Primary cause of mortality in the armyworm larvae simultaneously parasitized by parasitic wasp and infected with bacteria.

Parasitoid wasps never kill their hosts before the wasp larvae emerge from the host. However, almost 100% of the host armyworm larvae Pseudaletia separata die within 2-3 days by parasitization with the wasp Cotesia kariyai or by injection of polydnavirus, the wasp symbiont virus, when they are simultaneously infected by the pathogenic bacterium Serratia marcescens. The present study was conducted to elucidate the crucial factor causing this larval mortality. An insecticidal protein has been shown to exist in the hemolymph of dying host larvae; it has been purified by procedures consisting of reverse-phase column extraction, gel filtration and ion-exchange column chromatography. The purified protein showed a strong insecticidal effect with a median lethal dosage (LD50) of 13 pmol/larva and was estimated to have a molecular mass of 57 kDa. The amino acid sequence of the insecticidal protein was partially characterized and used for isolation and sequencing of the genomic DNA. The deduced amino acid sequence for this protein revealed striking similarity with the metalloprotease of S. marcescens enterobacter.