Spikelet flowering time: cause of sorghum resistance to sorghum midge (Diptera: Cecidomyiidae)

Resistance of sorghum, Sorghum bicolor (L.) Moench, to sorghum midge, Stenodiploxis sorghicola (Coquillett), has not been well understood. In this study, possible causes of resistance were examined by comparing time of oviposition with times spikelets of resistant and susceptible sorghums flowered and thus were vulnerable to damage. Spikelets of resistant genotypes began flowering during the night, at 0100 hours (CDT), and most were closed and no longer vulnerable by the time sorghum midges began to oviposit in the field at 0800 hours. Peak flowering of resistant sorghums was between 0300 and 0700 hours. Most spikelets of susceptible sorghums, however, were flowering after daylight, at 0800 hours, when sorghum midge abundance began to increase. At 1000 hours, the time most sorghum midges (2.5 per panicle) were in the field, percentages of spikelets penetrable by a pin the diameter of a sorghum midge ovipositor were 32 and 62% for resistant and susceptible sorghums, respectively. Damage to panicles of resistant sorghums averaged 29.6%, whereas susceptible sorghums were more than twice as damaged (68.2%). Delaying for 5 h the daily flowering period of a resistant hybrid to coincide with time of peak sorghum midge abundance in the field resulted in 3.2 times as many sorghum midges emerging and 2.2 times as much damage as to panicles of the resistant hybrid that flowered at the normal time. Damage and numbers of sorghum midges emerging were not significantly different between normal and changed panicles of a susceptible hybrid. Most spikelets of resistant sorghums flowered and closed early and thus evaded damage before ovipositing sorghum midges were in the field. These results support the hypothesis that resistance is caused by asynchrony between time of sorghum spikelet flowering and presence of sorghum midge.