,  , The entomopathogenic bacterium, Xenorhabdus sp., was isolated from an entomopathogenic nematode, Steinernema monticolum. When these bacteria were injected into the hemocoel of the diamondback moth, Plutella xylostella, they caused significant mortality. However, the bacterium was not pathogenic when it was administered orally. This study showed that Xenorhabdus sp. significantly enhanced oral pathogenicity of Bacillus thuringiensis (Bt) against the last instar larvae of P. xylostella. Different ratios of culture broth of Xenorhabdus sp. and Bt showed significantly different pathogenicities against P. xylostella. In field tests, the optimal bacterial mixture significantly enhanced control efficacy against P.xylostella compared to Bt treatment alone. These results demonstrated that Xenorhabdus sp. culture broth can be developed as a potent biopesticide by enhancing the insecticidal efficacy of Bt.

An entomopathogenic bacterium, Xenorhabdus sp., symbiotic to Steinernema monticolum was investigated in its insecticidal activity. The bacteria induced septicemia of two lepidopteran insects (Plutella xylostella, Spodoptera exigua), a coleopteran insect (Tribolium castaneum), and a hemipteran insect (Riptortus clavatus) when they were injected into hemocoel. The bacterial culture broth contained immunosuppressive factor(s) that inhibited hemocyte nodulation in response to heat-killed bacteria. The immunosuppressive activity appeared to be caused by inhibition of two main immune-associated enzymes, phospholipase A2 (PLA2) and phenoloxidase (PO). HPLC analysis of the bacterial culture broth contained several PLA2 inhibitors. The bacterial culture broth significantly enhanced Bt pathogenicity. There results support a novel insect pest control strategy using eicosanoid-biosynthesis inhibitors.