| Sorghum midge, Stenodiplosis sorghicola (Coquillett), is a major insect pest of sorghum, Sorghum bicolor (L.) Moench. Adult sorghum midges emerge in the morning, lay eggs only in flowering spikelets and die the same day. Larval feeding prevents kernel development. Development of hybrids with higher and more durable resistance to sorghum midge requires understanding resistance mechanisms. Sorghum resistance to sorghum midge seems caused by early spikelet flowering and glume closure. Most spikelets of resistant genotypes flowered earlier than susceptible sorghums and completed flowering before sorghum midges were ovipositing. Damage to resistant sorghums was to spikelets not yet tightly closed when sorghum midges were present. Higher temperatures resulted in earlier spikelet flowering of most lines but did not affect sorghum hybrids. Only 0.2 or fewer sorghum midges per panicle were present between 0100 and 0700 h, when resistant sorghums were flowering. By 1000 h, when sorghum midge abundance averaged 2.5 per panicle, 83.0% of spikelets of resistant sorghums had completed flowering and had tightly closed glumes. Glumes of only 56.5% of spikelets of susceptible sorghums were closed at 1000 h, when sorghum midges were most abundant. Susceptible sorghums were significantly more damaged (55.5%) than resistant sorghums (25.0%).;More than three times as many sorghum midges emerged per panicle (75.4) and damage rating was more than double (3.1) for a resistant hybrid changed to flower 5 h later (1100) than for the same hybrid flowering at the normal time (23.8 sorghum midges and 1.4 damage rating). A susceptible sorghum hybrid exposed to the same treatment was as damaged as the nontreated counterpart.;Glume tightness was estimated at 1000 by attempting insertion of a number 00 insect pin to mimic a sorghum midge ovipositor. More glumes of resistant (62.0%) than susceptible sorghums (32.0%) were tightly closed. Resistant sorghums had shorter stigmas (1.08-1.16 mm), longer (1.18-1.20 mm) and wider (1.03-1.05 mm) ovaries, shorter filaments (0.99-1.01 mm), and shorter anthers (1.45-1.47 mm) than susceptible sorghums, (1.27-1.41 mm), (1.13-1.16 mm), (0.99-1.01 mm), (1.25-1.37 mm) and (1.52-1.65 mm), respectively. However, no relationship could be determined among sizes of floral parts and resistance to sorghum midge. |
