PESTS OF CORN/SORGHUM


Extracted from INSECT and related PESTS of FIELD CROPS (AG-271)
Dated 5/82
Placed on the Web 6/95 by the Center for Integrated Pest Management, NCSU

In 1980, corn was planted on 708,502 hectares (1,750,000 acres) of North Carolina farm land concentrated in the Coastal Plain, but also scattered throughout the Piedmont and Mountains. Grain sorghum accounted for an additional 46,559 hectares (115,000 acres) in the Piedmont and south central counties of this state. The corn and sorghum planted on this extensive acreage are subject to infestation by a diverse array of insect or related pests.

Although these two crops are unlike in many respects, (e.g., sorghum being much more drought tolerant than corn), they have many pests in common. When present, many of these pests go unnoticed because they are hidden within the soil, stalk, ear, seedhead, or whorl. Few corn and sorghum pests feed on exposed sites where they can be easily detected and controlled before damage occurs. As a result, much attention needs to be focused on early recognition of both pests and their injury so control measures may be employed at the most appropriate time.

Key to Pests of Corn and Sorghum

A. Insects that feed on seed, roots, lower stems, or surface debris.

  1. Beetles - These chewing insects have hard, shell-like forewings (wing covers or elytra) which meet in a straight line down the middle of the back (Fig. A, B). They may or may not be able to fly.

    1. Billbugs - These black, gray, or brown snout beetles are 10 to 16 mm long (Fig. A). They pierce young seedlings near the ground with their "beak," causing stunting, tip dieback, and often death of corn seedlings. Plants which survive display yellow streaks, suckering, and rows of holes transverse across the blades.

    2. Sugarcane beetle - This black, hardshelled, dome-shaped beetle (Fig. B) is about 13 mm long. As it feeds below the ground line on the lower stalks of corn seedlings, a large ragged hole is left in the stalk. Damage is most common in corn following sod.

  2. Beetle larvae - These larvae are either legless or have three pairs of short legs near the head (Fig. A to D).

    1. Billbug larvae - Several species of these cream-colored, brown-headed, legless grubs (Fig. A) tunnel in the basal area of the stalk, dwarfing and often killing plants. Billbug larvae range from 2 to 15 mm long.

    2. Southern corn rootworm - This wrinkled, yellowish-white larva (Fig. B) has three pairs of short legs and measures 15 to 16 mm long when mature. The last segment of the body has a brown, dorsal shield.

    3. White grubs - These C-shaped, brown-headed grubs (Fig. C) may be as long as 45 mm, and have three pairs of legs and a slightly enlarged abdomen. They feed heavily on roots of grass crops, stunting and sometimes killing plants.

    4. Wireworms - Injury by these slender, short-legged larvae (Fig. D) produces stunting, tip dieback, and chlorotic blades. Taproots of recently infested plants usually display an irregular feeding hole with jelly-like plant secretions. Damage to germinating corn is common. Larvae of most species are cream to copper in color.

      1. Melanotus communis - This pale yellow to reddish-tan wireworm is 21 to 25 mm long, and its last abdominal segment has blunt scalloped edges (Fig. A). This pest usually feeds on the roots of seedlings. Older infested plants are stunted and may not tassle.

      2. Southern potato wireworm - This white, cream, or yellow-gray larva has a red-orange head capsule and may be as long as 17 mm. The last abdominal segment terminates in a closed, oval notch (Fig. B).

      3. Tobacco wireworm - This brown-headed, white, yellowish or cream-colored wireworm, 14 to 19 mm long when mature, is characterized by a V-shaped notch in its last abdominal segment (Fig. C).

  3. Corn root aphid - This typically wingless, pale green to blue-green aphid has a black head and black or reddish eyes. Some females are gray and pink with a white, powdery coating. Length varies from 0.3 mm for small nymphs to 2.0 mm for mature adults. Usually associated with anthills in corn fields, this aphid feeds on the roots causing the foliage to develop a yellowish to reddish tinge.

  4. Caterpillars - These moth larvae have three pairs of legs near the head and five pairs of prolegs.

    1. Cutworms - These fat, darkly colored caterpillars burrow in soil and debris during the day and cut off or partially sever seedlings near the soil line at night.

      1. Black cutworm - This caterpillar, 3.5 to 45.5 mm in length, varies from light gray to black in color, often appearing greasy. The skin of this cutworm is granulated (as seen under a 10X hand lens), the granules resembling rounded, flattened pebbles (Fig. A). After cutting off a seedling, this cutworm commonly pulls it into the entrance of its burrow and feeds on it during the day.

      2. Granulate cutworm - This caterpillar, 2.0 to 38.5 mm long, has a pale brown head, a dark brown band down its back, and brown sides with faint stripes. The skin granules of this cutworm are like blunt cones as high as they are wide (Fig. B).

    2. Lesser cornstalk borer - This larva may be found feeding on leaves or roots but eventually tunnels into the plant's stem usually at or near the soil line. A silken, soil-covered tube is often connected to the plant stem at the entrance hole. This small, slender, bluish-green caterpillar has brown bands around each body segment and measures about 19 mm long when mature.

    3. Southern cornstalk borer - This larvae, which reaches a length of 25 mm, is creamy yellow during the winter and white with black spots in the summer. Although it feeds in the whorl early in its development, it later causes severe taproot and lower stalk injury by tunneling within the plant.

    4. Webworms (Sod Webworm, Sorghum Webworm) - When mature, these pinkish-white, yellow, or light-brown worms are 6 to 19 mm long with thick bodies, coarse hairs, and paired dorsal and lateral spots on each segment. They feed on the roots of seedlings, and bore into the stalk near the soil line. Such stalk damage causes the plant to be deformed and the newly emerged leaves to be ragged and distorted. Fine silken strands entangled with plant debris are usually present near the entrance hole.

  5. Seedcorn maggot - This tiny, legless, white to yellow maggot is 5 to 7 mm long, tunnels in seeds and the tender root tissue of seedlings. Many injured seeds do not germinate and those which do produce weak, spindly seedlings which rarely survive.

B. Small pests that extract juices from foliage and stems (Fig. A to C).

  1. Chinch bug - This black bug, up to 4 mm long, has opaque wings, each marked with a black triangle (Fig. A). Feeding anywhere from the roots to the upper leaves, adults and smaller, wingless, red and black nymphs cause dwarfing, lodging, and yield reduction. Seedlings may wilt and die during severe infestations.

  2. Corn leaf aphid - Up to 2 mm long, the wingless form of this pear-shaped insect (Fig. 70B) is pale bluish-green with a dark area around the base of the cornicles. Its body often seems to have a powdery coating. The insects feed in groups of 5 to over 2000. Tassels, leaves, and silks on which this aphid feeds become black with mold. Leaf deformation, yellowing, and cast aphid skins often are associated with heavy infestations.

  3. Spider mites - Several species of these eight-legged (larvae six-legged), almost microscopic pests (Fig. 70C), congregate and feed on the underside of leaves and spin webs on the foliage. A silver speckling of the leaves, especially during dry periods, is associated with these pests. In North Carolina, they are usually a problem on corn rather than sorghum. Severe infestations resemble drought stress since damage progresses from the bottom of the plant up.

    1. Carmine spider mite - The 0.4 mm long, oval female is red, with two or four dark, dorsal spots; the male is smaller and slightly diamond shaped.

    2. Twospotted spider mite - The 0.4 mm long, oval female is yellow dark green, with two or four dark, dorsal spots; the male is smaller and diamond shaped. Webbing is initiated near the midribs of the host plant and gradually envelopes the plant as the population increases.

C. Caterpillars that bore into the whorl, stalk, or ear. All larvae described in this section have three pairs of legs near the head and five pairs of prolegs.

  1. Corn earworm - When fully grown, this larva is green, reddish, or brown with pale, longitudinal stripes and scattered black spots. Early instars are cream colored or yellowish-green with few markings. Larval instars vary from 1.5 to 44 mm in length and have five pairs of prolegs. As injured leaves unfurl, they display ragged holes and soggy, brown frass. This worm also feeds on the tip kernels of the ear. Round emergence holes are sometimes evident in the shuck.

  2. European corn borer - This cream to light-pink caterpillar with a reddish-brown to black head reaches a length of 25.5 mm. Its boring into the stalk weakens the plant and causes tassel and stalk breakage, ear dropping, and small ears. Light tan frass entangled with silk, tunneled midribs, and pin holes in the leaves indicate the presence of this pest.

  3. Fall armyworm - This green, brown, or black worm reaches a length of up to 40 mm and often has a pale, but distinct, inverted "Y" on its head capsule (Fig. A). It has a black, longitudinal stripe down each side of its body and a yellowish-gray stripe down its back. It feeds in the whorl causing injury similar to that of the corn earworm. It also attacks ears near the shank and when mature may bore into the stalks. Unlike the corn earworm, it rarely occurs in North Carolina before July.

  4. Lesser cornstalk borer - See section A.4.b. for description. Especially during dry periods, this caterpillar bores into the stalk within 5 cm of the soil surface leaving a soil-covered, silken tube attached near the entrance site. Young injured plants become distorted and fail to produce normal stalks and ears.

  5. Southern cornstalk borer - See section A.4.c. for description. During early stages of development this larva feeds in the whorl, causing emerging leaves to have rows of irregular holes and sometimes damaging the growing point. Later it tunnels into the stalk causing severe taproot and lower stalk injury.

  6. Stalk borer - This caterpillar may be solid white, light purple, or cream colored with a dark brown band around the middle and brown or purple longitudinal stripes. Reaching a length of 31.8 mm, it may enter the stalk at ground level and sever leaves as it moves upward or it may climb the plant and enter through the bud and tightly coiled leaves. Tassels are usually destroyed and the upper plant deformed. This insect is most common in no-till or weedy fields.

D. Chewing insects that feed exposed on foliage or silks.

  1. Armyworm - This smooth-skinned, 30- to 35-mm long caterpillar is basically yellowish or greenish brown with three dark, longitudinal stripes and five pairs of prolegs. The tan or greenish-brown head capsule has markings like those of the fall armyworm, but on the armyworm, these markings are lighter or less intense (Fig. B). Feeding primarily at night and spending the days under ground litter, this insect may damage both tender and mature foliage. In corn, this insect characteristically removes foliage from the ground up, leaving only the midribs.

  2. Beetles - These chewing insects have hard, shell-like forewings (wing covers or elytra) which meet in a straight line down the middle of the back when the beetles are not in flight (Fig. A to C).

    1. Corn flea beetle - This oval, 1.3- to 2.5-mm long insect (Fig. A) is about the size and shape of a #5 shotgun pellet. It chews the foliage of seedlings, leaving small round holes and bleached-out spots or stripes. Basically black, this beetle is tinged with bronze or bluish-green and usually jumps very actively.

    2. Japanese beetle - This shiny, metallic green, 13-mm long beetle has coppery brown wing covers (Fig. B). It feeds on the silks of corn and in very severe infestations may reduce pollination and kernel formation, although yield reduction has not been documented in North Carolina.

    3. Spotted cucumber beetle - About 6-mm long, this elongate, yellowish-green beetle has black legs, antennae, and head as well as 12 black spots on its back (Fig. C). It feeds on most above ground plant parts but causes greatest damage when it severs newly emerged, unpollinated silks. As a result, ears of corn are sparsely filled.

  3. Grasshoppers - Adults of various grasshopper species, varying from 3.8 to 4.4 cm in length, feed from the outer edges of leaves inward. Stalks, ears, and silks may also be attacked if infestation is severe.

E. Insects that feed in the heads of sorghum.

  1. Caterpillars - These soft-bodied, moth larvae have three pairs of legs near the head and either four or five pairs of fleshy prolegs.

    1. Corn earworm - See section C.1. of this key for larval description. This caterpillar bores deep into the sorghum seed head, feeding on seed contents and flower stems. It is usually no longer than 45 mm.

    2. Fall armyworm - See section C.3. of this key for description.

    3. Sorghum webworm - Feeding on grain, this hairy larva is green with four reddish-tan stripes and reaches a length of 14 mm. It has four pairs of prolegs, three pairs of abdominal prolegs and one pair of anal prolegs. It webs flowers and seeds together.

  2. Sorghum midge maggot - This tiny maggot, no longer than 2 mm and pink to orange in color, feeds inside sorghum kernels. The orange adult is similar to a mosquito.