Relative potency of a novel acaricidal compound from Xenorhabdus, a bacterial genus mutualistically associated with entomopathogenic nematodes.

Our study aimed to identify the novel acaricidal compound in Xenorhabdus szentirmaii and X. nematophila using the easyPACId approach (easy Promoter Activated Compound Identification). We determined the (1) effects of cell-free supernatant (CFS) obtained from mutant strains against T. urticae females, (2) CFS of the acaricidal bioactive strain of X. nematophila (pCEP_kan_XNC1_1711) against different biological stages of T. urticae, and females of predatory mites, Phytoseiulus persimilis and Neoseiulus californicus, (3) effects of the extracted acaricidal compound on different biological stages of T. urticae, and (4) cytotoxicity of the active substance. The results showed that xenocoumacin produced by X. nematophila was the bioactive acaricidal compound, whereas the acaricidal compound in X. szentirmaii was not determined. The CFS of X. nematophila (pCEP_kan_XNC1_1711) caused 100, 100, 97.3, and 98.1% mortality on larvae, protonymph, deutonymph and adult female of T. urticae at 7 dpa in petri dish experiments; and significantly reduced T. urticae population in pot experiments. However, the same CFS caused less than 36% mortality on the predatory mites at 7dpa. The mortality rates of extracted acaricidal compound (xenocoumacin) on the larva, protonymph, deutonymph and adult female of T. urticae were 100, 100, 97, 96% at 7 dpa. Cytotoxicity assay showed that IC50 value of xenocoumacin extract was 17.71 μg/ml after 48 h. The data of this study showed that xenocoumacin could potentially be used as bio-acaricide in the control of T. urticae; however, its efficacy in field experiments and its phytotoxicity need to be assessed in future.