Entomopathogenicity of endophytic Serratia marcescens strain SRM against larvae of Helicoverpa armigera (Noctuidae: Lepidoptera)

Abstract

An endophytic Serratia marcescens strain SRM (MTCC 8708) isolated from the flowers of summer squash was found to be entomopathogenic against the larvae of Helicoverpa armigera. Natural epizootic of this bacterial strain on the larvae collected from summer squash flowers ranged from 19.9 to 72.3%. Under laboratory conditions, a dose of 6 × 1010 c.f.u./ml diet induced 66.3% mortality of first instar H. armigera larvae. Similarly all other growth and development parameters of the insect were severely retarded in a dose-dependent manner. The bacterium invaded the entire alimentary canal and haemolymph with successful replacement of all other gut-associated microflora. There was a great reduction in midgut proteinase activity due to inhibition of five major proteinase isozymes by S. marcescens infection. Further, a synergistic interaction between chitinases isolated from this strain and Bacillus thuringiensis Cry1Ac toxin was observed. The present findings suggest that this plant-associated S. marcescens strain SRM could be suitably exploited for the management of H. armigera.