Insects Harmful to Sorghum

Description And Damage. The sorghum midge is, at times, a severe pest of grain sorghum in Alabama. The adult midges are tiny, fragile-looking, orange-colored gnats or flies. Damage to sorghum results from larvae that hatch from eggs deposited by the female midge on the flowering grain head. The pinkish orange larva or maggot feeds on the developing seed. Those heads damaged by midges are often referred to as "blasted".
     Blasted heads may also result from drought, disease, high temperature, or infertility. When the sorghum midge causes this damage, tiny pupal skins can often be seen attached to the empty grain seed.
Field Scouting. Since the adult midge lays eggs only during the time that sorghum is blooming (yellow anthers exposed on the seed head), that is the critical time to scout for midges.
     A single sorghum head requires about 9 days, beginning at the tip and progressing toward the base of the head, to completely pollinate. However, not all sorghum plants within a field will uniformly head-out at the same time; therefore, a field may have heads blooming for several weeks and will require scouting throughout that time.
     Fields should be scouted daily from early to mid morning to coincide with midge activity. Begin scouting when the field has reached 20-percent bloom and continue until 90-percent bloom. Randomly select and examine at least 100 heads across a field for the presence of midges on or around the grain head. Be careful not to disturb the plant.
     The plastic bag technique can also be used. Taking a half-gallon, clear plastic bag, walk through the field and select a sorghum head to test. Quickly place the bag over the head and close the bag. Shake the sorghum head vigorously into the bag. Then, carefully lift the bag and examine the contents for midges.
When To Treat. The sorghum midge overwinters in the "seed husks" of host plants in the maggot, or larval, stage. Host plants, in addition to grain sorghum, include sweet sorghum, johnsongrass, sudangrass, broomcorn, and other wild or uncultivated grasses. Adult emergence occurs in spring from these hosts about the time that johnsongrass begins to bloom.
     The adult midges mate, and then the female lays up to ten eggs in each head. A female may deposit 250 eggs over her 1- to 2-day lifetime.
     Midge populations increase in the spring on wild hosts and move into early plantings of grain sorghum. These early plantings are not usually sufficiently damaged to warrant control, but they do contribute to the buildup of the midge population. The increased populations are then capable of heavily damaging successive grain sorghum plantings.
     Within a local area, early uniform planting is the best way to avoid severe midge damage. Once significant populations of midges are found in grain sorghum fields, there is little chance for the later-blooming fields to escape damage unless they are treated.
     Pesticide treatments should be applied when 20 to 30 percent of the field has begun to bloom and an average of one midge per head is found. If adults are still active 3 to 5 days later, a second treatment may be needed, as well as a third treatment 3 to 5 days later if the midge population is high and the yield potential is good.
     Remember, treatments applied after pollination of the grain head will be too late to prevent any damage from the midge.

Description And Damage. Webworms are small, hairy caterpillars which reach a maximum length of 0.5 inch. They are greenish tan with well-marked darker stripes down the length of the body. The webworms feed on the developing seeds from the milky stage until seed maturity. Usually, several (5 to 15) will be found on a head. These pests will hollow out and consume many individual seeds in a head. The caterpillars will spin a web over the grain during feeding; however, these webs may not be conspicuous.
     Webworm populations are favored by cool, wet weather during late summer and early fall. Tight or close-headed varieties will usually have more webworms than the open-headed varieties. In some fields, damage from webworms may be unevenly distributed over the field with most significant damage occurring late in the season.
Field Scouting. After sorghum begins blooming, heads should be examined twice weekly for the presence of these caterpillars. To sample an individual head, bend it over and shake it into a bucket.
When To Treat. Apply insecticides for webworm control when you find an average of five or more small caterpillars per head in 25 percent of the heads examined.

Description And Damage. Fall armyworms and corn earworms are the most common large caterpillars found feeding on grain sorghum in Alabama. They may be similar in shape and size, reaching 1.5 inches in length, but they vary widely in color.
     They can usually be separated based on the color and markings of the head capsule. The head capsule of the fall armyworm is usually dark with a cream to white inverted Y on its "face" while the corn earworm has a yellow to orange head capsule without the white inverted Y.
     Corn earworms and fall armyworms feed on the foliage of grain sorghum as well as the grain. Prior to head emergence, they feed in the whorl as "budworms. " This foliage damage may appear devastating but can be tolerated well by the plants. The most serious damage by these worms is caused when they feed on the head, consuming or damaging large numbers of individual seeds.
     Late-planted or double-cropped sorghum is usually more seriously damaged by these two insects.
Field Scouting. Until head emergence, check weekly for evidence of budworms or damage. Although treatments are seldom warranted, excessive feeding on small sorghum plants may reduce stands or delay head emergence.
     Check for corn earworms or fall armyworms in the head the same way as for webworms.
When To Treat. Insecticide treatments are seldom justified for budworms. When 75 percent or more of sorghum plants are being damaged or an average of one worm per whorl is found, treatments may be economically feasible. However, treatments must be applied by ground equipment, directing 25 to 30 gallons spray into the whorl, to obtain effective control.
     For corn earworms or fall armyworms feeding in the head, treat when an average of one worm per head is found.

Greenbugs And Other Aphids. These soft-bodied insects are often called "plant lice". They cause damage by sucking plant juices. They may severely damage and stunt small seedlings or cause problems at harvest by secreting "honeydew" on the grain head. This causes grain to become sticky. A black fungus may grow on the honeydew, reducing the feed quality of the grain.
     Greenbugs or other aphids seldom require insecticidal control in Alabama. Generally, lady beetles, other predators, and certain parasites control aphid populations. However, if aphid feeding results in the death of two or more leaves per plant, insecticidal treatments may become necessary.
Lesser Cornstalk Borers. These insect pests are particularly important on late or double-cropped plantings during the hot, dry period of mid summer. Dry, sandy soil enhances the possibility of damage from these pests.
     The active larvae are blue-green with prominent transverse brown bands. They bore into the stems of the tiny seedlings and attach silken tubes covered with soil particles to the plants from which they feed.
     Under ideal conditions for the borer, heavy stand losses may be sustained. Once infestations are discovered, attempts at control are usually ineffective. Treating at planting or as plants emerge gives best results.
Chinch Bugs. The adult chinch bug is approximately 0.125 inch long and has a black body with silver-colored wings. The immature chinch bugs resemble the adults in shape but are reddish with a white band across the back.
     The adult and immature chinch bugs suck juices from sorghum plants, causing leaf reddening, wilting, and stunting of small plants. Heavy infestations may cause significant stand losses. Large numbers of chinch bugs may migrate in the spring from wheat or other hibernation sites to invade young sorghum fields nearby.
     Chinch bugs usually congregate at the base of plants; thus, insecticidal sprays should be directed to this area for effective control. Treat only the infested areas of the field.
European Corn Borer. The European corn borer may sometimes feed in the whorl of sorghum, but control is usually not necessary.
Sugarcane Borer. This insect may attack sorghum in the southernmost counties of Alabama. Injury tends to occur after injury from lesser cornstalk borer. There are several generations per year. Conventional at-planting treatments for lesser cornstalk borer can exacerbate sugarcane borer problems. In fields planted near sugarcane or that have had problems in the past with this insect, preserve natural enemies by avoiding at-planting treatments. The pesticides most effective against sugarcane borer are not registered on sorghum.