Phoretic mites do not affect burying beetle reproductive success, despite eliminating fly competitors

Abstract Interspecific interactions exist along a continuum from parasitism to mutualism, but the fitness costs and benefits of these associations may be fluid, depending on factors like abiotic conditions and interactions with other organisms. Phoretic mites ( Poecilochirus spp.) use burying beetles ( Nicrophorus spp.) as a means of locating and traveling to carrion, the site of both organisms' reproduction. Several species of Dipteran fly also reproduce on carrion, competing with beetles and mites. Mites could benefit their beetle hosts by reducing the abundance of these flies, but experimental evidence of this benefit is mixed. We experimentally tested the fitness effects of Poecilochirus mites on N. orbicollis and determined if these effects are dependent on the presence of fly competitors by experimentally manipulating both fly and mite presence with beetles breeding in a semi‐natural environment. Fly colonisation of carrion reduced beetle reproductive success, and mites reduced fly numbers. Despite this, the fitness cost of flies was not ameliorated by mites. Mites did not affect beetle reproductive success, regardless of fly presence or absence. Although beetle reproductive success was not affected by mites, beetles were physically compromised by high mite loads; beetles with more mites had slower righting times than beetles with fewer mites. Our results suggest that these mites are commensals of burying beetles, although unmeasured benefits of reductions of flies could tip this relationship to a mutualism, and unmeasured fitness costs of compromised locomotion could tip it to parasitism.