Evaluation of a Laboratory Bioassay for Determining Resistance Levels to Sorghum Midge Contarinia sorghicola (Coquillett) (Diptera: Cecidomyiidae) in Grain Sorghum

The level of resistance to the sorghum midge, Contarinia sorghicola, in a range of grain sorghum hybrids was assessed: in the laboratory, by measuring oviposition (eggs/spikelet and % spikelets infested); in the glasshouse, by measuring oviposition and seed set; and in the field, by measuring panicle weight loss per ovipositing female per day. The levels of oviposition and seed set determined in the laboratory and glasshouse trials were significantly correlated with the field parameter. Number of eggs per spikelet was the laboratory parameter most highly correlated with the field result (r = 0.93, P < 0.01). Studies on the rate of oviposition under the conditions of the bioassay confirmed 6 h as an appropriate interval to expose panicles to midges. A laboratory method based on estimating the number of eggs produced by 5 midge females on 50 flowering spikelets in 6 h at 25°C and 75%orh is suggested as a practical method of testing for level of resistance in those sorghum hybrids showing ovipositional antixenosis.