It’s no secret that honey bee populations have been struggling to survive in recent years.  Hypotheses abound in regards to the causes of honey bee decline, and it’s likely that no single factor will prove to be responsible, but it’s clear that the presence of Varroa mites (Varroa destructor) in honey bee hives weakens bees, and contributes significantly to colony demise.

The shiny brown disc attached to the thorax of this honey bee, is the parasitic Varroa mite (click any image to enlarge)

The Varroa mite is an obligate parasite of various species of honey bees.  This mite was first introduced into the United States from Asia in 1987.  Previously Varroa’s host had been the eastern honey bee (Apis cerana), which was well adapted to this parasite, and seemed to suffer few ill effects from it.

Low temperature scanning electron micrograph (LTSEM) of Varroa destructor on a honey bee host (Image Source: Public Domain - USDA Agricultural Research Service)

Then Varroa found a new host in the European honey bee (Apis mellifera).  Unlike the Asian bees, European honey bees were not adapted to living with the mite, and shortly after its introduction to the United States, colonies, both feral and managed, all across the country were devastated.  If you’ve read the Aesculus californica post, you might recognize the common theme of co-evolution between these posts.  Novel environmental influences, be they parasitic, or toxic, can have detrimental effects on non-adapted populations.

This honey bee has a Varroa mite on her thorax. She was found crawling on the ground outside the hive, ataxic, uncoordinated, and unwilling to fly

Some former beekeepers we’ve spoken to have told us that they had given up beekeeping by the early 1990s because they found it impossible to sustain their bee colonies in the presence of Varroa.  Entire apiaries were routinely wiped out.  Beekeeping has changed since Varroa’s arrival, and although the situation is improving, many beekeepers still lose their hives due to overwhelming Varroa populations.

The Varroa mite is here to stay.  These mites are now found in honey bee colonies worldwide, and they’ve become established on every continent except Australia.[1]  Our hives are no exception.

Two of these festooning worker bees have Varroa mites on their abdomens

All beekeepers here have Varroa, at least to some extent.  Some colonies are showing some resistance to the presence of the mite, but many honey bee colonies are still being significantly, and adversely, affected by Varroa mites.

Varroa’s Life Cycle

In order to understand how any parasite thrives and survives, it’s imperative to understand its life cycle.  Varroa mites can only reproduce within the hives of honey bees.  Although they’ve occasionally been found on wild native bees, like bumble bees, they can’t survive on wild bee hosts as they depend on the structure of honey bee hives to successively reproduce.  They are perfectly adapted to life within a honey bee hive.

Female mites are approximately 1.0 mm long and 1.5 mm wide, and must enter the brood cells where honey bee larva reside before the cells are capped, and have a preference for larger drone brood cells that are capped for a longer period of time.

This drone pupa had a parasitic Varroa mite feeding on it within the capped cell

They feed on the hemolymph of developing bee pupa, and complete their entire life-cycle within the cell before the bees emerge as adults.  The following video, although not short, provides an excellent overview of the life-cycle, both of the honey bee, and of the Varroa mite. It may take a minute to load, but if you watch it, you’ll understand how Varroa is able to exploit the honey bee’s development cycle to its advantage.

Detecting Varroa in the Hive

As new beekeepers we knew that our colonies would have Varroa, but we needed to learn how to evaluate the overall burden of Varroa within our hives.  A number of detection methods are available for assessing mite populations within a colony.

Mites attached to adult bees, called phoretic mites,  may periodically be groomed off of the bees, and naturally fall to the hive floor.  Monitoring of this natural mite-fall can be used to gauge severity of Varroa infestation in the hive.  All of our hives are fitted with IPM screened bottom boards.  By using screened bottom boards in the hive, as the mites fall off the bees, most of the mites will fall through the screen onto the ground below.  By inserting a monitoring grid (usually called a ‘sticky board’) under the screen, the mites will land on the board, and the rate of mite fall can be monitored for each hive.

Positioning this board under the screen of the hive floor, mites that land on the board can be counted

To accelerate these counts, some beekeepers will motivate grooming in bees by applying a dusting of powdered sugar to bees within the hive.  This stimulates grooming, and increases the rate of mite drop.  However, we have a lot of ants near the apiary that we’re already struggling to keep at bay, so we avoided the sugar dusting method.

Instead, last week we completed our first passive ‘sticky board’ counts for our colonies.  We first labeled each board, as we have multiple hives.  Each board was then sprayed with a light even coating of cooking oil.

A food grade spray oil is safe for use under the hive, traps mites on the board, but is easy to clean up

The oil will trap any mites that fall, especially live mites, and prevent them from crawling away before they’re counted.  Some advocate the use of conventional orchard sticky-trap sprays, or Vaseline, but we’ve found the advantage of cooking spray is it’s very easily wiped off when you’re done with the mite counts, and doesn’t leave any unwanted residues.

There is a slot under the screen of the screened bottom board that the monitoring board slides into at the bottom of the hive

We installed the oiled boards under each hive for 48 hours.  Some beekeepers only leave the boards in for 24 hours, others for 5-7 days.  Leaving the boards in slightly longer should give a  more accurate measure of natural mite-drop over time.  However, it’s advisable not to leave the boards in for too long, especially under large colonies, as more than mites will fall through the screen, complicating the counts.

After 48 hours, each board was removed, and the mites on the board were counted.  We determined that two tools help tremendously for doing the counts, especially when you have multiple hives.  A magnifying source (we used loupes, but a strong hand lens will work), and a strong light source.

A magnifying lens is helpful to more clearly distinguish mites (yellow arrows) from board debris

Each board has lettered rows, and numbered columns, making it easy to be methodical while performing counts.

The boards when they’re first removed will have a varied amount of debris, not just mites.  Pieces of pollen, and flakes of wax from comb building and remodelling, also fall on the board.

Developmental stage of the mite, orientation of the mite, and amount of debris on the board can all serve to confound getting accurate mite counts. This mite was hiding under some debris...

Unless you have perfect magna-vision, be prepared to spend some time doing the counts.  It takes time to train your eye to see the mites amidst the debris field.

Excessive debris may be seen on boards placed under large colonies, or if left in for too many days during the count

We counted the total number of mites for each hive, and then divided that number by 2 to determine our 24 hour mite count.

Chamomile Count: 1; 1/2 = 0.5 mites per 24 hours
Rosemary Count: 8; 8/2 = 4 mites per 24 hours
Lavender Count: 25; 25/2 = 12.5 mites per 24 hours
Salvia Count: 180; 180/2 = 90 mites per 24 hours

The Salvia mite count at first is alarming, and it is too high.  Depending on who you talk to, or which papers you read, threshold counts vary, but generally any natural drop count over 60 mites in 24 hours is considered potentially damaging to the colony.

This is how the distribution of mites looked on the sticky board from the Salvia hive (click image to enlarge)

However, natural mite-fall counts are far from perfect.  One key factor these counts don’t correct for is the size of the colony.  This means we can’t compare the results of Salvia (our largest colony), to Chamomile (our smallest).  The difference in population of bees could account for the part of the difference in the number of mites.

A somewhat more quantitative approach for counting mites is to do an alcohol wash.  Approximately 300 bees from the brood nest are placed in a jar, and washed down with 250 mls of isopropyl alcohol.  The number of mites in the resulting supernatant provides an indication as to the number of total (phoretic) mites, per 300 bees.  Note though, this still doesn’t count mites trapped inside the capped cells with the larva/pupa.  As such, at some times of year these counts may still woefully underestimate total Varroa burden within the hive.  However, as these are mites per (roughly) the same number of adult bees, it is a little easier to compare Varroa load hive to hive.

This alcohol wash was done at a friend's apiary two weeks ago. The Queen was gone, and there was a laying worker in the hive. This failing colony yielded 80+ mites from approximately 300 bees. That translates to a 26.7% infestation rate (a little over 1 in 4 bees)

Despite imperfections with these survey methods, repeat regular counts do help us as beekeepers to gauge when mite populations are increasing considerably, and can help to guide when to intervene, if the beekeeper desires to do so (more on that later).

As we’ve only done a single sticky board count thus far, we’ll repeat these counts every two weeks to gauge how rapidly our mite populations are increasing.

Varroa Vectored Diseases

Varroa mites alone can weaken honey bee colonies, but they also leave them susceptible to numerous opportunistic pathogens.  Varroa mites have the capacity to vector specific diseases to bees, including viruses, one of which is now commonly seen in colonies with high mite burdens.  Deformed Wing Virus.

The Varroa mite can weaken bees, but it's also a vector for other diseases

Deformed Wing Virus is one of at least 18 different viruses known to afflict the European honey bee.  This virus can be passed from Queen to egg, or between bees orally, but is most prevalent in hives strongly associated with high Varroa mite populations.  Virus has been detected in significant concentrations within Varroa mites who are capable of vectoring the disease while attached to developing pupa. [2,3,4,5]

Even before our first Varroa counts, we knew something was wrong when we spotted this worker bee crawling on the ground in front of the Salvia hive.

Two weeks ago we found bees crawling on the ground with deformed wings in front of our large swarm colony hive, Salvia.  Generally the belief is that survivor stock is more robust than commercially bred bees, but this colony shows that even survivor stock isn’t immune to Varroa infestation. We suspected that although up until this point the colony seemed healthy and robust, that trouble was brewing, and this is what prompted us to begin surveying for Varroa levels in our hives.

She wasn't alone, we also found this bee...

...and watched as two worker bees cast this deformed drone pupa out of the hive

Based on our recent observations at the Salvia hive entrance, we would have been surprised if this hive didn’t have a high mite count.

So with the presence of significant numbers of Varroa mites in the Salvia hive, and those bees overtly expressing signs of being infected with Deformed Wing Virus, we now need to decide which methods of Varroa control we’ll employ in our apiary.  We’ll discuss our management options, and the controversies surrounding treatment, in a future post…


[1] Australian Government: Department of Agriculture, Fisheries, and Forestry.  Varroa Mite.

[2] Fievet, J; Tentcheva, D; Gauthier, L; De, Miranda, J; Cousserans, F; Colin, Me; Bergoin, M (March 2006). “Localization of deformed wing virus infection in queen and drone Apis mellifera L“. Virology journal 3: 16.

[3] Tentcheva D, Gauthier L, Zappulla N, et al. (December 2004). “Prevalence and seasonal variations of six bee viruses in Apis mellifera L. and Varroa destructor mite populations in France“. Appl. Environ. Microbiol. 70 (12): 7185–91.

[4]Highfield, AC, A El Nagar, L Mackinder, ML Laure, J Noël, MJ Hall, SJ. Martin, and DC. Schroeder (2009) Deformed wing virus implicated in over-wintering honeybee colony losses. Appl. Environ. Microbiol 75(2): 7212-7220.

[5] Runckel, C; Flenniken, M. L.; Engel, J; Ruby, G J; Ganem, D; Andino, R; DeRisi, J. L.  (June 2011) Temporal Analysis of the Honey Bee Microbiome Reveals Four Novel Viruses and Seasonal Prevalence of Known Viruses, Nosema, and Crithidia.  Plos One Online. Vol 6 Issue 6.