Evaluation of planting date, sorghum hybrid, and insecticide treatment on sorghum midge (Diptera : Cecidomyiidae) management in northeast Louisiana

The combined effect of planting date, insecticide treatment, and host-plant resistance was studied in northeast Louisiana for management of the sorghu m midge, Stenodiplosis sorghicola (Coquillett), during 1994 and 1995. Signi ficantly higher numbers of sorghum midges were observed visiting flowering spikelets of the midge-susceptible sorghum hybrid (Delta and Pine Land 'DP1 552') than those of the midge-resistant sorghum hybrid (DeKalb 'DK-60'). Nu mbers of midges averaged 1.2 and 0.6 per flowering panicle in the susceptib le and resistant sorghum hybrids, respectively, in 1994 and 1.8 and 1.0, re spectively, in 1995. Midge densities increased significantly as the sorghum flowering season progressed. Sorghum midge reached peak densities during t he first half of August in 1994 and 1995. The length of the flowering perio d in the early-planted (mid-March) sorghum was significantly longer compare d with the flowering periods in the mid-April, mid-May, or mid-June planted sorghums. This resulted in prolonged exposure of flowering panicles to ovi positing midges and increased midge damage in the early-planted (mid-March) sorghum. Damage by sorghum midge was significantly higher in the early-pla nted (mid-March) sorghum hybrids than in the late-planted (mid-June) sorghu m hybrids. The midge-susceptible hybrid produced highest yields when plante d in mid-April and mid-May (optimum period) and lower yields when planted v ery early (i.e., mid-March) or late (i.e., mid-June). No significant differ ences were observed in yields for the resistant hybrid at any planting date in 1994. However, in 1995, significantly lower yields were recorded in res istant sorghum planted in mid-June. Levels of sorghum midge damage and sorg hum seed yields in the untreated resistant hybrid were not significantly di fferent than those observed in the insecticide-treated susceptible hybrid. Numbers of adult midges captured on sticky traps were positively correlated to numbers of visual estimates of ovipositing midge females visiting flowe ring spikelets.