Insect Management: Grain Sorghum

Many insects attack sorghum and small grains in Mississippi. Some producers let insects rob them of profit because they do not use adequate pest control.

Planting at the proper time is important. Plant sorghum in the shortest time possible, preferably at the same time your neighbor plants his crop (so there is uniform blooming).

Check fields weekly or more often, depending on pest and damage. Apply proper control measures according to economic thresholds for optimum economic returns. Insecticides are most effective if you apply them when insects are in early growth stages and the economic threshold is reached.

Leaf- and stem-feeding insects*

Greenbug

Mature female greenbugs are approximately 1/16 of an inch long, with the typical "pear" shape of aphids. They are pale green with a darker green stripe down the middle of the back. The legs and cornicles are also green, except for the tips that are usually black.

Damage*

Greenbugs injure small grains and sorghum in three ways:

1. They extract sap with their piercing-sucking mouthparts, thereby depriving the plant of water and nutrients;
2. They inject a chemical during the feeding process, and this causes enzymatic destruction of cell walls that leads to reddening and yellowing and eventually necrosis (browning) of leaf tissue;
3. They may transmit devastating viruses, such as barley yellow dwarf virus, in small grains and maize dwarf mosiac virus (MDMV) in sorghum, or the plants may be predisposed to other diseases, such as charcoal rot of sorghum.

Greenbug infestations in small grains are often first detected by the red- spotted, yellow, or orange leaves that appear on a few plants in localized spots within a field. (These symptoms should not be confused with rust disease.) As greenbug density increases, more plants show the typical damage symptoms. Inspection for the presence of greenbugs on plants will distinguish greenbug injury from the damage caused by nitrogen deficiency or moisture stress. Greenbugs most often feed in colonies on the undersides of the lower leaves and stems of small grains but may feed on plant parts at or slightly below the soil surface.

Seedling sorghum plants are very susceptible to greenbug injury and turn yellow or reddish when only lightly infested. Seedling death results in stand loss. In larger plants, greenbugs cause stunting and kernal-weight reduction and increase the likelihood of plant lodging.

Economic Threshold

Plants 6 inches or less -- treat when greenbug aphid colonies appear on lower leaf surface.

Plants 6 inches to preboot -- treat before any entire leaf is killed.

Plants preboot and larger -- treat when death of more than two lower, normal-sized leaves occur.

Oat bird-cherry aphid

The aphid is broadly oval, and its color ranges from mottled yellowish or olive green to greenish black. Often there are reddish patches around the bases of the cornicles. The antennae are entirely black, but the legs and cornicles are green with black tips.

Nymphs and adults of the oat bird-cherry aphid suck juices from the leaves of small grains. There is no obvious toxin associated with its feeding, and damage symptoms are not readily apparent. Chemical control is rarely justified for the oat bird-cherry aphid.

Corn leaf aphid

This aphid is dark blue green and about the size of a greenbug or slightly smaller. Its legs, antennae, and cornicles are entirely black. It has the typical "pear" shape of aphids.

The corn leaf aphid is less injurious than the greenbug, primarily because it does not inject a potent toxin during feeding. It sucks juices from plants and secretes honeydew. The aphid commonly feeds in the plant whorl, where it often becomes extremely abundant and may fill the whorl of the middle leaf. The aphid feeds in colonies on the undersides of leaves and may coexist with greenbugs. Feeding causes a yellowish mottling of the leaves. The mottling is apparent, however, only after the leaves creating the whorl have extended. Some marginal leaf necrosis also may be associated with corn leaf aphid feeding, but this is likely the symptom of maize dwarf mosaic virus, which the aphid transmits. In rare instances corn leaf aphids stunt plants, interfere with panicle exertion, and, if they are abundant in the panicle, may affect harvest. Heavy infestations during the seedling stage may cause plant death and stand loss. However, in spite of these exceptions, chemical control of the corn leaf aphid is rarely justified.

English grain aphid

The English grain aphid usually is grass-green but can be yellow or pink with a brown head. There is often a dusky dorsal patch on the abdomen. The long cornicles and antennae are entirely black.

The English grain aphid sucks plant juices and causes damage similar to that of the oat bird-cherry aphid. This aphid does not inject any toxins during feeding and seldom causes yield losses. After plant heading, the aphid can be found clustered in the heads of small grains.

Fall armyworm

The most common features of the fall armyworm larvae are prominent, white, inverted Y-shaped marks on the front of the head and longer hairs coming from black spots (tubercles) on the body.

Southwestern corn borer

All mature larvae are dull white with a regular pattern of dark brown to black spots, except overwintering forms.

Corn earworm

Larvae range in color from light green, brown to almost black, with light and dark stripes running lengthwise of the body. The head capsule is usually orange.

Damage

"Whorlworm" refers to worms that attack the whorl of the sorghum plant before blooming. Damage may be caused by corn earworm, fall armyworm, and southwestern corn borer. All of these may be in the same field at the same time. Whorlworm damage to the whorl often appears as ragged shot-hole damage.

Plants can stand a considerable amount of whorlworm damage. Crop loss is when infestations are heavy enough to cause "dead-heart" or severe stunting of plants. When worms infest young plants shorter than 24 inches high, the center of the plant may be killed.

Economic Threshold

Control of fall armyworm and other whorlworm infestations is critical on young plants less than 15 inches high and on plants approaching heading. Between these two stages, the plant can withstand considerable defoliation or leaf loss. Ground application, using 20 gallons of water per acre, has given the best results.

Control is needed when an infestation averages one worm or more per plant. For satisfactory control, apply the insecticide in sufficient water (volume) to penetrate deeply into the whorl of the plant. Directed sprays over the whorl are essential for effective control.

Economic losses probably do not occur unless populations reach or exceed one larva per plant.

Lesser cornstalk borer

Larva is light-bluish-green with transverse brown bands.

Damage

Eggs are laid by moths at the base of seedlings. Larvae form silken tunnels covered with soil particles, and these are attached to the base of plants. The larva burrows into the stalk. Damage may range from stunting of plant growth to reduction of stand (plant death). Hot, dry conditions combined with sandy soils enhance the damage potential by this pest.

Economic Threshold

Do not allow stands to be reduced below the suggested plant population for the row spacing being used. Cultural controls should include the following:

• maintaining clean, weed-free field borders;
• conducting tillage practices to increase moisture and decaying organic matter;
• planting early;
• rotating crops.

Chinch bugs

Adults are black and white with whitish wings marked by a dark triangle on their outer margins. The legs and base of antennae are red. Nymphs are bright red but become darker with maturity.

Damage

The nymph and adult stages have piercing-sucking mouthparts, and damage is caused by removal of plant sap that results in severe stunting and sometimes death of the plant. Hot, dry weather early in the season, before plants reach 18 inches in height, favors the buildup of chinch bugs. When scouting, check the base of seedlings, behind the leaf sheath, and the soil at the base of the plant for nymphs and adults.

Economic Threshold

Randomly select several sites in a field to determine the level of infestation and presence of damage. If you find an average of two or more adult chinch bugs on 20 percent of the seedlings less than 6 inches high, apply an insecticide.

Use ground equipment with a directed spray at lower 1/3 of plant and 20 to 30 gallons of water per acre.

Grain-head feeding insects

Corn earworm or Fall armyworm

See descriptions under Leaf- and Stem-Feeding Insects.

Damage

Egg deposition by moths begins at about the bloom stage. Tight-head varieties offer an excellent environment for larval development and are the varieties where more severe infestations usually occur. Larvae reduce yield by destruction of developing grain.

Economic Threshold

Young larvae may be protected in partially opened heads or in varieties that have tight heads. Spray coverage is very important.

Treat when you find an average of two or more worms, less than ½ of an inch, per head or one worm, greater than ½ of an inch, per head.

Sorghum midge

Overwintering stage -- Larvae overwintering inside cocoons spun within the spikelets of sorghum, Johnsongrass, or other host-plant residue. Time and percentage of emergence in the spring are influenced by tempera ture, cultural practices, and several other factors.

Adult stage -- The sorghum midge is a small orange-colored fly that is difficult to see. The edges of its wings appear fringed under magnification. The female lives one day, laying 30 to 120 eggs, singly, in the glume. About 90 percent of the eggs are laid during the 4 days following plant-head emergence. The life cycle of the midge requires about 14 to 18 days. The male lives only a few hours.

Damage

Sorghum is susceptible to damage from midge only during the bloom period. The interval of blooming for a field depends on how uniform blooming occurs across a field. Once blooming begins, an individual head is susceptible to damage for 4 to 9 days. Adult midges do not damage sorghum; however, the female flys to blooming sorghum and crawls over seed heads in bloom in search of egg-laying sites.

The female sorghum midge deposits a single egg between the glumes of a floret. Larvae destroy the seed, resulting in "blank" or shriveled seed coats that often appear discolored. Heads with severe midge damage appear small and compressed with blank areas. For more information, refer to MSU-ES Information Sheet 1331, Sorghum Midge Control.

Economic Threshold

To help control sorghum insects, plant the crop between April 15 and May 10 and as uniformly as possible (depth and date). A crop planted under these conditions rarely has problems with worms (bollworm, fall armyworm, sorghum webworm) or midge. Sorghum planted after these dates or in staggered plantings over 6 to 10 days is much more susceptible to insect infestations. The staggered plantings or uneven emergence provide ideal situations for sorghum midge reproduction. During the bloom period, check the crop daily for sorghum midge infestations. Check fields for midge activity from mid morning until shortly after noon. Windy weather conditions will make the midge more difficult to locate and sample accurately.

Treat when one adult midge per head is observed after 20 percent to 30 percent of the heads have begun to bloom. Consider using midge-resistant varieties for some production areas.

Sorghum webworm

The larval stage ranges from pale green to tan in color and is thickly covered with spines and hairs. The back has four red to brown longitudinal stripes.

Damage

The larvae feed on the individual grain kernels and consume the contents, leaving the outside hull only. Prolonged hot, dry weather during the season appears to be a factor in lowering the level of damage.

Economic Threshold

Treat if there are two worms per head and if heads are 4 to 6 inches apart. Two webworms equal one corn earworm.

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Publication 1757
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director