|Sorghum Midge: Stenodiplosis sorghicola (Coquillett)|
|Description and Biology: The sorghum midge probably is the most widely distributed of all sorghum insect pests and one of the most damaging to sorghum in the southern United States. It occurs in almost all regions of the world where the crop is grown, except Southeast Asia. The adult sorghum midge is a 1.3-mm-long, fragile-looking, orange-red fly, with a yellow head, brown antennae and legs, and gray membranous wings. During the single day of adult life, each female lays about 50 yellowish-white eggs between the glumes of flowering spikelets of sorghum. The cylindrical eggs are 0.1 to 0.4 mm long and hatch in two to three days. Initially, larvae are colorless, but, when fully grown, are dark orange. Larvae complete development in nine to 11 days and pupate between the glumes of the spikelet. Shortly before adult emergence, the pupa works its way toward the upper tip of the spikelet until three-fourths the pupal length protrudes between the glumes at the tip of the spikelet. When the adult emerges, the clear pupal skin remains at the tip of the spikelet. The pupal period is completed in three days. A generation is completed in 14 to 16 days. The insect's rapid development permits multiple generations to occur during a season and results in high infestation levels when sorghum flowering times are extended by a wide range of planting dates or sorghum maturities. Sorghum midge diapause to overwinter as larvae in cocoons in spikelets of host grasses which are, in the United States, exclusively sorghum and johnsongrass. When plants are shredded or spikelets shatter, spikelets fall to the ground and are disked into or covered with the soil. Sorghum midges that emerge during the spring infest johnsongrass before flowering sorghum is available. The insect increases in abundance as the season progresses, especially if flowering sorghum continues to be available. Sorghum midge abundance decreases later in the season.|
|Symptoms and Damage: Damage to sorghum is caused by sorghum midge larvae feeding on the newly fertilized ovary, preventing kernel development and resulting in direct grain loss that can be great. Glumes of a sorghum midge-infested spikelet fit tightly together because no kernel develops. Typically, a sorghum panicle infested by sorghum midge will have various proportions of normal kernels scattered among nonkernel bearing spikelets depending on the degree of damage. An insecticide application would be justified when there is one sorghum midge per panicle of susceptible sorghum or about five sorghum midges per panicle of resistant sorghum.|
Monitoring: The presence of adults must
be determined when assessing sorghum midge abundance in a field. To do
so, fields should be inspected at midmorning when the temperature reaches
approximately 85 F, when sorghum midge adults are most abundant on flowering
sorghum panicles. Because adult sorghum midge live less than one day,
each day a new brood is present. This fact requires sampling almost daily
during the time sorghum panicles are flowering. Sorghum midge adults can
be seen crawling on or flying about flowering panicles. The simplest and
most efficient technique for detecting and counting sorghum midge is careful,
close inspection of all sides of randomly selected flowering panicles.
Panicles should be handled carefully during inspection to avoid disturbing
ovipositing sorghum midges. Other methods, such as placing a clear plastic
bag or jar over the panicle as a trapping device, can be used when sampling
Management: Effective control of sorghum midge requires
integration of several practices to avoid and reduce sorghum midge abundance.
Early and uniform planting of sorghum in a locale is the most effective
cultural management method. Planting hybrids with uniform maturity early
prevents late flowering of panicles and avoids damaging infestations.
Cultural practices that promote uniform panicle exertion and flowering
in a field also are important in sorghum midge management, in making treatment
decisions, and in achieving acceptable levels of chemical control. Eliminating
johnsongrass inside and outside the field with cultivation and/or herbicide
applications also will help suppress sorghum midge abundance. Deep plowing
sorghum residue kills some overwintering larvae, reducing sorghum midge
abundance the next year.