Friday, May 1, 2015

Concrete mites

Mites are among the tiniest arthropod pests to challenge the pest management professional. Tiny relatives of ticks (Class Acarina), mites play many roles in their microscopic world.  Some are predators of insects, insect eggs or other mites. Others are plant feeders, weakening plants and spreading disease.  Some mites feed exclusively on decaying plant material.  And still others are parasites, hitching rides on spiders or beetles, feeding on birds and mammals and even humans.

Insects are hard enough to identify because of their small size. But most insects are huge compared to mites. Larger mites may reach 1-2 mm in length; most are much smaller.  Scabies mite, one of the only mite parasites to exclusively feed on humans, are among the smallest of mites (0.18-0.45 mm-long) and visible only through magnification. Most mites are less than 1 mm-long, though velvet mites (the largest of the mites) can reach lengths of 4 mm, as long as a termite worker.

Clover mites are distinguished by their long front legs. Photo
by Rayanne Lehman.
For size reasons, I'll admit that I groan a little inwardly when I receive a mite specimen to identify. Mounting mites on glass slides is not one of my strong skills, and takes extra time.  So I was pleasantly surprised this week when a promised mite sample arrived and it was actually on the big size, at least for a mite.

I was initially ready to identify my client's mite as a clover mite, one of the common springtime mite pests in Texas. The clover mite, Bryobia praetiosa, is a reddish brown mite with very long pair of front legs, about 2X the length of the other legs. Though mostly harmless, the clover mite is an occasional nuisance pest indoors when it migrates from its normal feeding sites in grass and weeds outdoors through windows and under doors.  But there was something about these mites that didn't quite fit the clover mite profile.  

Balaustium (or concrete) mite. Photograph
by Lyle J. Buss, University of Florida
The one-millimeter-long, bright red mites were being found all over decorative stone and concrete in a Tyler, Texas backyard.  The homeowner thought he was being bitten, perhaps by mutant chiggers from you-know-where. But although chigger mites are also bright red, they are tiny (.3 mm) and barely visible to the naked eye. These were not chiggers.

An online search led me to one of my favorite online resources, Bugguide.net. There I discovered a mite genus I had never heard of before.  Concrete mites, Balaustium species, were a dead ringer for the specimens I had under the microscope.  So called, because certain species in this genus are commonly found wandering on concrete sidewalks, foundations and walls, and stonework, Balaustium mites are apparently common throughout the U.S.; but I had never heard of them before.

The University of Florida Entomology Circular Series (also an excellent source of detailed information about urban insects, especially plant-feeding pests) had a 1995 publication by W. C. Welbourn on Balaustium mites in Florida. According to the circular, at least one species of Balaustium is "commonly found in urban areas where they appear in large numbers on sidewalks and walls for a brief period during the spring and early summer. It is during this time that they sometimes enter homes and buildings and become pests."  

One of, if not the only, food source of these urban mites appears to be pollen. Balaustium mites have been found clustered in large numbers on anthers of flowers.  And based on the numbers of times I've had to wash my car and BBQ grill of oak pollen in the past month, I'm imaging that these mites have been having a good time of it lately.  This would also explain why the mites can be found on nearly any outdoor surface, including sidewalks and roofs--because pollen is everywhere.

Some species of Balaustium are predators on other insects, others feed on plants.  I was surprised, however, to read that there may be some association between our otherwise peaceful, pollen feeding, house-invading concrete mites and bites on people.  An entomologist named Irwin Newell in 1963 reported four cases of human "biting" involving Balaustium, three of which were associated with structures. The evidence for the bites was very strong, including a sample submitted by entomologist who was bitten on the arm while working in an entomology museum (how dare they!). 

The curious thing about the story is that while Dr. Newell stumbled across several cases in a relatively short period of time, prompting him to predict this mite was a growing problem, it's been crickets (silence) since then.  None of the major texts or reference books I scanned have even a mention of this family of mites, in spite of being common in the landscape. Even Ed Riley, Texas A&M University's crack assistant museum curator, was not familiar with these mites--though he admits, like me, to not paying too close attention to red mites he sees outdoors.  

Newell admits in his paper that he had no idea what species of Balaustium he was dealing with in his biting cases. And I don't believe anyone know what species are common in Texas. But I will be more interested and paying closer attention the next time I see my brick mailbox covered with red mites. 

[Request: I have little doubt that some of you get questions from customers about little red mites crawling on the sides of homes and sidewalks this time of year.  I would be interested to know if you or anyone you meet believes they have experienced bites from these little critters.] 

1 comment:

Glen Ramsey, BCE said...

Awesome post, Mike! I have wondered a bit about these being clover mites for a while (even though I kept calling them that). I didn't really see why they would be in such great quantity on concrete and brick away from heavy plantings. I haven't heard of them biting, but we get a fair amount of phantom bites reported in and while I understand the root cause of most of them I have to wonder about some of the others. This could be an explanation that needs further research. Thanks a bunch!