NPIPM:Cutworms on corn
Authors: Buyung Hadi, Robert Wright, Thomas Hunt, Janet Knodel, Phillip Glogoza, Mark Boetel, R. Jeff Whitworth, Holly Davis, and J.P. Michaud
Cutworms are larvae of various moths species (Lepidoptera: Noctuidae). There are several species of cutworms that can be found on corn. Some of the cutworms commonly found in the Northern Plains are described under identification and life cycle sections. The management section will discuss the overall approach to manage cutworms on corn.
Identification
Black cutworm
Black cutworm larvae (Agrotis ipsilon) range in size between 0.2-1.5 in (6-38 mm) long. The color varies from greasy gray to black above the spiracles (small circular openings on the caterpillar's sides) with lighter shades on the underside. A broad light gray band runs down the middle of the back. Convex and shiny skin granules (tubercles) of various sizes appear conspicuously on the upper side of the body.
Western bean cutworm
A fully-grown western bean cutworm larvae (Striacosta albicosta) is light tan to pink in color with crosshatch markings on its back. Western bean cutworm of third or older instar (development stage) has characteristic dark markings behind the head capsule. Historically western bean cutworm is found in western Great Plains but an eastward spread of this insect has been documented since the late 1990s. As of 2010, economic damage on corn because of western bean cutworm feeding has been documented from states as far east as Indiana, Michigan, and Illinois. .
Bristly cutworm
A fully-grown bristly cutworm larvae (Lacinipolia renigera) is about 1 in (25 mm) long, dull gray-brown in color with conspicuous stripes on the sides and stiff hairs (bristles) along the top of the body.
Dingy cutworm
Dingy cutworm larvae (Feltia ducens) are pale gray to reddish brown, mottled in appearance with faint v-shaped markings visible on the back. When fully grown, the larvae are about 1.25 in (32 mm) long. A diagnostic trait to differentiate dingy cutworms from black cutworms is the size of raised skin granules (tubercles) along the top of the abdominal segments. The tubercles of dingy cutworm are similar in size while the ones on black cutworm are of variable sizes.
Glassy cutworm
Fully-grown glassy cutworm larvae (Apamea devastator) are about 1.5 in (40 mm) long with translucent green-white body (thus the name 'glassy') and red-brown head. The pronotum segment of the glassy cutworm has a red-brown plate at the top of the body.
Sandhill cutworm
Sandhill cutworm larvae (Euxoa detersa) are white to pale gray, with white stripes on its back. The head is dull red-brown. The larvae are about 1.3 in (35 mm) long when fully grown.
Spotted cutworm
When fully grown, spotted cutworm larvae (Xestia spp.)are about 1.3 in (35 mm) long, with gray-brown body color and wedgelike dark markings on the larva's back.
Life Cycle and Seasonal History
Black cutworm
Adult black cutworms migrate to the northern states from southern latitudes in early spring. These migrants deposit eggs singly or in clusters on grasses, broadleaf weeds and crop residues. Female migrants seem to prefer dense low-lying weeds such as curly dock and yellow rocket to lay their eggs. Thus, agronomic practices and field characteristics that allow for crop residues or weed growth on a particular field in early spring may increase the risk of black cutworm infestation in the subsequent crop.
The resulting black cutworm feed on the base of corn plants. Black cutworms develop through six to nine instars depending on temperature and diet. When the larvae reach the forth instar, they are big enough to actually cut the stalk of young corn plants. The larvae rest underground during the day and feed on plant parts at night. Upon maturation, the larvae dig deep underground and pupate in earthen cells. Adult moths crawl up to the surface through the tunnel made by the last instar
Temperatures during pupal developmental is thought to trigger long range migration behavior of the resulting adult moths. It is hypothesized that when the pupae are exposed to temperatures between 32 and 96.8°F (0 and 36°C) during their development, the resulting adults will stay within a region. However, if the pupae are exposed to higher temperatures, for an extended period of time, (as in early spring in the southern latitudes) the adults will enter nocturnal low-level jet winds that blow south to north; thereby migrating to the northern states. On the other hand, if the pupae are exposed to temperatures lower than this range for a period of time, as expected during fall in the northern latitudes, adults actively enter the airflow related to cold fronts that move southward; thereby initiating a north to south migration.
Western bean cutworm
Western bean cutworms survive the winter in the Northern Plains in dormant prepupal stage underground. Pupation begins in late spring and adults emerge in summer. After mating, adult moths of western bean cutworm lay eggs on field corn, popcorn or sweet corn. Eggs are laid in groups (between 5-200 eggs per group) on the upper surface of the leaves. Female moths of western bean cutworm prefer corn plants in whorl stage to lay their eggs. Within a week the eggs hatch. The resulting larvae are quite mobile and may move to adjacent plants. In pre-tassel corn, the larvae move to the whorl and feed on the flag leaf and the flowers of the tassel. Once tasseling begins, the larvae feed within the tassel, leaf axils or the green silks of the developing ears. The larvae later move into the ear to feed on the kernels. Feeding injuries by western bean cutworms expose the plants to higher risk of secondary fungal infection. Unlike common stalk borer, Western bean cutworm larvae are not cannibalistic, thus more than one larva may be found within an ear.
Other cutworms
Other cutworms described here can survive the winter in parts of the Northern Plains. Bristly cutworms survive the winter as larvae with the resulting adults emerging between May and July. Bristly cutworms produce two generations per year. The larvae feed below ground on many host plantsincluding corn, fruits and vegetable crops.
Dingy cutworms produce one generation per year. They survive the winter as larvae in diapause. The larvae begin to become active again in March or April. Typically, the timing of larval activity coincides with the emergence of corn seedlings. Corn planted in weedy fields, or in fields previously planted with alfalfa are of especially higher risk of dingy cutworm exposure. Fully matured larvae pupate in the soil and the resulting adults emerge during mid-August to September. The adults mate and lay eggs on leaf sheaths positioned near the soil surface.
As the name implies, the sandhill cutworm prefers sandy soil. Sandhill cutworms survive the winter as larvae in diapause. The larvae become active again in May and begin to feed below ground on underground plant parts. They pupate in the soil and the adults emerge in August. One generation of sandhill cutworm is produced per year.
Spotted cutworms overwinter as large larvae. The adults emerge between June and August, mate and lay eggs on the leaves of host plants. Spotted cutworms prefer vegetables, but they are generalist feeder that can also feed on fruit trees and cereal crops. Spotted cutworms produce 2-3 generations per year.
Plant Injury and Damage
The black cutworm is often considered as the most damaging cutworm species on corn in the Corn Belt. However, the importance of each cutworm species varies with geographic location. Black cutworms feed at the base of corn plant and may cut or severely damage the the stalk. The damage due to black cutworm infestation is considered the highest when corn is preceded by soybean and in corn planted in flood plains. In corn, black cutworm feeding on plants in developmental stages after V6 do not cause any serious damage. Thus although there are more than one generation of black cutworm per year, only the first generation causes significant damage on corn since the emergence of the second larvae usually occurs when corn is already in advance developmental stages
Western bean cutworm is considered an occasional pest of corn in the Northern Plains. Unlike other cutworms described here, western bean cutworm can feed on corn ears. Ear feeding by western bean cutworm reduces yield weight and exposes the ears to fungal infection. One larva per plant at the dent stage is estimated to cause an average yield loss of 3.7 bu/acre. In years with severe infestations, two or more larvae per plant may be common. In such situations, as much as 50 to 60 percent of an ear's kernels may be damaged by western bean cutworm feeding.
The other cutworms are considered to be minor pests of corn in the Midwest. Bristly, glassy and sandhill cutworms feed primarily below ground and may cut small plants below the soil surface. Dingy cutworms feed on young corn leaves and only rarely cut young corn plants. Spotted cutworms climb on corn plants and feed on the above ground plant parts
Management Approaches
Scouting and Threshold
Soil-applied insecticide treatment during planting time is not recommended since it is difficult to predict whether a given field will have economically damaging levels of cutworm infestation. Cutworm management program relies on field scouting and usage of post-emergence 'rescue' insecticides when the economic threshold is reached. For black cutworm, a scouting program utilizing adult pheromone trap has been devised. Adult pheromone traps are used to estimate the timing critical scouting phase of black cutworm later in the season. Commercial pheromone traps are available. The timing to setup pheromone trap varies with regions, but generally it falls between March and April. Using pheromone traps, the number of black cutworm moth captures can be monitored nightly. A significant capture is defined as eight or more moths per trap captured over two nights. The timing of significant capture is used as the starting point to count degree days to estimate the time of critical larval scouting. When 300 degree days (using the base temperature 50°F or 10°C) have accumulated after this date, fourth instars are expected to be potentially cutting corn stalks. At this time more intensive sampling for black cutworms larvae is warranted. The damage due to cutworm feeding must be determined by field scouting. The absence of significant black cutworm activity does not mean that field scouting is not necessary since there are other cutworm species that can cause damage. Economic threshold varies with regions and readers are referred to regional information of black cutworm management available under the Resources section below.
For western bean cutworm, pheromone traps are recommended to monitor the flight of adult moths. Adult pheromone traps can be setup in late June and the number of adult moths trapped should be monitored daily. When adult moths start to be captured on consecutive days in increasing number, field scouting for western bean cutworm should begin. To scout for western bean cutworm, choose 10-15 locations across the field. In each location, check the leaves of 10 adjacent plants for western bean cutworm eggs and/or small larvae. After scouting, calculate the percentage of plants with eggs and/or small larvae out of all sampled plants. If 5-8 percent of total sampled plants have egg masses and/or small western bean cutworm larvae, consider an insecticide application.
Other cutworms should be scouted once or twice a week beginning with plant emergence. On corn, cutworm scouting can end once the plants reached V6 stage (6-leaf stage) since at this point the plant is too big to cut. Observe plants at several locations in each field for evidence of leaf feeding or stem cutting. Pay special attention to poorly drained areas, areas where weeds were present before planting and areas with soybean stubble. If cut plants were found, examine the soil around the plants for cutworms. Cutworms maybe found on the ground, below the surface or under debris during the day. Take note of the size and species of cutworms found. If it is determined that a given field has a cutworm problem, it may help to monitor the same set of plant during the scouting period. To do this, mark off 100 plants in a row with stakes or flags. Do this at several locations across the field, and continue to monitor these marked plants for injuries. Monitoring the same set of plants a more reliable picture of black cutworm activity can be developed.
Cultural Methods
Weedy or residue covered fields may attract oviposition of mated female adult moths in early spring. Consequently, weedy fields are associated with higher risk of cutworm infestation. Tillage and herbicide application just before planting do not always help since the eggs are already present. A delay of seven days or more in planting after seedbed preparation has been recommended in states with routine cutworm problems. The delay period is intended to give time for the cutworm larvae to emerge, starve and die due to the absence of a food source.
Biological Control
Opportunistic natural enemies help reduce black cutworm populations in the field. These enemies include birds, ground beetles, small insect-feeding mammals, caterpillar-feeding insects and ground-dwelling spiders. Naturally-occurring soil-predator complex has been shown to be factor that regulates black cutworm population in the field.
Chemical Control
Neonicotinoid insecticides applied as seed treatment (e.g. clothianidin and thiamethoxam) were reported to reduce feeding damage caused by black cutworm, albeit erratically. Insecticidal seed treatments require growers to make decisions before confirming the actual black cutworm infestation in the field, and thus not advisable.
Post-emergence insecticide applications guided by field scouting is the recommended approach. Applying post-emergence treatment in the evening increases the chance of exposure since the cutworm larvae are active at night. Under dry soil conditions, a light surface tillage such as rotary hoeing may help move some insecticide into the soil zone. Pyrethroid insecticides should not be incorporated into the soil.
Transgenic Crops
Bt-corn expressing the Cry1F protein (e.g. Herculex I, Hercules Xtra and SmartStax) are labeled for control of black cutworm and suppress (not completely controlled) western bean cutworm populations. Other cutworm species that overwinter as partly grown caterpillars may not be controlled adequately.
Other Online Resources
Insecticide Treatment Options for Cutworms in Field Corn - University of Nebraska
Corn cutworms - University of Nebraska
Western bean cutworm - University of Nebraska
Black cutworm - Kansas State University
Corn insects of North Dakota affecting the crop after emergence - North Dakota State University
Corn insects - North Dakota Field Crop Insect Management Guide - North Dakota State University
Related NPIPM/HPIPM Resources
Black cutworm on soybean - Northern Plains IPM Guide
Variegated cutworm on field corn - High Plains IPM Guide
Western bean cutworm on field corn - High Plains IPM Guide
