Resistance modality of resistant hybrid sorghum to sorghum midge (Diptera: Cecidomyiidae)

Typescript (photocopy).Using the the sorghum midge, Contarinia sorghicola (Coquillett), sorghum midge resistant hybrid ATx2755 X Tx2767 and the sorghum midge susceptible hybrid ATx2752 X Tx430, the resistance mechanisms and several aspects of sorghum resistance to sorghum midge were examined under field conditions from 1982 to 1984 at College Station, Texas. Based on the number of adult female midges captured per flowering panicle, about 1/3 more female midges visited flowering panicles of the susceptible than the resistant hybrid. Also, about 1/4 more female midges were captured on flowering panicles of either hybrid which had not been previously midge infested compared to when panicles had already been infested. The number of eggs laid in the spikelets by 20 caged females per panicle or by naturally occurring females in the field, was from 4 to 5 times greater in spikelets of the susceptible than the resistant hybrid. Sorghum midge females searched flowers of the resistant hybrid more rapidly than flowers of the susceptible hybrid; however, the time spent attempting to oviposit was longer on the resistant hybrid. Also, the frequency of ovipositional success was much lower, about 4 times, in flowers of the resistant than the susceptible hybrid. These results support that nonpreference for oviposition is an important resistance mechanism of resistant hybrid to sorghum midge. The development of immature midges in spikelets of the resistant and susceptible hybrid was evaluated by periodically collecting samples of spikelets from controlled panicle infestations, and by dissecting subsamples of these spikelets under a stereomicroscope. Proportionally, a greater number of midges reached the late immature instars (large larvae and pupae) in spikelets of the susceptible than the resistant hybrid. In addition, large larvae and pupae were positioned next to the carvopsis in a higher proportion in spikelets of the susceptible than the resistant hybrid. However there was no difference of the size of pupae developed in spikelets of resistant and susceptible hybrid. The number of immature midges that developed per spikelet can only partially explain the large reduction in caryopsis size of infested spikelets of either hybrid..