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 for adults.
Because they are relatively weak fliers and rely on wind currents to aid their dispersal, adult sorghum midge usually are most abundant along edges of sorghum fields. For this reason, plants first should be inspected along field borders, particularly those downwind of fields of earlier flowering sorghum or johnsongrass. If few sorghum midges are found on sorghum panicles along field edges, there should be little need to sample the entire field. However, if sorghum midge numbers in border areas of a field equal or exceed the economic threshold level, additional panicles from the entire field (avoiding plants within 45 m of field borders) should be inspected. Average sorghum midge abundance should be calculated based on these additional samples. At least twenty panicles for each eight hectares in the field should be sampled.

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.
Sorghum hybrids resistant to sorghum midge, within limits, provide an additional management tool. At similar infestation levels of ovipositing sorghum midge females, resistant hybrids generally are only one-fifth as damaged as susceptible hybrids, meaning that resistant sorghum hybrids have economic injury levels five times higher than susceptible hybrids. Multiple insecticide applications directed at sorghum midge adults primarily are used to prevent damage when sorghum is planted too late to escape damaging infestations. To determine the need for chemical control, crop development, yield potential, and sorghum midge abundance should be assessed daily during sorghum flowering. Because sorghum midge lay eggs in flowering sorghum panicles (yellow anthers exposed on individual spikelets), damage can be caused until the entire sorghum panicle or all plants in the field have flowered. The period of susceptibility to sorghum midge may last from seven to nine days (individual panicle) to two to three weeks (individual field), depending on uniformity of flowering. Peak flowering of panicles in a field occurs on the eighth day of flowering, when 19.2% of the sorghum in the field is flowering.
Need for insecticide treatment is based on the number of adult sorghum midges during the sorghum flowering period. If adults still are present three to five days later, immediately apply a second insecticide treatment. Several insecticide applications at three-day intervals may be justified if sorghum midges are abundant.

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