Mites that hitchhike

While I was leading a community wildlife walk across some cattle-grazed water-meadows yesterday, one of the younger members in the group used his youthful sharp vision to spot a small dung beetle (Aphodius fimetarius) which looked a little unusual with what appeared to be tiny bumps on the elytra. Although some sort of deformation is presumably possible, the 'bumps' were (as suspected) phoretic mites of the genus Macrocheles. 'Phoretic' means they practice 'phoresy' i.e. use other species as transport rather than feeding on them as a parasite might.

Aphodius fimetarius with phoretic mites on the pronotum and elytra. 1 square = 5mm.

Aphodius fimetarius with phoretic mites on the underside, especially of the abdomen. 1 square = 5mm.

On this occasion I haven't taken microscope photos of the mites as I don't intend to identify them beyond genus. Although these mites are possibly more familiar from larger dung beetles, research in France (Glida & Bertrand, 2002) suggests that, as they are active all year, Aphodius are important in distributing and establishing populations of Macrocheles mites because these beetles are active throughout the whole year, including cold periods. At least some Macrocheles species are predatory on fly larvae (and some are phoretic on adult flies), hence being moved between manure piles/cowpats presumably delivers them safely to new hunting grounds. Also, although the beetle presumably bears some burden in carrying sometimes large numbers of mites, there may be a trade-off if the mites reduce the populations of dung-feeding fly larvae and therefore increase the amount of dung available to their dung beetle hosts. I'm sure there's a potential experiment in there somewhere...

Reference

Glida, H. & Bertrand, M. (2002). The occurrence of Macrocheles mites (Acari: Macrochelidae) in relation to the activity of dung beetles: a field study in Southern France. In: Bernini, F., Nannelli, R., Nuzzaci, G. & de Lillo, E. (eds.). Acarid Phylogeny and Evolution: Adaptation in Mites and Ticks. Proceedings of the IV Symposium of the European Association of Acarologists, pp. 199-207.