INTERACTIONS BETWEEN PANICLE SIZE, INSECT DENSITY, AND ENVIRONMENT FOR GENOTYPIC RESISTANCE IN SORGHUM TO HEAD BUG, CALOCORIS-ANGUSTATUS

Studies were undertaken on interactions between panicle size, insect d ensity, host plant, and the environment for sorghum head bug, Calocori s angustatus Lethiery on five sorghum genotypes in terms of bug popula tion increase, grain damage and loss in grain mass across four panicle sizes (5, 10 or 20 branches/panicle and whole panicle), and three inf estation levels (5, 10 and 15 pairs of bugs/panicle). Head bug numbers increased and grain damage decreased with an increase in panicle size in the head bug susceptible cultivars, CSH 1 and CSH 5. However, the increase in bug numbers or decrease in grain damage was not significan t in head bug resistant genotypes, IS 17610 and IS 17645. Head bug num bers increased with an increase in infestation level in CSH 1 and CSH 5, however, such an increase was not substantial in IS 17610 and IS 17 645. Grain damage was significantly lower in IS 17610 and IS 17645 com pared with CSH 1 and CSH 5 across infestation levels. Head bug populat ion increased at a greater rate during the rainy season compared with the dry season. Panicle size and infestation levels accounted for grea ter variation in grain damage and percentage loss in grain mass during the rainy season than in the dry season. To identify reliable sources of resistance to insects, it is important to study insect host plant- interactions across panicle sizes (levels of food availability), infes tation levels and seasons.