PROJECTEDBU/AYIELDLOSS=_______YIELDLOSSFACTORX_______%PLANTSCUT(DECIMAL)X_______EXPECTEDYIELD(BU/A)

PROJECTED$LOSS/A=_______BU/ALOSSX$_______(PRICE/BU)

PREVENTABLELOSS/A=$_______PROJECTEDLOSS/AX_______%CONTROL**(95%CONTROLWITHADEQUATEMOISTURE,80%CONTROLWITHLIMITEDMOISTURE

$RETURN(+/-)FORINSECTICIDETREATMENT=$_______PREVENTABLELOSS/A-$_______CONTROLCOST/A

1. Determineaverageinstaroftheblackcutwormlarvaeandcornleaves(collars).2. Considersoilmoistureinadequateifthetop3-4inchesaredryandrainisnotforecast.

BTHYBRIDS,SEEDAPPLIED,ANDAT-PLANTINSECTICIDESBt 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, they

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 neonicitinoid 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 insecticides 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 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.

T-band applications for granular insecticides,

if so labeled, are sometimes more effective on cutworm than in-furrow applications.

However, the banded insecticides are not

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.

Fortunately, 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 the best defense against

yield loss from black cutworms.

CulturalcontrolMaintaining good early-season weed control

can reduce the attractiveness of fields to egg-laying females.

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, a yield-avoiding planting strategy.

OTHERCUTWORMSPECIESINMINNESOTACORNBlack 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.

There is good evidence that similar to many

agriculturalists, some black cutworm moths

migrate south to warmer climates in the fall.

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 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 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.

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.

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 feeding to

kill corn plants makes black cutworm a

threat. When larger larvae tunnel into the

growing point, corn as large as 5 or 6 leaves

can be killed.

With a bit of practice, the two 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 (Figure 22).

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 20).

Table6:SomecutwormspeciesthatcanbefoundinMinnesotacorn

SPECIES EGGSLAIDIN #GENERATIONS OVERWINTERSAS LIKELYHABITAT

BLACK SPRING-SUMMER 3 ADULTSMIGRATE LATE-TILLEDFIELDS,EARLYWEEDS

BRONZED FALL 1 EGGS/LARVAE AFTERSOD

CLAYBACKED FALL 1 LARVAE AFTERSOD

DARKSIDED SUMMER 1 EGGS AFTERWEEDYCROP

DINGY SUMMER-FALL 1 LARVAE AFTERSOD,ALFALFA,WEEDYFIELDS

GLASSY SUMMER-FALL 1 LARVAE AFTERSOD

REDBACKED FALL 1 EGGS AFTERWEEDYCROP

SANDHILL SUMMER-FALL 1 LARVAE SANDYSOILS

VARIEGATED SPRING-SUMMER 2 ADULTSMIGRATE INANDAFTERALFALFA,WEEDS

Figure13.Bronzedcutworm.Notethedistinctstriping.Photo:WhitneyCranshaw,ColoradoStateUniversity,Bugwood.org

Figure14.Claybackedcutworm.Distinctlightbandalongthebacktuberclesasblackcutworm.Photo:JamesKalisch,UniversityofNebraska,Bugwood.org

Figure15.Darksidedcutworm.Notethewhitelateralstripabovethelegsanddarkspotsonhead.Photo:FrankPeairs,ColoradoStateUniversity,Bugwood.org

Figure16.Dingycutworm.OneofthemostcommonMNcutworms.Notesimilarsizedtuberclesanddiagonalslashesalongtopedgeofbodysegments.Photo:JohnCapinera,UniversityofFlorida,Bugwood.org

Figure17.Glassycutworm.Notethesquat,paletranslucentbodywithoutanydistinctivemarkings.Photo:JosephBerger,Bugwood.org

Figure18.Sandhillcutworm.Notepalecolorandfaintstripeswithinternalorgansvisiblethroughskin.Photo:JamesKalisch,UniversityofNebraks,Bugwood.org

Figure19.Variegatedcutworm.Notetheyellowdiamondsalongbackandorangestripealongside.Colorrangesfromolivetoblack.Photo:BrucePotter,UniversityofMinnesota

Figure20.Craneflylarvae.Notacutwormorapest.Notethelackofheadcapsules,legsandtheheadareataperedtoapoint.Photo:EdLehman

WHATABOUTOTHERCROPS?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 more 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 will move to beet seedlings as oats and weeds are killed by

herbicides.

Figure22.Blackcutworm.NotethedefensiveC-shape.Findthelarva'shead,thetruelegsandprolegs.Examinethetubercles,thesmallblackbumpsoneachsegmentnearthecutworm'sback.Theunequalsize,withthereartuberclemuchlargerischaracteristicofblackcutworms.Theoverallappearanceisablackand“greasy.”Photo:BrucePotter,UniversityofMinnesota

Figure21.Somecutwormsreadilyfeedabovegroundasdefoliators.Thisisavariegatedcutworm.Spottedcutwormalsofeedsinthecanopy.Photo:BrucePotter,UniversityofMinnesota.

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