Watch for sorghum midges in late-blooming sorghum

Tuesday, August 12, 2014

Oklahoma sorghum producers should be on the lookout for sorghum midges in late-blooming fields.

Tom Royer, Oklahoma State University Cooperative Extension entomologist and Integrated Pest Management coordinator, recently noted high populations of sorghum midge adults actively swarming on some late-blooming heads in sorghum test plots at the OSU Division of Agricultural Sciences and Natural Resources’ Cimarron Research and Extension Center in Perkins.

“Favorable climate and abundant Johnsongrass may allow them to become a problem in late-planted sorghum this year,” he said. “Johnsongrass establishes and spreads well in disturbed areas, such as crop fields and roadsides. As the season progresses, midge numbers build on Johnsongrass and sorghum, and will concentrate on later plantings, underscoring the need to control Johnsongrass in and around sorghum fields.”

A sorghum midge is a tiny fly, measuring less than 1/32 inches long. It has a reddish abdomen with one pair of grayish transparent wings.

“The female fly lays eggs in open florets and the maggots feed inside the developing seed,” Royer said. “The contents of the seed are usually completely consumed. The lifecycle from egg to adult is completed in 14 to 16 days.”

Heads that have sorghum midge injury are referred as “blasted.” The adults live for one day, but one female can lay 30 to 100 eggs.

“Scouting a field is essential to achieve effective control with an insecticide,” Royer said. “Use a 10x magnifying hand lens to aid in in identification.”

There are two ways to scout: One is to carefully move to a plant without disturbing it, quickly put a plastic bag over the head, and shake it vigorously. Remove the bag and contents and look for midges inside the plastic baggie.

“The other way is through direct observation,” Royer said. “Without disturbing the plant, look for small gnat-sized flies moving about the head or laying eggs on flowers with extended anthers. A sorghum midge is most active from 9 a.m. to 11 a.m., so that is the best time to scout.”

Producers should begin scouting when sorghum heads first emerge and begin pollinating, continuing to scout every three days until the field is finished blooming. The economic threshold insecticide application is one midge per head for susceptible plant varieties and five midges per head for resistant varieties. There are numerous insecticides labeled for control.

“Apply the first when the threshold is reached, and 25 percent to 30 percent of the heads are blooming,” Royer said. “A second application may be necessary in three to five days, and producers should always take care to follow all label restrictions.”

For a list of registered insecticides, consult OSU Current Report CR-7170, “Management of Insect and Mite Pests in Sorghum,” available online at http://osufacts.okstate.edu.

Royer said producers can help mitigate sorghum midge problems next year by employing the following management strategies, listed from most to least desirable:

First, plantings of sorghum should occur early and uniformly in an area. By planting early, the crop typically will avoid an infestation. If all plantings flower at the same time, existing midge populations become diluted. If sorghum is planted as a double crop, and there are full-season sorghum crops nearby, consider planting a variety that will flower before August 15.

“In general, the risk of a sorghum midge infestation increases for each day past August 15 the sorghum blooms,” Royer said.

Second, control Johnsongrass in and around sorghum fields. Controlling Johnsongrass before the sorghum blooms can reduce resident midge populations.

Third, choose a resistant sorghum variety, of which a limited number are available from seed companies. Resistant lines generally suffer only a fifth the level of injury that susceptible lines receive from the same number of adults.

“Consider a resistant variety for double crop sorghum that will bloom after Aug. 15,” Royer said.

The value of the U.S. grain sorghum crop is approximately $1.6 billion annually. Oklahoma is one of the nation’s leading producers of sorghum, along with Texas, Kansas, Louisiana, Arkansas, South Dakota, Mississippi and Nebraska.