Synergistic effect of Xenorhabdus nematophila K1 and Bacillus thuringiensis subsp. aizawai against Spodoptera exigua (Lepidoptera: Noctuidae)

Xenorhabdus nematophila K1 (Xn) is the symbiotic bacterium of an entomopathogenic nematode, Steinernema carpocapsae, and causes fatal septicemia of the target insect. Our study showed that when orally ingested, the symbiotic bacterium could exhibit insecticidal toxicity against young larvae of Spodoptera exigua. The oral toxicity of Xn alone was analyzed in terms of its antibiotic activity to the target insect’s intestinal bacterial flora because Xn alone did not penetrate into the hemocoel of the insect. An intestinal bacterium was isolated from S. exigua and identified as Bacillus cereus K1. Owing to the symbiotic role of B. cereus for development and survival of young S. exigua larvae, the inhibitory activity of the orally ingested Xn against B. cereus can be suggested as a pathogenic pathway. Bacillus thuringiensis subsp. aizawai (Bta) effectively killed the third instar larvae of S. exigua, but did not cause high mortality of the fifth instar larvae. A mixture of Xn and Bta fed to the fifth instar larvae resulted in significant synergistic effect. The synergistic effect of the Xn and Bta mixture could be explained by the passage of Xn into the hemocoel through the damaged midgut cells caused by the Bta infection. Our study suggests a novel insecticidal activity of Xn in the gut as well as being a hemocoelic pathogenic agent. It also demonstrates a synergism between two entomopathogenic bacteria whereby Bta assists in allowing Xn to penetrate into insect’s hemocoel.