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Studies were conducted on components of resistance to sorghum midge on four resistant (DJ 6514, AF 28, TAM 2566 and IS 15 107) and two susceptible cultivars (CSH 1 and Swarna). Data were recorded on the numbers of eggs, larvae, emerged adults and grain damage in panicles of different genotypes infested with 60 midge females/panicle under no-choice conditions. The size of floral parts (glume, lemma, palea, lodicule, stigma, style, ovary and anther), rate of grain development and tannin content of grain were measured. The lengths of glume g l and 82, lemma L1 and L2, palea, lodicule, anther, style and stigma were positively associated with susceptibility to sorghum midge. Rate of grain development (between 3rd and 7th day after anthesis) was negatively associated with susceptibility to sorghum. Tannin content of grain was also negatively correlated with midge susceptibility, although there were distinct exceptions (e.g. DJ 6514 is highly resistant bur has a low tannin content).

Sorghum midge, Contarinia sorghicola Coq. (Diptera: Cecidomyiidae) is an important pest of grain sorghum, and host-plant resistance is one of the most effective means of controlling this pest. We studied the antibiosis mechanism of resistance in sorghum to C. sorghicola in a diverse array of midge-resistant and midge-susceptible genotypes. Data were recorded on adult emergence, postembryonic developmental period, number of mature eggs in the ovary, fecundity, larval survival from artificially implanted eggs; and the tannins, soluble sugars, and protein content of 10-day old and mature grains during the 1982-91 rainy and post-rainy seasons.Adult emergence was significantly lower in the midge-resistant genotypes compared with the susceptible controls. Initiation of adult emergence was delayed by 4-8 days on DJ 6514, IS 8571, IS 9807, IS 10712, IS 19474, IS 19512, ICSV 830 and ICSV 197. Postembryonic 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 emergence or the extended developmental period was not observed during the post-rainy season in some genotypes. These differences in the expression of antibiosis to midge in resistant genotypes over seasons may be attributed to the effect of environmental conditions on the insect development and chemical composition of sorghum grain. Amounts of tannins and proteins were generally greater in the midge-resistant lines compared with the susceptible ones (except tannins in DJ 6514) while the soluble sugars were low in the midgeresistant lines (except TAM 2566). These differences in chemical composition of the grain between genotypes and variations over seasons have been discussed in relation to the expression of antibiosis mechanism of resistance to the sorghum midge. Antibiosis to sorghum midge was also evident in terms of smaller size of larvae, lower number of eggs in the ovary, reduced fecundity, and larval survival. Midge-resistant lines have diverse effects on the biology of this insect. Antibiosis along with other components of resistance can be used to develop cultivars with stable resistance to C. sorghicola.

Gene action for resistance to sorghum midge (Contarinia sorghicola Coq.) was studied in a diverse array of midge-resistant and midgesusceptible females and males under natural infestation and under uniform infestation with a no-choice headcage technique. Gene action for glume and grain characteristics associated with resistance to sorghum midge was also studied to understand their role in expression of resistance to this insect. Gene action for resistance to midge is largely governed by additive gene action. Genotype x environment interaction was significant for midge damage rating under natural infestation but nonsignificant under no-choice headcage screening. The GCA effects of midge-resistant cytoplasmic male-sterile (CMS) females (PM 7061 A and PM 7068 A) were significant and negative, and such effects for the midge-susceptible CMS females ICSA 42 and 296 A were positive. Similar results were observed for the males (except for CS 3541 and MR 750 for midge damage in one out of two seasons). Dominance (mid-parent heterosis) was also important midge resistance in some cross combinations. For genotypic nonpreference by the midge females, the SCA effects were greater than the GCA effects. The SCA effects for genotypic nonpreference were negative for PM 7061 A. The GCA effects were significant and negative for glume length in PM 7061 B, glume hardness for 296 B, and glume hairiness for PM 7061 B. The GCA effects were significant and positive for glume length, glume hairiness, and glume hardness of ICSB 42. Resistance is needed in both the parents to produce midge-resistant hybrids.

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