Many mites are very specific for particular habitats, whether it is the inside of the lip of a bat or a flower bud of a single plant species. We have established this for many plant, insect or vertebrate associates. But knowing where exactly the mites are on a host is fairly easy. What about mites living is less discrete situations, like the litter layer? We are fairly sure that litter mites also have fairly specific microhabitats, but this is much more difficult to demonstrate.
In the acarology lab we have been looking at one subgroup of litter habitats, tree holes. In this case we define tree hole as any cavity in the trunk of a tree that is not directly connected with the underlying soil. Tree holes in general may provide more stable microclimates, in terms of temperature, moisture, humidity and sun exposure, than standard litter habitats. All types of tree holes contain mites, and those mites tend to be specific to tree holes. Wet tree holes, containing water or just very wet litter, have been studied quite extensively because they are breeding grounds for certain species of mosquito, but we are particularly interested in dry tree holes. Initially we became interested in this microhabitat because we wanted to know more about distribution and habitat restrictions of Uropodella, a rare genus that had been found only in tree holes. The genus Uropodella is most diverse in Chile, with only 1-2 species in North America. These mites appear to be phoretic on Tenebrionidae, and, fitting with that association, are found in very dry tree holes, containing nothing more than pulverized wood.
Mites in treeholes can also tell you something about other inhabitants of that treehole. Finding Aeroglyphidae in a S. Carolina tree hole indicated that there were probably bats roosting in that hollow tree, while the numerous Glycyphagidae in a tree hole in Columbus were consistent with the squirrel nest found in that same tree hole.
But this early research was largely anecdotal.
One of us, George Keeney, followed up in a big way by systematically sampling a large number of tree holes, some several times during different seasons. The focus for this study was a quite diverse group, the Uropodina. We found that tree holes in Central Ohio not only have a quite diverse uropodid fauna, but that the species in tree holes tend to be tree hole specialists. A few species have been associated with a wide variety of tree species, while a number of other species have only been encountered only once or twice. The two most commonly encountered tree hole species are Allodinychus nr. cribraria and Vinicoloraobovella cf. americana. George affectionately calls the former species the “elf hat mite”, due to its fanciful resemblance to such! That being said, we do not have enough information yet to determine whether these tree hole uropodines are specific for a given tree species, a given tree hole inhabitant (e.g. squirrels, birds, bats), particular exposure, tree hole size, tree hole litter moisture, season, etc., etc. To find out, we are now following up on the early survey, by recording more of these details, and especially by sampling many more tree holes.
Some tree species are more prone to developing dry tree holes in their trunks, usually at the site of branch removal or other injury. Silver maples, magnolias, American beeches and American sycamore are some notable examples of such trees. In Ohio, these species can be common in urban plantings and therefore, tree hole mite sampling can be quite productive in parks, campuses, street boulevards and other urban areas as opposed the more rural areas. Such park trees are often mature and may have had large branches removed by landscaping and maintenance, providing the initial germ of many tree holes. Sampling tree holes involves gathering the detritus from within the hole, though the hole should be thoroughly inspected before placing ones hands inside, as one may often be intruding upon the abode of a wary raccoon or testy gray squirrel! So if you see anybody carefully trying to get “goo” out of a tree hole, it is not just fun, it might be research.
And there is always the option of finding something that is truly unexpected. One of the most spectacular for us was a hole in a broken branch of a box elder near campus at the Olentangy River Wetland Research park. It contained a large population of Philodana johnstoni, a very odd species of Trigynaspid mite described from Ontario and New York, with no additional published records. It appears to be associated with the tenebrionid Neatus tenebrioides.
It is a very appropriate find, given the connection of this species with Ohio State University. Philodana johnstoni was named by John Kethley in honor of the previous director of the Acarology Laboratory, Don Johnston, and is a double honorific, as Philodana combines “philos” (=loving) and Dana, Don’s wife.
About the Authors: Dr. Hans Klompen is professor in the Department of Evolution, Ecology and Organismal Biology and Director of the Ohio State University Acarology Collection. George Keeney is Manager of the Acarology Collection and the OSU Insectary.