Black Cutworm

Black cutworm (Agrotis ipsilon) is a migratory pest that arrives in Iowa with spring storms each year. It is sporadic and unpredictable, making it essential to scout to determine whether larvae are present in a field or if management is required.

Identification

Adult: Adult black cutworm moths are approximately 1.5 inches long. Forewings are dark brown to black in color with a light band at the end of each wing. Key identification features include a small black dagger mark that extends from a bean-shaped spot near the area of the wing where the light band begins (Photos 1 and 2). Wing markings are most distinct on newly emerged moths; moths may be difficult to identify as they age or become damaged.

Egg: Eggs are white, globular, and ribbed. They are typically laid in clusters of 10-30 but may be laid singly. Usually, adults lay eggs on dense, low-lying vegetation near larval food sources.

Larva: Black cutworm larvae are caterpillars with three pairs of legs near the head and four pairs of fleshy prolegs on the abdomen. Their skin is grainy and light grey to black, and they have a dark brown head (Photo 3). There are pairs of dark tubercles (wart-like bumps) along the side of the body. The tubercles are used to distinguish black cutworms from other cutworm species that rarely cause economic injury in corn. For black cutworm, the tubercle closest to the head on each body segment is approximately 1/3 the size of the tubercle closest to the rear. As a comparison, for dingy cutworm, the tubercles are roughly the same size (Photo 4). Black cutworm larvae are 2 inches long at maturity.

Pupa: Pupae are approximately ¾ inch long and are found in the soil. They are orange-brown in color but become darker brown as they develop.

Biology

Black cutworm is native to North America and widely distributed. Even so, black cutworms cannot survive winters in regions with freezing temperatures. Therefore, infestations of black cutworms in Iowa and other northern latitudes depend on annual migrations of moths from southern overwintering sites in Texas and Mexico. Black cutworm moths use nocturnal jet streams to help them passively migrate long distances at altitudes up to 5,000 feet.

Depending on the time of spring arrival and summer temperatures, black cutworm may undergo one or more generations. In Iowa, moths typically arrive as early as late March. Females lay up to 1,000 eggs, either singly or in groups, on grasses and weeds. Females may also lay eggs in crop debris, but they seek low-lying and weedy areas to lay eggs. Eggs hatch in about a week, depending on temperature. Larvae typically pass through seven instars and feed for 4-5 weeks before pupating. Fully-grown larvae burrow into the soil and form an earthen cell to pupate for approximately two weeks. Late summer temperatures experienced during the pupal stage trigger the moths to migrate to southern overwintering sites. The entire life cycle takes 35-60 days, depending on temperature and food quality.

Plant Injury

Black cutworm larvae can feed on several broadleaf and grass crops in Iowa, but the main concern is in corn. Young larvae create irregular holes in corn leaves (Photo 5) or may cut small weeds. Feeding by young larvae is usually not significant but indicates the potential for severe injury later. Older, larger larvae may feed on stems below the soil surface (Photo 5), which may cause plants to wilt and die. However, the typical injury by large black cutworm larvae is cutting plants, and a single larva may cut 4-5 corn plants during its lifetime.

Black cutworm larvae typically cut plants (Photo 6) at or below the soil surface (compared to dingy cutworm, which cuts plants at or above the soil line). If cutting occurs above the growing point, plants generally recover. Usually, corn in Iowa is only susceptible during the first generation of black cutworm after moths migrate north in the spring. Black cutworm larvae must reach the 4^th instar to be large enough to cut corn plants and cutting occurs until plants reach the V5 growth stage. At this time, plants are usually large enough that cutworms cannot cut them. Once plants are too large to cut, large larvae tunnel into the stem, which may kill the plant by damaging the growing point or reducing water and nutrient uptake. Yield reduction by black cutworm is primarily from reducing plant stands.

Although black cutworms can cut soybeans below their growing point, it is not considered an economic pest in soybean because of the crop’s ability to compensate for stand loss. However, if stands are reduced to a yield-limiting level due to other factors and heavy infestations of black cutworm are present, management may be necessary.

Risk Factors

Areas that have had significant flights of black cutworm moths are at increased risk of larval injury, but some fields are at higher risk as well. Poorly drained, low-lying, or weedy areas may have a higher risk of black cutworm injury. It is common to find black cutworm larvae where winter annual or early-spring weeds are emerging. Additionally, fields near perennial vegetation, fields with reduced tillage, or fields with cover crops may be at higher risk. Green cover crops are attractive to egg-laying females. Dry conditions can encourage cutting belowground, and any situation that delays emergence can contribute to cutting prior to emergence. Late planted fields are also at higher risk since the susceptible growth stages may better align with larval growth stages that can cut plants.

Scouting

Trapping moths: Pheromone traps use a synthetic sex pheromone to attract unmated males arriving in an area. We send wing-style pheromone trapping kits to volunteers of the Iowa Moth Trapping Network each spring to help us monitor black cutworm migration into Iowa. We can provide timely scouting information and predict cutting dates based on significant flights that occur throughout the state. A significant flight is when eight or more moths are captured in a wing-style pheromone trap over a two-night period, and this indicates that a population is large enough that a particular area is at higher risk for black cutworm injury in corn. Capturing black cutworm moths in a pheromone trap does not necessarily mean there will be economic infestations in a particular location. Scouting fields is the only way to determine if black cutworms are present and whether management is warranted.

Scouting for larvae: Fields should be scouted every 7-10 days until corn reaches V5. Begin scouting several days before estimated cutting dates (see Tracking Development in the Management section). Early scouting is important because local larval development varies due to weather variation within a climate division, and regular scouting is important because additional flights could prolong the presence of black cutworm larvae in a field. Scouting for black cutworm is easily combined with stand assessments or scouting of other early-season pests.

Examine 10 corn plants in five areas of the field (50 total plants). Ensure sampling is random and completely covers the field. Look for wilting, leaf discoloration or damage, and missing or cut plants (corn or emerged weeds). Black cutworm larvae sometimes drag cut plants under soil clods or into the furrow to continue feeding during the day (Photo 7). If weeds are being fed upon or disappear without an herbicide application, black cutworm may be feeding there. Flag areas with suspected black cutworm feeding and return later to assess further injury.

Leaf feeding or missing plants is easy to notice, but it is important to take the time to look for larvae to determine their size and how long they will continue to feed. Larvae are nocturnal feeders and will hide during the day; they can be found by carefully excavating the soil around a damaged plant, looking under residue or soil clods, or looking at the leaves for small larvae. Remember to watch for indications of other early-season pests such as wireworms, white grubs, and seedcorn maggots.

The generic economic threshold for BCW in corn is 2-3% of plants cut when larvae are less than ¾ inch long and 5% of plants cut when larvae are greater than ¾ inch long. However, this threshold can be adjusted depending on corn price, final corn stand, and input costs. A dynamic threshold calculator for BCW may be used to aid management decisions.

Management

It is uncommon to have black cutworm infestations in Iowa that reduce yield, so preventative insecticide treatments are not recommended. Scouting and applying rescue treatments as necessary is the most economical approach for black cutworm management.

Tracking development: We can predict when black cutworm will be large enough to cut corn plants (4^th instar) using a combination of moth trapping data (see Trapping Moths in the Scouting section) and growing degree days (GDD). A GDD model for black cutworm development predicts that larvae will have reached the 4^th instar once 300 GDD (base 50°F) have accumulated since a significant flight occurred (Table 1). Each year, we predict cutting dates across the state based on data from the Iowa Moth Trapping Network in ICM News.

Table 1. Black cutworm development and associated activity.

Cultural: Early-season weed control and timely termination of cover crops can reduce the attractiveness of a particular field to egg-laying females and remove food sources for young caterpillars. Tillage after egg-laying is not effective unless fields are kept unplanted for several weeks to starve larvae. This may not be a good strategy to ensure yields.

Biological: Larvae have many predators and parasites, but their role in management is not well understood. Parasites include fly, wasp, and nematode species, and viral and bacterial pathogens may also infect larvae. Larval populations may also be impacted by birds, mammals, and ground beetles. Adults are often fed upon by birds and bats.

Plant-incorporated: Some Bt hybrids provide black cutworm suppression, but larvae can still cut young plants. Look for hybrids with Vip3A or Cry1F proteins. Use the Handy Bt Trait Table to find appropriate hybrids or see if a hybrid has effective traits.

Chemical: Preventative insecticide treatments are a questionable practice since black cutworm infestations are sporadic. Seed-applied diamides and high rates of neonicotinoid seed treatments can be effective on many seedling pests, including black cutworm, but may not be satisfactory under high populations. Rescue treatments are usually the most efficient and economic approach to managing black cutworms.

Black cutworm is controlled well with foliar insecticide applications, including compounds in the pyrethroid, organophosphate, carbamate, and diamide groups. Consider spot treatments if infestations seem to be isolated to a particular area of the field when scouting. If soils are dry, larvae may be lower in the soil profile and insecticides may need to be incorporated to be effective. Additionally, proper coverage is important. If spraying late in the afternoon to target larvae when they are feeding, be aware of temperature inversions that may affect insecticide efficacy.

If significant stand loss occurs, replanting the field is an option. A replant decision should be based on percent stand loss and cost of additional seed. Resources for replant decisions are available:

Replant resources for Corn

Replant resources for Soybean