ANTIBIOSIS COMPONENT OF RESISTANCE IN SORGHUM TO SORGHUM MIDGE, CONTARINIA-SORGHICOLA

Sorghum midge, Contarinia sorghicola Coq. (Diptera: Cecidomyiidae) is an important pest of grain sorghum, and host-plant resistance is one o f the most effective means of controlling this pest. We studied the an tibiosis mechanism of resistance in sorghum to C. sorghicola in a dive rse array of midge-resistant and midge-susceptible genotypes. Data wer e recorded on adult emergence, postembryonic developmental period, num ber of mature eggs in the ovary, fecundity, larval survival from artif icially implanted eggs; and the tannins, soluble sugars, and protein c ontent of 10-day old and mature grains during the 1982-91 rainy and po st-rainy seasons. Adult emergence was significantly lower in the midge -resistant genotypes compared with the susceptible controls. Initiatio n of adult emergence was delayed by 4-8 days on DJ 6514, IS 8571, IS 9 807, IS 10712, IS 19474, IS 19512, ICSV 830 and ICSV 197. Postembryoni c developmental period was prolonged on DJ 6514, IS 15107, IS 3461, IS 7005, IS 19474, ICSV 831 and ICSV 197. However, the delay in adult em ergence or the extended developmental period was not observed during t he post-rainy season in some genotypes. These differences in the expre ssion of antibiosis to midge in resistant genotypes over seasons may b e attributed to the effect of environmental conditions on the insect d evelopment and chemical composition of sorghum grain. Amounts of tanni ns and proteins were generally greater in the midge-resistant lines co mpared with the susceptible ones (except tannins in DJ 6514) while the soluble sugars were low in the midge-resistant lines (except TAM 2566 ). These differences in chemical composition of the grain between geno types and variations over seasons have been discussed in relation to t he expression of antibiosis mechanism of resistance to the sorghum mid ge. Antibiosis to sorghum midge was also evident in terms of smaller s ize of larvae, lower number of eggs in the ovary, reduced fecundity, a nd larval survival. Midge-resistant lines have diverse effects on the biology of this insect. Antibiosis along with other components of resi stance can be used to develop cultivars with stable resistance to C. s orghicola.