Natural occurrence of entomopathogenic nematodes (Steinernema and Heterorhabditis) and Pristionchus nematodes in black truffle soils from Spain

The European truffle beetle Leiodes cinnamomeus is the most important pest in black truffle (Tuber melanosporum) plantations. Current control methods against it are inefficient, so entomopathogenic nematodes (EPNs) could play an important role in their population regulation due to their efficacy against many soil-dwelling insect pests. A survey of EPNs and Pristionchus nematodes was conducted in truffle soils of Spain, considering environmental and physical-chemical soil factors. A total of 164 soil samples were collected from forests, productive plantations and null-low productive plantations, representing three distinct black truffle-growing habitat types. EPNs were isolated from seven soil samples (4.3%); four nematodes were identified as Steinernema feltiae and three as Heterorhabditis bacteriophora. Both species were sampled in three types of soil texture (loam, sandy loam or sandy clay loam), characterized by alkaline pH (7.5 to 8.5) and high organic matter (2.1-11.04%). The presence of these EPNs was influenced by habitat type and organic matter content. Pristionchus nematodes were isolated from truffle soil, around truffle fruit bodies and under the elytra of L. cinnamomeus, with Pristionchus maupasi being the most commonly identified species. No significant associations were found between environmental and soil factors and the occurrence of Pristionchus nematodes. These nematodes were found in alkaline soils (pH 7.75 to 8.7), across all seven sampled soil textures, with variable organic matter content (0.73%-5.92%). The ecological trends and the presence of Pristionchus may affect the occurrence of EPNs and their prospective use as biological control agents against L. cinnamomeus in black truffle plantations.