Over the past few days, I’ve received several e-mails about the backsides of pants being stained with red dots after people have sat on concrete park benches or other concrete surfaces. A few messages referenced benches made of wood or recycled plastic.
The culprits behind the red-dotted behinds are so-called “concrete mites.” They make an annual appearance each spring in Ohio meaning we need to be looking before we sit.
These tiny, fast-moving bright red mites scurry around on sunny surfaces such as on picnic tables, patios, sidewalks, concrete retaining walls, and on the outside walls of homes and buildings. They belong to the genus Balaustium (family Erythraeidae) and are commonly called concrete mites owing to the locations where they tend to congregate.
Concrete mites are predaceous and eat other mites as well as small insects. They are also capable of supplementing their meat diet with pollen. Research has revealed that the mites often start the season as pollen-feeders and switch to a meat diet later in the season as more prey becomes available.
The mites are primarily a nuisance pest producing small red stains on clothing when sat upon. There have been reports in various medical journals that bites from these predaceous mites can produce dermatitis. However, the reports focused entirely on mites invading homes and other structures which seems rare in Ohio.
It is unclear why concrete mites appear in large numbers in sunny locations in the spring. The mass gatherings appear to be highly seasonal and short-lived. Even if their numbers swell this spring, the onslaught will quickly subside. However, should these mites threaten to spoil outdoor events involving an abundance of white clothing, such as a spring wedding, they can be suppressed with a surface application of a pyrethroid insecticide such as products containing bifenthrin (e.g. Talstar).
Do not confuse Red Grasshopper Mites (Trombidium holosericeum, family Trombidiidae) for concrete mites. This predaceous solitary mite is one of the largest-sized mites found in the U.S. They are sometimes called "red velvet mites" because of the fine, soft hairs covering their bright red body, making them look like they're covered in velvet.
Red grasshopper mites are commonly found in Ohio woodlands crawling on tree bark in search of prey which includes insects. Immature red velvet mites feed like tiny ticks as ectoparasites on other arthropods such as grasshoppers and harvestmen (= daddy longlegs).
References
Muñoz-Cárdenas K, L.S. Fuentes-Quintero, D. Rueda-Ramirez D, C.D. Rodríguez, and R.F. Cantor. 2015. The Erythraeoidea (Trombidiformes: Prostigmata) as biological control agents, with special reference to Balaustium. In: Carrillo D, de Moraes GJ, Peña JE, editors. Prospects for biological control of plant feeding mites and other harmful organisms, Progress in Biological Control. Vol. 19. Basel (CH): Springer International Publishing. p. 207–239. doi:10.1007/978-3-319-150420_8
Yodera, J.A., A.J. Jaicka, P.M. Tomkoa, A.E. Rosseleta, K.M. Gribbinsa, and J.B. Benoitb. 2012. Pollen feeding in Balaustium murorum (Acari: Erythraeidae): visualization and behaviour, International Journal of Acarology Vol. 38, No. 8, 641–647