Compatibility of entomopathogenic nematodes and essential oils: A new step for integrated pest management of the truffle beetle

The European truffle beetle, Leiodes cinnamomeus, is the most important pest in black truffle (Tuber melanosporum) plantations. Entomopathogenic nematodes (EPNs) are promising biological control agents against L. cinnamomeus. Essential oils (EOs) are also recently being investigated for the control of the adults of this pest. Therefore, both control methods could be combined in Integrated Pest Management (IPM) programs to enhance their efficacy. However, limited information exists regarding the effects of the EOs on EPNs and so their compatibility. The aims of our work were to study the effects of three previously described insecticidal and nematicidal essential oils, Allium sativum, Mentha suaveolens, and Satureja montana, on the survival, infectivity, reproduction, and attraction behaviour of three EPN species: Steinernema feltiae, Steinernema carpocapsae and Heterorhabditis bacteriophora. Therefore, we conducted three experiments under laboratory conditions to observe the lethal and sublethal effects by direct contact, the lethal effect by fumigation, and the chemotaxis response. Allium sativum caused the highest mortality rates in all three EPN species at 24 and 72 h post application whether by direct contact (97–99%) or fumigation (40–42%), and it also reduced their infective capacity on Galleria mellonella. Satureja montana EO caused low mortality rates compared to A. sativum, but it was significantly more lethal (6–8%) than the control (0–3%) at 72 h in the direct contact assay. It also displayed repellent properties against S. feltiae and H. bacteriohora in the chemotaxis assay. In contrast, M. suaveolens EO exhibited minimal impact on the survival, infectivity and reproduction of all three EPN species. Therefore, our results suggest M. suaveolens oil may be the most compatible EO for use integrated with EPNs. Further validation under field conditions and in the presence of L. cinnamomeus is necessary to confirm the practical applicability of these findings.