New records of Micromegistus bakeri, Trägårdh 1948 (Acari: Mesostigmata: Parantennulidae), a mite symbiotic on carabid beetles, and notes on the species’ distribution and host specificity

Micromegistus bakeri is a purported commensal mite that spends most of its life on the ground beetle Scarites subterraneus. It is thought to sustain itself by feeding on the beetle’s dermal fluids and opportunistically scavenging on nearby organic debris, although the nature of this relationship, and whether it’s beneficial, detrimental or neutral to the host, is unclear. This species has previously been recorded in Mississippi, Texas, and Kansas. This study reports the first record of M. bakeri in Pennsylvania based on our collecting, as well as the first records for Ohio, Michigan, Missouri, Arizona, and Canada, based on previously unpublished museum records. The literature and our own survey of carabid beetles indicate that M. bakeri has a strong preference for S. subterraneus as a host and is not common on other carabids. Our own data shows a prevalence of 27% of S. subterraneus (10 out of 37) carrying M. bakeri, with an average of 3.1 (range 1–8) mites per infested beetle; none of the other 31 carabid species collected during this study carried M. bakeri. This study represents the first publicly available COI barcode sequence for this taxon. We also provide photographs of live mites and summarize suspected sightings from additional locations across North America obtained via the citizen science websites BugGuide and iNaturalist. These records indicate that M. bakeri is present throughout the known distribution of its beetle host. Suspected M. bakeri were observed on other Scarites species in North America, but their presence on these hosts needs to be confirmed via examination of physical specimens. It should be noted that the westernmost record for the species is in Arizona, based on a male specimen that exhibits 6.6% divergence in COI barcode sequence with specimens from Pennsylvania. The specimen is otherwise essentially morphologically identical to males of eastern USA, and it is therefore uncertain whether it represents a cryptic species or merely high intraspecific genetic divergence over a large geographic distance.