Bottom-up effects of nematode prey on soil predatory mites (Acari: Mesostigmata)

Pest suppression is one of the desirable functions of a healthy soil. Predatory Mesostigmata soil mites, with their high degree of omnivory, are one of the most remarkable groups performing this beneficial role. Food web bottom-up effects are important for the fitness of these predators, which ultimately affects their potential as biocontrol agents. In below-ground systems, free-living nematodes (FLN) are an important diet for mites, providing essential nutrients such as omega 3 long-chain polyunsaturated fatty acids (ω3-LC PUFAs). Here we assess the food quality of nematodes for the maintenance of predator fitness, i.e. development, reproduction and lipid content. We selected two mite species, different in size and morphology, collected from vegetable crops in Israel. Additionally, five species of FLN were isolated from the same fields. Additionally, one plant-parasitic species, common across arable sites in Israel, and maintained on tomato as host, was tested. Depending on species and trophic group, nematodes varied in fatty acid profile, and both mite species generally reflected the fatty acid pattern of their diet. All nematode species were able to produce ω3-LC PUFA de novo, except the bacterial feeder Acrobeloides bodenheimeri, which had the highest proportion of ω6-PUFA. Interestingly, the latter lacking ω3-LC PUFA, was the best diet for both mites. Macrocheles aff. scutatus, a medium-sized mite with a holodorsal shield, fed and developed on all nematodes except the entomophilious Oscheius tipulae. It was able to complete its development feeding on the plant parasitic Meloidogyne incognita, yet the reproduction was almost zero. The smaller and slender Protogamasellopsis zaheri, with a split dorsal shield, developed and reproduced with all nematodes, with M. incognita as an intermediate diet. Our results demonstrate that nematode species vary in their nutritional quality as prey. However, food quality is predator species dependent, likely affecting biocontrol efficacy. Besides ω3-LC PUFA, ω6-PUFA can play an important role in the reproductive and developmental parameters of mesostigmatid mites; yet, other factors such as nematode size and behaviour have to be considered.