Acarology Laboratory
The Ohio State University
Biological Sciences Building
484 W. 12th Avenue
Columbus, OH 43210

Varroa jacobsoni External parasite of the honey bee.

Mites are a very important part of the animal kingdom. Their diminutive size belies the various roles: good, bad and neutral they play in nature. In fact organism size is not correlated with complexity. A cure for HIV would have been discovered long ago if that concept was true. If not for the negative impact created in the beehive by several specialized mite species, most of us would give their presence little thought.

How worried should we be, and how much should we know about these microscopic creatures?

Acarapis woodi Tracheal mites inside a the trachea of a honey bee (magnification 65X) *

Fifteen years ago the world of the honey bee was in grave danger, especially in North America. Invading Africanized honey bees from the south and tracheal mites, Acarapis woodi, eliminated what had been a relatively simple life for beekeepers. Today we are accepting the "killer bee" more as an agricultural problem than a personal health risk as it makes its way northward.

The mite story is not so encouraging. An even more troublesome external parasite, Varroa jacobsoni, was detected several years after the tracheal mite and together they have all but decimated the casual beekeeper and feral (wild) bees in North America. There has been a modest investment by beekeeping industries and the government in finding solutions. Unfortunately our efforts to design protection from these parasites is not "full armor" but more like a thin fabric covering.

The complexity and challenge of managing healthy colonies are reflected in the lack of solutions available today despite many years of struggle. Sole reliance on chemical control and abuses of those active ingredients has resulted in the development of resistance by the pest mites. Although some new materials show promise, there have been no quick fixes. Use of vegetable shortening along with granulated sugar is the closest thing to a cheap, effective and safe control of tracheal mites.

The long-term future of beekeeping now rests in the skilled hands of a few dedicated scientists and beekeepers worldwide who are striving to better understand the basic biology of these mites and their bee hosts.

Mites (Acarines)
The existence of acarology as a separate discipline (from zoology and entomology) is based almost solely on the problems mites (and ticks) present to humans in our day-to-day lives. Even the trained specialists who knew arthropods underestimated the wealth of mite species and their importance in nature.  Acarines have been known as pests since the 9th century B.C. where Homer refers to ticks. As the most widely distributed and numerous arachnids (spiders, scorpions, daddy longlegs) the number of mite species may rival that of insects (> 1 million), although only about 30,000 have been given names. Their biological diversity is impressive.

The Acarina (mites and ticks) are found in just about every habitat animals can occupy. To see for yourself just pick up a handful of organic soil and examine it under a dissecting microscope or good magnifying glass where you will find mite specimens of different shapes, sizes and activity. Handle a bird nest just after it has been abandoned by the nestlings and you may see dozens of small organisms (bird mites) scurrying across your hand. Fresh water streams, ponds, lakes and even hot springs have their own mite faunas.

Some economically damage host plants (e.g. spider mites), while others cause unsightly growths (galls). Some mites hitch a ride on other arthropods.  Daddy longlegs (Opiliones) often have bright red mites on their legs, which are immatures getting moved about on their mobile, active transport. The Madagascan hissing cockroach common to zoos, classrooms and laboratories has a visible mite that exists solely on these hosts, causing them no harm. Most of us, especially if you are an adult, carry follicle mites in our facial pores.

Mites make some of us sneeze. About 5 percent of the population have allergies to dust mite feces and their cast skins. Mites even invade the internal organs of humans, other vertebrates and invertebrates. These associations range from commensalisms (symbiotic benefits, host not helped or harmed) to parasitism (symbiotic benefits at expense of host, either inside or outside of the host; e.g. Varroa, Acarapis).

Mites living in temporary habitats often use other arthropods (especially winged insects) or vertebrates as vehicles of dispersion, as mites have no wings. For bee mites there is double the dispersal potential via the host and then humans who move bees on trucks, trailers, boats, airplanes and even in the pocket. Eva Crane's overview links the advent of air travel with the dispersal of mites globally.

Another feature that provides many mites with an advantage in life is their reproductive strategy. Many mite species have a system where the female controls which eggs are fertilized, much like honey bees. The fertilized egg becomes a female so she has the benefit of genetic material from both parents, while the unfertilized egg becomes a male and has half of the genetic complement (only mother's genes).

Acarapis woodi also uses a short-cut for development. Mites typically have the following: egg - larva - protonymph - deutonymph (sometimes a tritonymph) - adult, but the tracheal mite has only the egg, larva and adult stages. This greatly shortens the time for development to about two weeks so populations of Acarapis can grow very quickly.

An abbreviated development time and a high number of offspring allow mites to be very adaptable in changing environments. A changing environmental condition produced artificially is the addition of acaricides to the beehive for mite control. Reproductive capacity and shortened development provide an especially disturbing problem in that they speed up acaricide resistance. ApistanTM strips for Varroa control are becoming less effective in certain areas of the world and mite biology explains part of the problem. The other is the overuse of active ingredients causing greater selection pressure (killing) leaving only resistant mites to mate, which produce more resistant offspring.

Lifestyles and Body Forms
Diversity in lifestyles is especially reflected in body forms and life-history patterns. Their body plan is typical arthropod in design with an integument that is variously hard and dark, or light in color and very flexible. Their development is highly variable and may consist of numerous post embryonic instars, but unlike insects (with gradual metamorphosis, like grasshoppers) the immatures may be very different from the adults or even the other immatures. This provides great plasticity for living in different habitats or in habitats that change quickly. A well-known example is the chigger mite, which as a larva, feeds on lizards and occasionally bites humans, but the subsequent nymphs and adults are bright red free-living predators. As with other small arthropods, most mites are short lived and development occurs over days or weeks.

Just the size of mites like Acarapis woodi is a critical feature. Smallness has its advantages and disadvantages. Visualization of Acarapis by Apis is so problematical routine grooming and housekeeping activities probably miss the few tracheal mites found outside the host. An obvious advantage is the ability of A. woodi to scurry under the flat lobe that covers the bee's first thoracic spiracle, and once there to colonize the main tracheal trunk.

Since the distance separating Acarapis's internal tissues from outside air is so short there is no need for a respiratory opening. Rather they rely on simple diffusion of oxygen in and carbon dioxide out (thus their classification as astigmatids -- having no spiracle openings). This feature helps conserve water that would be lost by typical arthropod respiration, and liquids cannot enter via this opening to fill breathing tubes. Being so small allows more mites to occupy the limited space within the tracheae facilitating reproduction.

The primary disadvantage of diminutive size is the risk of dehydration, because mites have such a large surface area compared to their internal volume. This basic physical-chemical property is worse as mite size decreases. As a result, these mites cannot be outside the host for very long or they will rapidly desiccate and die. This limitation necessitates obligate parasitism and/or the acarine must be able to obtain moisture someway other than by feeding (e.g. vapor uptake from sub-saturated air, drinking water, producing metabolic water).  The problem with drinking for such a small creature is the risk of being entrapped by the surface tension of the liquid. 

From a practical standpoint, beekeepers know that the small size of Acarapis makes it impossible to visualize them while working their bees and thus diagnose a colony infestation. This parasitic life within the bee's breathing tubes makes reaching them with a miticide very difficult. If a volatile active ingredient (e.g. Menthol) is inhaled by a bee it must be lethal to the parasite but safe for the host. Since both are arthropods many of their basic physiological processes are similar so the window of opportunity for finding such toxicants is indeed quite narrow. Beekeepers know bees are also one of the most sensitive insects to accidental pesticide poisoning, so the host is especially vulnerable to many of the common active ingredients used for apiculture.

For the ectoparasite Varroa jacobsoni there are several key morphological features that make them successful. Dorsoventral compression (rather than lateral compression like fleas) of the female allows them to fit snugly beneath structures like the abdominal sclerites (plates), which reduces their vulnerability to grooming or being knocked off during a bee's active life. Also, being in this location probably slows water loss by transpiration across their integument. When adult mites enter the brood cell this same morphology allows them to fit between the cell wall and the brood body surface. Varroa are seen with their back (dorsal) surface down and entirely covered in liquid food being fed to larvae. A special snorkel-like respiratory structure extends through the fluid's surface to facilitate breathing while submerged. Like tracheal mites, Varroa can evade acaricides when the brood cell is capped just prior to pupation.

Mites are a common inhabitant of the bee colony. Some 86 mite species have been recorded in association with Apis and their nests, but most are neutral or benign in nature. "Hive" mites fall into four ecological categories: 

1. Mites scavenging on hive debris.

2. Predatory mites that eat the scavengers.

3. Mites that "hitch" a ride to flowers and other hives on foraging honey bees.

4. Mites that are honey bee parasites.

More research is needed to design novel control strategies that incorporate all we know about the bees and their mites. Finally, a statement that puts this into perspective is: "just because you may be small and ugly doesn't mean you're not complicated".