Mechanisms Of Sorghum Resistance To The Spotted Stalkborer, Chilo Partellus ( Swinhoe) Lepidoptera Pyralidae

Abstract

Stalkborers are some of the most important pests of cereal crops in the tropics and in particular the semi-arid areas. Chilo partellus, the spotted stalkborer, is one of the most notorious pests of sorghum in that it occurs in almost all the areas where sorghum is grpwn in the semi-arid regions of Asia and Africa. Lately it has been found that this pest is gradually increasing its range and importance. Of the various sorghum pest control methods available in the semi-arid regions none holds better prospects than the use of resistant varieties. In this study several experiments were conducted to elucidate the mechanisms of resistance of different sorghum cultivars and also to propose ~he methodology for mechanisms of resistance studies. Accordingly five criteria were used - for explaining these mechanisms. These criteria were : Ca) Preference or Non-preference for oviposition of the moth on different sorghum cultivars, Cb) Larval establishment of Chilo partellus larvae first instarl on different sorghum cultivars, Cc) Relative leaf damage and stem tunnelling in different sorghum cultivars by C. partellus larvae, Cd) Biophysical and preJimary biochemical studies that elucidate the differences in cultivar susceptibility to partellus, and (e) studies on different cultivar tolerances to C. partellus damage. The oviposition preference studies revealed that sorghum cultivars differedm their suitability as a C. partellus oviposition substrate. Cultivar IS 2205 was the least preferred while IS 18363 was the most preferred. Experiments to identify the factors responsible were inconclusive. Evidence pointed to both biochemical and biophysical factors. First instar larval establishment studies were also inconclusive even though it was d~mostrated that larval establishment was different among the cultivars used. The only definit€ evIdence for poor establishment in-some cultivars was biophys-ical. Different cultivars were significantly different in their susceptibility to leaf damage. The tendency to form deadheirts was also significantly different. But reasons for these differences were not apparent. Cultivars were not significantly different in their susceptibility to tunnelling even though they had different amounts of fibre, lignin and sucrose. The single most important factor for the different cultivar susceptibilities was in their different tolerances to C. partellus attack. Tillering, in particular, was demonstrated to play an important role in compensation for damage. The ability to flower and to produce seed inspite of having a high infestation were also very significant. Susceptible cultivars dried up before they had flowered. Others still, flowered but could not form seed. Using tolerance as a criterion for resistance showed IS 18520 to be the most resistant. Formation of multiple heads was also shown to be an important factor. Different cultivars had varying effects on the development of f.. p.arte:!)us(ant ibiosis) . However, the antibiotic effect was either insignificant or only just significant statistically