Post-exposure effects of Photorhabdus nematicidal secondary metabolites on the fitness of entomopathogenic nematodes

Previous studies conducted by our team have shown that three secondary metabolites (SMs) from Photorhabdus luminescens sonorensis, trans-cinnamic acid (TCA), (4E)-5-phenyl-4-pentenoic acid (PPA), and indole, exhibit nematicidal and/or nematistatic activities against root knot and citrus nematodes, with no discernible effects on non-target entomopathogenic nematodes (EPNs). To further explore the post-exposure fitness of EPNs, this study focused on the effects of these SMs on the virulence and reproductive fitness of three EPNs: Heterorhabditis sonorensis (the native host of P. l. sonorensis), H. bacteriophora, and Steinernema carpocapsae. The results indicate that the virulence of H. sonorensis was not affected by the tested SMs. TCA significantly reduced virulence of H. bacteriophora, by 31 %, delayed mortality progression to eight days post-infection, and prolonged progeny emergence by two days. Nevertheless, none of the tested SMs adversely affected progeny production in H. bacteriophora. In contrast, the virulence and reproductive fitness of S. carpocapsae were not affected by any of the tested SMs. PPA did not adversely affect the virulence and reproductive fitness of any of the tested EPN species. However, TCA showed some effects, particularly for H. bacteriophora. These results suggest that PPA and indole may be a more suitable option for managing plant-parasitic nematodes among the tested SMs, as it showed no negative effects on the tested beneficial nematode species. In contrast, TCA may require caution due to the observed species-specific effects, particularly at the high concentration used, which underscores the need for careful consideration of its application concentration.