When a beetle is too small to carry phoretic mites? A case of hydrophilid beetles (Coleoptera: Hydrophilidae) and Uropoda orbicularis (Acari: Mesostigmata)

The aim of our study was to determine the limit of a beetle's body size below which it is not able to carry phoretic mites. We tested the hypothesis that body size of the carrier is the vital factor in selecting a host in mites with facultative and nonspecific phoresy. The study was conducted on beetles of the family Hydrophilidae and deutonymphs of the mite Uropoda orbicularis (Muller, 1776). Among 20 species of hydrophilids collected, only 7 carried deutonymphs. The smallest hydrophilids with body lengths below 2.32 mm did not carry deutonymphs. These were Cercyon analis (Paykull, 1798), Cercyon nigriceps (Marsh am, 1802), Cercyon pygmaeus (huger, 1801), Cercyon terminatus (Marsham, 1802), Cryptopleurum crenatum (Kugelann, 1794), Cryptopleurum minutum (Fabricius, 1775), Cryptopleurum subtile Sharp, 1884, and Megasternum concinnum (Marsham, 1802). On the contrary, the largest beetles, of the genus Sphaeridium Fabricius, 1775, were most heavily and frequently occupied by phoretic deutonymphs. Most representatives of the genus Cercyon Leach, 1817 did not carry mites or had a very low mite burden. Our study indicates that beetle infestation by nonspecific phoretic mites is strongly influenced by beetle body size.