Effect of Combinations of Vairimorpha necatrix (Kramer) with Antibiotics on the Susceptibility of Spodoptera litura (Fabricius)

Vairimorpha necatrix (Kramer) was combined with different antibiotics to increase its pathogenicity. It was administered to third instar larvae of Spodoptera litura (Fabricius) under laboratory conditions by diet surface treatment and the mortality and spore yield was studied. Reproductive spores of V.necatrix at 10⁶ spores/ml were mixed with 100 ppm of antibiotics viz. gentamycin, oxytetracycline, ampicillin, kana mycin and chloramphenicol. The results showed that the larval mortality due to various Vairimorpha antibiotic combinations ranged from 10 to 62.07 per cent and it was high compared to prepupal and pupal stages. Assessment of spore yield in the infected larvae, pre-pupae and pupae revealed that larval stages recorded higher production of spores which ranged from 0.23 to 2.36 x 10¹⁰ spores for larva, 2.26 to 5.28 x 10⁹ spores for pre pupa and 0.28 to 2.23 x 10⁹ spores for pupa. Among the various combinations, Vairimorpha-gentamycin combination produced higher mortality and spore yield of V. necatrix.