Flowering events in sorghum in relation to expression of resistance to sorghum midge, Stenodiplosis sorghicola

Abstract

Sorghum midge (Stenodiplosis sorghicola Coquillett) is an important pest of grain sorghum, and host plant resistance is one of the most effective means of controlling this pest. Several studies have shown that resistance to sorghum midge is associated with short and tight glumes, faster rate of grain development, and tannins. However, some recent studies suggested that time of flowering is the principal component of resistance to sorghum midge. Therefore, we conducted a series of experiments under laboratory and field conditions on the flowering behaviour of a diverse array of midge-resistant and midge-susceptible genotypes to quantify the contribution of time of flowering in genotypic resistance to sorghum midge. Time of flowering under field and laboratory conditions did not show any differences between midge-resistant and midge-susceptible genotypes. Time to maximum flowering varied considerably between the rainy and the post-rainy seasons. Under field conditions, most of the spikelets in the midge-resistant lines opened between 0000 and 0400 h, while most of the spikelets in the susceptible check, CSH 1, opened at 0200 h. Under light and dark conditions in the laboratory, most of the spikelets opened at 0300 h in all genotypes, irrespective of their level of resistance to the sorghum midge. However, flowering events continued for a longer period under dark conditions outside the laboratory. There were no differences in oviposition in panicles on the midge-resistant genotype, ICSV 745, infested between 0200 and 0600 h, and significantly more eggs were laid in spikelets of the susceptible check, CSH 1, than in the midge-resistant, ICSV 745, at all infestation times. The peak oviposition was recorded at 0945 h irrespective of the level of resistance to sorghum midge, and significantly more eggs were laid in the spikelets of CSH 1 and Swarna than in ICSV 745 and ICSV 197. There was no evidence of change in the susceptibility of sorghum midge-resistant genotypes when infested at different times in relation to time of flowering. Therefore, flowering behaviour of sorghum genotypes seems to play little role in genotypic susceptibility to sorghum midge.