“Best food” does not apply to all: Quality of nematode diet for development and fitness of their predator Parasitus sp. (Mesostigmata: Parasitidae)

Long-chain polyunsaturated fatty acids (PUFAs), especially ω3 forms, are essential nutrients for many animals and are typically obtained from dietary sources, yet there is limited availability in terrestrial ecosystem. In soil food webs, free-living nematodes are an important source of these PUFAs. Predatory mites (Mesostigmata) play a key role in regulating invertebrate populations, including free-living and plant-parasitic nematodes. Recent studies suggest that diets including nematodes improve the fitness of predatory mites and enhance their function as biological control agents within soil food webs. The predatory mite Parasitus sp. isolated from Mediterranean agricultural fields showed nematode consumption and colony establishment with a mixture of nematode species. This study investigated the effects of single nematode diets on the fitness of Parasitus sp., focusing on the roles of ω3- and ω6-PUFAs. This included one plant-parasitic and five free-living bacterial-feeding species, mostly co-occurring in the same Mediterranean fields, with one species unable to synthesize ω3-PUFAs. The results showed that nematodes rich in ω3-PUFAs supported Parasitus sp. development, but those high in ω6-PUFAs and deficient in ω3-PUFAs did not. However, mite survival and moulting to adulthood were generally low, possibly indicating a need for a specific stimulus or a more varied diet. This study further demonstrated ω3-PUFA de novo synthesis in Mesostigmata. Overall, the nematode diets influenced the development of Parasitus sp. differently, which emphasizes the importance of diverse nematode populations to support the fitness of higher trophic levels such as predatory mites. • Nematodes rich in ω3-PUFAs supported Parasitus sp. fitness. • ω6-PUFAs-rich nematode diet was of poor quality. • first evidence of ω3-PUFA synthesis in Mesostigmata predatory mites. • nematode diversity, i.e. access to various prey species, is essential for mite development.