Sorghum Midge, Contarinia sorghicola, Diptera: Cecidomyiidae

Description

The sorghum midge is generally the most damaging insect pests of grain sorghum in Arkansas and without management may devastate the crop. The adult sorghum midge is a minute (<1/8 inch long) orangish “gnat-like” fly which can be detected on flowering sorghum heads. Larvae are minute orange maggots found in the seed.

Distribution, damage and impact

The sorghum midge occurs worldwide and throughout Arkansas. Larvae feed directly on the developing seed and may destroy all seed within each sorghum head (Photo 7). Where high sorghum midge populations are left unmanaged, resulting damage may be severe and yield reduction may approach 100 percent.

Life History

When temperatures climb into the 70's in late winter or spring adult flies begin to emerge from old seed heads in which larvae have overwintered. Wild grasses including johnsongrass are the initial host plants and several generations can develop within these hosts prior to flowering of grain sorghum. Because of the limited availability of the wild hosts, midge populations generally remain below damaging levels in early summer. As early planted grain sorghum begins to flower, adult flies migrate into these fields, especially into the edge of the sorghum field immediately downwind from the wild host. The flies mate and each female midge can deposit up to 250 eggs in less than two days. Larvae emerge from the eggs within two days and burrow into the developing seed. Pupation occurs on the seed and lasts for about three days. The time required for a complete generation varies from two to three weeks depending on temperature. Thus midge populations can rapidly increase and later sorghum plantings may be severely infested.

Management

The first choice of management should be early and uniform planting. In areas or even within fields, where the period of sorghum flowering is extended, the sorghum midge can complete multiple generations on grain sorghum and build to damaging levels in late blooming fields. Because only the flowering stage is susceptible to midge attack, delaying subsequent grain sorghum plantings for five or six weeks may disrupt population increase. Destruction of wild host plants including johnsongrass early during the season may slow midge population development. Some grain sorghum hybrids possess some level of resistance to sorghum midge. In fields with the potential for high yields and profit, it is important to scout for sorghum midge as soon as the plants begin to bloom. Scouting should be initiated when about 25% of the field begins to bloom. Sorghum heads on the field edges should be searched for the orangish flies. This can be accomplished by slowly approaching the plant and visually searching all sides of the head for adult midges. Also, placing a clear half-gallon plastic bag over the head and shaking the head in the bag will dislodge and capture adults. By carefully removing the bag and examining the contents, adults can be counted. Mornings with reduced winds are the best time for sampling. Within each field (80 acres or smaller) 100 heads should be counted with emphasis placed on the field boarders. The current economic threshold in Arkansas is an average of 1 adult sorghum midge per head. If the field possesses the potential for economic profit and the threshold is surpassed, insecticide application is likely warranted. Additional scouting should be completed at about two day intervals until the soft dough stage. Adults detected at this stage are unable to infest the seed. Following harvest, grain sorghum crop residue should be destroyed as larvae overwinter within seed.

Insecticide recommendations for sorghum midge on grain sorghum

See Table 2 for insecticide recommendations current at time of publishing. Current updated recommendations can be found in the Cooperative Extension Service publication MP-144 and on the world wide web at WWW.cdms.net/manuf/default.asp. Always follow instructions on pesticide labels.