REDUCTION OF SHOOT FLY DAMAGE IN IRRIGATED POST-RAINY SEASON SORGHUM BY MANIPULATING IRRIGATION

Soil moisture was manipulated in an attempt to control shoot fly (Athe rigona soccata Rondani) incidence in irrigated post-rainy season sorgh um grown under a rainout shelter (ROS) and in field conditions. After uniform irrigation at sowing, the plants were subjected to water stres s at young seedling stage (7-28 days after emergence, DAE) for differe nt lengths of time. Soil water had profound effects on the production of water droplets on the surface of the central whorl leaf of seedling s (leaf surface wetness, LSW) of sorghum genotypes. LSW, which facilit ates movement of the larvae, was more drastically affected in suscepti ble (CSH 5) than in moderately resistant (IS 1054) sorghum genotypes. Shoot fly oviposition (infestation) and deadhearts (crop damage) were much higher in treatments with full irrigation (control) than in treat ments to which less water was applied during the first 3 wk after seed ling emergence. This resulted in higher plant biomass and overall grai n yield in the latter treatments than in the control. Using insecticid es to control shoot fly infestation, it was shown that a simple cultur al practice of inducing plant stress by reduced soil moisture content during early plant growth gave the same or better control of shoot fly damage and the same or higher grain yield than insecticide-protected plots with full irrigation. Thus the costs associated with irrigation requirement and insecticide can be greatly reduced in the former manag ement option compared with the latter. It is suggested that manipulati on of soil water content during the vulnerable early stages of crop gr owth can reduce shoot fly damage in irrigated post-rainy season sorghu m.