Relationship of Sorghum Midge (Diptera: Cecidomyiidae) Density to Damage to Resistant and Susceptible Sorghum Hybrids

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Abstract
Three methods were used in 1980 and 1981 to investigate the relationship between adult sorghum midge, Contarinia sorghicola (Coquillett), density and yield loss in one susceptible, ATx2752 × RTx430, and two resistant, ATx2755 × RTx2767and ATx2761 × RTx2767, sorghum hybrids. By using natural infestation (method 1) and a sequential model (method 3), similar results were obtained: 1.5 g of grain lost (42 to 48 damaged spikelets) per ovipositing midge for the susceptible and 0.32 g (9 damaged spikelets) for the resistant hybrids. A technique using caged midges (method 2) resulted in less loss per midge than the other two methods: 0.54 g (16 damaged spikelets) for the susceptible and 0.14 g (4 to 6 damaged spikelets) for the resistant hybrids. This was due to reduced oviposition by midges in cages. However, the relative differences in midge damage to resistant and susceptible sorghum hybrids were similar by all three methods. Resistant hybrids suffered about one-fifth as much damage as the susceptible hybrid because of a reduction in oviposition (45%), in the proportion of egg-infested spikelets that failed to produce kernels (52%), and in weight loss per ovipositing midge (80%).Under the conditions of these experiments, an economic threshold level of one adult midge per flowering panicle of susceptible sorghum and five for resistant sorghum is suggested. The 5-fold increase in the economic threshold level for resistant hybrids is of major significance to integrated pest management strategies.

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