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

Resistance of sorghum, Sorghum bicolor (L.) Moench, to sorghum midge, Stolo diplosis sorghicola (Coquillett), has not been well understood. In this stu dy, possible causes of resistance were examined by comparing time of ovipos ition with times spikelets of resistant and susceptible sorghums flowered a nd thus were vulnerable to damage. Spikelets of resistant genotypes began f lowering 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 fiel d 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 incre ase. 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 s orghum midge ovipositor were 32 and 62% for resistant and susceptible sorgh ums, respectively. Damage to panicles of resistant sorghums averaged 29.6%, whereas susceptible sorghums were more than twice as damaged (68.2%). Dela ying for 5 h the daily flowering period of a resistant hybrid to coincide w ith time of peak sorghum midge abundance in the held resulted in 3.2 times as many sorghum midges emerging and 2.9 times as much damage as to panicles of the resistant hybrid that flowered at the normal time. Damage and numbe rs of sorghum midges emerging were not significantly different between norm al and changed panicles of a susceptible hybrid. Most spikelets of resistan t sorghums flowered and closed early and thus evaded damage before oviposit ing sorghum midges were in the field. These results support the hypothesis that resistance is caused by asynchrony between time of sorghum spikelet fl owering and presence of sorghum midge.