Tritorphic interactions in sorghum, midge (Stenodiplosis sorghicola) and its parsitoid (Aprostocetus spp.)

Studies on tritrophic interactions involving sorghum genotypes, midge (Stenodiplosis sorghicola) and the predominant parasitoids (Aprostocetus spp.) at ICRISAT Asia Center were conducted using three midge resistant (ICSV 745, ICSV 89058 and IS 10712) and three susceptible (Swarna, CSH 9 and ICSV 112) genotypes during the post-rainy (1992/93) and rainy (1993) seasons. A coimbatorensis, the predominant parasitoid during post-rainy season, preferred mid-late midge larvae for parasitization, while A. gala, which was predominant during the rainy season, preferred early-mid larval stages. Variations in the preference of A. coimbatorensis and A. gala for the developmental stage of their host larvae indicate good prospects for the biological control of sorghum midge populations. There were significant differences in parasitization level of midge by Aprostocetus spp. between resistant and susceptible sorghum genotypes, and season. Higher parasitization was observed on susceptible genotypes than on resistant ones during both post-rainy and rainy seasons. However, the level of parasitization was greater in post-rainy than in rainy seasons. There was also evidence of higher midge infestation in rainy than in post-rainy seasons. Susceptible genotypes attracted more parasitoids because of high levels of midge infestations. Low parasitoid density in midge resistant sorghum under glasshouse and field conditions was associated with low midge infestations in these genotypes. However, parasitoids were always associated with their host in spite of low midge densities in resistant genotypes. The present study revealed that the interaction between host plant resistance and parasitoids of sorghum midge would thus be synergistic and complementary and could result in successful integration of these two important pest management tactics.