Sorghum Midge
Contarinia sorghicola (Coquillett), Cecidomyiidae, DIPTERA

DESCRIPTION

Adult - The sorghum midge is an orange fly, the male measuring approximately 1.3 mm in length and the female 1.6 mm. Color plate.

Egg - Each white, cylindrical egg, 0.3 by 0.6 mm, is attached to the host spikelet by a slender, tapering stalk.

Larva - The newly hatched larva is colorless. As it feeds on the developing grain, it gradually becomes pale pink to a deeper pink, then orange, and finally a darker orange or red-orange. The full-grown larva, 1.5 to 2.0 mm long, is slightly flattened and spindle-shaped, tapering to a point at the head.

Pupa - At first, the pupa is uniformly dark orange, but after a few hours the head, antennae, legs and thorax darken until they become black. Only the abdomen retains the orange color.

BIOLOGY

Distribution - The sorghum midge occurs in nearly all areas of the world where sorghum is grown. In the United States, it occurs from Virginia to Florida and as far west as Texas. It is an important full-season pest in Texas and a threat to late-planted sorghum in southeastern states such as North Carolina. Areas where sorghum has been grown for several years and where Johnson grass is prevalent are typically infested.

Host Plants - Johnson grass and grain sorghum are the primary host plants of the sorghum midge. Although the midge has been reported on and reared from 14 other grasses, these hosts generally are not considered suitable for normal midge development.

Damage - Larvae of the sorghum midge feed on the ovary thereby preventing normal seed development. Infested heads appear blighted or blasted and produce small, malformed grain.

Life History - Sorghum midges overwinter as larvae in aborted sorghum spikelets. They spin cocoons inside the spikelets where they may remain in a resting stage, resistant to cold and desiccation for as long as 2 or 3 years. Under favorable conditions, however, pupation and emergence take place the following spring at about the time Johnson grass begins to bloom. After mating, each female then deposits 30 to 120 eggs, singly, in the flowering spikelets of this grass. The eggs hatch 42 to 60 hours later, depending upon the temperature. The first two generations of the sorghum midge can be found on Johnson grass, after which a migration occurs to the flowering sorghum spikelets. Under normal summer temperatures, the complete life cycle requires 14 to 16 days. Under warm (26 degrees C), humid conditions, at least nine generations per year are possible in North Carolina.

CONTROL

Several cultural practices have been recommended for control of the sorghum midge. These include preventing Johnson grass or other hosts from producing heads in and around sorghum fields before the crop blooms, planting at the time of year best suited for the variety selected, and destroying crop residues which may contain overwintering larvae. If early- and late-maturing varieties are to be grown in close proximity, some benefit may be derived by placing fields so that prevailing winds blow toward the earlier field. In North Carolina, early planting is the most logical control practice.

When a large adult population is detected at bloom, an insecticide treatment is warranted. Since the larvae are protected in the seed and spikelets, spray applications should be directed toward the ovipositing females which are particularly abundant for several days after the sorghum head emerges. For specific information concerning insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.