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 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 (Color Plate 11).
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 eleven days and pupate between the glumes of the spikelet. Shortly before adult emergence, the pupa moves upward until three-fourths the pupa protrudes between the glumes at the tip of the spikelet.
After the adult has emerged, the clear pupal skin remains at the tip of the spikelet. The pupal period lasts three days. A generation is completed in fourteen to sixteen days. The insect’s rapid development permits multiple generations during a season and results in high infestation levels when sorghum flowering is extended by a range of planting dates or sorghum maturities. Sorghum midge diapause to overwinter as larvae in cocoons in spikelets of host grasses that, in the United States, are exclusively sorghum and johnsongrass. When plants are shredded or spikelets shed, 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 during the season, especially if flowering sorghum continues to be available. Sorghum midge abundance declines late in the season.
Symptoms & Damage
Sorghum midge larvae feed on the newly-fertilized ovary, preventing kernel development and causing direct grain loss. Glumes of a sorghum midge-infested spikelet fit tightly together because no kernel develops. Typically, a sorghum panicle infested by sorghum midge will have, depending on the degree of damage, various proportions of normal kernels scattered among nonkernel-bearing spikelets.
The economic impact of different abundance levels of sorghum midge infesting resistant and susceptible sorghum hybrids is shown in Figure 7. In1998 economics, a $10 insecticide application would be justified when there was about one sorghum midge per panicle of susceptible sorghum or about five sorghum midges per panicle of resistant sorghum.
Abundance of sorghum midge adults must be assessed. To do so, fields should be inspected at midmorning when the temperature reaches approximately 28 C, when sorghum midge adults are most abundant on flowering sorghum panicles. Because adult sorghum midge live less than one day, a new brood is present each day. Sorghum midge abundance should be monitored almost daily during panicle flowering. Sorghum midge adults crawl on or fly about flowering panicles. The easiest and most efficient way to detect and count sorghum midge is carefully inspecting of all sides of randomly-selected flowering panicles. Panicles should be handled carefully 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.
Because they are weak fliers and rely on wind currents to aid their dispersal, adult sorghum midge usually are most abundant along borders of sorghum fields. For this reason, plants first should be inspected along field borders, particularly fields downwind of earlier flowering sorghum or johnsongrass. If few sorghum midges are on sorghum panicles along field borders, there is little need to sample the entire field. However, if sorghum midge numbers along field borders 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 should be sampled for each 8 ha in the field.
Effective management of sorghum midge requires integration of practices to avoid or reduce 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 and avoids damaging infestations. Cultural practices that promote uniform panicle exsertion and flowering in a field also are important in sorghum midge management, in making treatment decisions, and in achieving acceptable levels of insecticidal control. Using cultivation or herbicides to eliminate johnsongrass inside and outside the field also helps suppress sorghum midge abundance. Deep plowing sorghum residues kills some overwintering larvae, reducing sorghum midge abundance the next year.
Sorghum midge-resistant hybrids, within limits, provide an additional management tool. At similar infestation levels of sorghum midge females, resistant hybrids are only one-fifth as damaged as susceptible hybrids, meaning that resistant hybrids have economic injury levels five times higher than susceptible hybrids.
Multiple insecticide applications directed at sorghum midge adults 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 spikelets (yellow anthers exposed), damage can be caused until all spikelets on a panicle or all plants in a 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 percent of the sorghum in a field is flowering (Figure 8).
Need for insecticide is based on the number of adult sorghum midge per panicle during the flowering period. Economic injury levels for susceptible and resistant sorghum hybrids are presented in Table 5. If adults still are present three to five days after the first applications of insecticide, immediately apply insecticide again. Several insecticide applications at three-day intervals may be justified if sorghum midges are abundant.