Abstract: Broth culture of Xenorhabdus nematophila and Photorhabdus luminescens have been observed lethal to the nymphs of locust Schistocerca gregaria when injected into the abdomen, applied orally, mixed with bran or applied to the foliage of food plants as compared to broth alone (control). A hyperbolic relationship was observed between different bacterial application methods of cells and their metabolites and time intervals. Bacterial cells and their metabolites caused more or less similar damage to the locust but it is supposed that the insect death was probably due to the toxic metabolites present in the bacterial cells. These bacterial cells were also recovered from the abdominal haemocoele indicating that bacterial symbionts do have a free-living existence and can enter in the haemocoele in the absence of nematode vector. If these bacterial symbionts are to be used for insect control, X. nematophila would be the most appropriate as it has never been previously reported from clinical specimens.