[1996 Wenner, A.M. and W.W. Bushing. Varroa mite spread in the United States. Bee Culture. 124:341-343.]

Adrian M. Wenner and William W. Bushing

Early in the 17th century, beekeepers introduced European honey bees to the United States, forever altering relationships between plants and insects in this country. Previously, hummingbirds, bumble bees, other native bees (mostly solitary) and various insects pollinated native plants. As with a great many other invading species, honey bees soon became a major factor in the flowering plant ecosystem.

More than 300 years after the initial introduction, a beekeeper also imported honey bees (and a very serious problem), perhaps from Brazil, despite a quarantine that had existed for several decades. That person was not the first to break the quarantine; all too often the quarantine had been ignored by others hoping to "improve their strain." Unfortunately, this time the imported bees harbored a voracious parasitic mite, Varroa jacobsoni, soon to populate all contiguous states and Alaska.

In September, 1987, colonies in some hives transported from Florida to Wisconsin experienced colony failure - the first recorded case of Varroa infestation in this country. A spot check around the nation that Fall revealed the presence of Varroa mites already in a dozen states.

Florida seems likely to have been the initial introduction point. Subsequent queen and package distributions, as well as movement of colonies by beekeepers (e.g.. for pollination and overwintering), hastened the spread of Varroa mites throughout the nation. At the local level, swarm movements, robbing of weakened hives, incidental drift between colonies, freedom of drone movement between colonies, and some mite transfer by bees visiting blossoms all contributed to a rapid, near-universal infestation.

Attempts to check the spread in the United States came too little and too late. After a flurry of reported finds in this country, officials in most states apparently rather suddenly ceased keeping records of new finds - perhaps overwhelmed by the speed of the inevitable wide-spread disaster. As Sanford wrote: "...beekeepers did not believe what we in extension or [regulatory agencies] told them about the mites. Thus, they did no treatments and then were surprised by losing a great many colonies..."

Not only in the United States but elsewhere, the rapid and nearly worldwide spread caught nearly everyone by surprise, though it shouldn't have. As early as 1975, Akratanakul and Burgett published a warning about the threat posed by Varroa mites.

Decline of managed bee colonies during the past half-century. The sudden plummet after 1985 coincides with first appearance of Varroa mites. This figure is adapted from one published by Hoff and Willett (1994).​


The Rapid U.S. Spread

Some individuals kept rather extensive records during the first few years of Varroa infestation in the United States, notably Stephen Bambara (Entomology, North Carolina State University), I. Barton Smith (Secretary, Apiary Inspectors of America) and Jim Pheasant (USDA/APHIS). Queries broadcast over e-mail networks (e.g., BEE-L) yielded yet other information. Finally, Bee Culture's publication of resource people by state (April, 1995) enabled us to contact officials in undocumented areas, thereby gaining information to fill in the remaining gaps.

The information gathered permitted us to generate a composite map that illustrates earliest known establishment of Varroa mites in each state. A study of that map allows one to speculate about probable distribution patterns. In a few cases, reports did not mesh with one another, but such conflicts did not differ by more than a year.

Although not shown on the map, Canada has not escaped Varroa mite infestation, In the late 1980s, isolated cases appeared along the U.S. border in New Brunswick and Manitoba. By 1992 in Manitoba and 1993 in New Brunswick, Varroa seemed to have become established in a few operations and were later (1993) found in Alberta among some colonies that had been overwintered in southern British Columbia. Other Alberta finds in 1994 occurred in bee yards containing colonies that had overwintered in British Columbia areas distant from the U.S. border. By 1995, more general finds were recorded from a few bee operations in Alberta, Manitoba, Nova Scotia and Saskatchewan. Most beekeepers in Canada, however, still remain unaffected by those mites.

During the last few decades, the number of maintained honey bee colonies in the United States plummeted, largely as a consequence of the combined effect of tracheal and Varroa mite infestations. Between 1945 and 1990, the number of managed colonies dropped to about one-half of its former level. However, that circumstance does not represent the true level of devastation to agriculture. From all indications, feral bee colonies (see below) have effectively disappeared in all areas of Varroa mite infestation.

Year of first documented incidence in domestic colonies by state in the United States. Varroa mite infestations were first noticed in Wisconsin and Florida, but within a month or two, inspections revealed mite presence in 10 other states.​


The Feral Bee Reservoir

By their extermination of feral honey bee colonies that had existed previously, perhaps for many years, the tracheal and Varroa mite invasions eliminated a primary pollination source for most urban gardeners and other growers - who suddenly lost virtually all pollination services taken for granted earlier. However, cavities in which those "wild" bees formerly resided can repeatedly become filled with swarms from nearby managed colonies, feral colonies that will weaken and die with time from their combined parasite load. Robbin Thorp, a California bee researcher, refers to this rather rapid reoccupation of cavities and subsequent death as "annualization" of feral populations.

The rather temporary feral colonies in reoccupied cavities provide a source of Varroa mites for contamination of nearby managed colonies because mites travel freely between managed and feral colonies on drifting and robbing bees. This situation thereby complicates efforts to manage commercial colonies effectively. Whereas the beekeeper can depopulate managed hives of mite parasites to a great extent by judicious use of oil patties and Apistan strips, nearby newly re-established feral colonies remain untouched by that treatment. In the case of absconding swarms, the mite load can be considerable, since colonies often abscond when mites become too abundant.

A rapid mite buildup in feral colonies soon leads to their early demise and a vicious cycle. Highly infested and weakened feral colonies then become robbed out by bees from managed colonies, requiring more frequent miticide treatment of managed colonies. Lack of a uniform program for Varroa mite control efforts exacerbates the problem, in that infestations can also surge back and forth between apiaries managed by different beekeepers. Furthermore, misuse of Apistan strips (as apparently occurs) can be expected to lead to fluvalinate-resistant mites.

Prospects for the Future

With each new introduced problem, beekeeping has become ever more challenging in the United States and elsewhere. The foulbrood plagues, followed by indiscriminate pesticide use devastated the beekeeping industry in mid-century. Each time, beekeeping managed to rebound and furnish profit and/or pleasure for those who persisted.

The influx of tracheal and Varroa mites (particularly the latter), however, poses problems far greater than any faced before. From all indications, we can breed strains of tracheal mite-resistant bees and can do so ever better as we gain greater understanding of the interaction of those parasites with their bee hosts. Varroa mites, by contrast, feed on the blood of larvae, pupae and adults and can reproduce astonishingly fast; faster, that is, than the rate at which bee colonies can replace their losses.

Can one breed a Varroa resistant bee? The major problem here stems from the fact that virtually all our bee strains are actually quite highly interbred. That is, when queens mate in midair with nearly a score of drones of different genetic makeup, the resultant colony has a host of different characteristics. Perhaps some small percentage of the bees in a colony have a hygienic behavior suitable for ridding the colony of Varroa mites. Finding and isolating any such useful feature remains a formidable task under the circumstances.

Neither can we expect much success at finding Varroa-resistant strains by letting nature take its course in a joint population of managed and feral colonies. Our best hope along this line would be to find some relatively isolated population consisting of many feral colonies and follow the progress of Varroa infestation through time. If any of those colonies would survive, they could serve as a source of breeding material if kept isolated from contamination by importation of other bees from outside the area.

We are currently examining this possibility (resistance) on Santa Cruz Island and (hopefully) on Santa Catalina Island, two large islands that lie off the coast of Southern California. Contamination of the bee strain on Santa Cruz Island (currently a very homogeneous strain) by material from the neighboring mainland appears remote. We do not yet know enough about the Santa Catalina Island honey bee operation to determine whether that population might have potential for Varroa resistance.

With the advent of tracheal and Varroa mites, beekeepers may well have to rethink the degree to which they embrace the relative freedom they earlier practiced. We as individuals can no longer focus exclusively on the health of individual colonies, or even beeyards. We must now deal with contamination of the entire system of all managed and feral colonies within flight range (as with the early influx of foulbrood). It may well be that we will have to impose a region-wide and coordinated application of Apistan or other treatment to insure that we do not have mite infestations merely surge about among managed bee operations and feral colonies.

The alternative? We could merely cease all apiculture inspection programs and let the strong survive, whether it be the strong colonies or the most resourceful beekeepers. The danger of this last approach, of course, is that we can end up with the possibility of producing contaminated honey and beeswax by those who will try anything to keep their colonies alive.

Mite Detection and Colony Treatment

One can detect Varroa mites in a colony by a variety of methods, some more effective than others:
  1. Insert an Apistan® strip into brood nest for a couple of hours and then remove it - with a sticky board(e.g., vegetable oil coated) in place under brood combs to catch any mites that fall during that period.
  2. Inspect advanced drone pupae by spearing and lifting them out of a comb with a capping scraper.
  3. Look for mite fecal accumulations in recently emptied brood cells.
  4. Employ the standard ether roll technique - use a quart glass jar with vegetable oil, drop in a few hundred bees, spray ether (from auto supply store) into the jar, rotate the jar, and look for mites stuck on the jar walls.
  5. Slip a horizontal sticky card (overlaid by a coarse screen so bees will not become mired) into the entrance and keep it underneath the brood combs, inspecting it each few days.
  6. Look for mites on young bees as they walk about on brood combs.
  7. Inspect advanced-stage drone brood extracted from cells with perforated cappings.
  8. Search for deformed young bees in the hive and outside on the ground.

Beekeepers around the world employ a number of chemical treatments (tobacco smoke, fluvalinate, formic acid) to control Varroa mites, but those in the U.S. can legally use only fluvalinate incorporated into plastic strips (Apistan®). In Canada and Germany, one can also legally use formic acid, an inexpensive but quite dangerous substance that is reportedly not always particularly effective - especially if one does not have sticky boards in place underneath the brood combs during treatment.


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Dr. Wenner, Professor Emeritus (Natural History) in the Department of Ecology, Evolution, and Marine Biology at the University of California, Santa Barbara, continues his honey bee research on Santa Cruz Island. Mr. Bushing of the same department is finishing his doctoral studies on the application of geographic information systems to marine habitats; he now serves as the Catalina Island Conservancy's Director of Science, Education, and Ecological Restoration.
Many individuals provided input for this article. In particular, we thank Steven Bambara, I. Barton Smith, Hachiro Shimanuki, Jim Pheasant, Tom Sanford, Robbin Thorp, and Kenn Tuckey.