In our last bee post we noted that despite the outward appearance of our large swarm colony hive, which seemed to be thriving, that the colony in fact was coping with a significant, and potentially deadly infestation of Varroa mites.  Even though the hive currently has almost 4 full hive bodies of brood, and two honey supers, left unchecked, this hive could still be wiped out completely by early winter.

Occasionally it's the strongest hives, those producing the most brood, that ultimately collapse due to Varroa infestations. Looking at our Salvia Hive from the outside it seems to be a thriving colony

To find the mites, we had to actively look for them.  Passive observation of mites on bees is not a reliable way to gauge whether or not Varroa has a foothold in a colony. 

Until performing mite counts, we’d only noticed an occasional mite on the bees in our Salvia hive, but the counts proved they were there in far greater numbers than we’d passively observed during hive inspections. 

The first sticky board count from the Salvia hive revealed numerous mites

Our first set of mite drop counts was alarming, but nothing compared to what we observed this weekend. 

Our  priority in the apiary for the remainder of summer is Varroa control, and doing it early enough that our colonies can recoup losses of hive population, and still have time to build up some winter food reserves.

We have Varroa mites, now what?

There are numerous methods of Varroa control available to beekeepers. Synthetic chemical insecticides, organic acids, essential oils, drone trapping, sugar dusting…the list goes on.  Which method is employed depends on the level of mite infestation, time of year, seasonal temperature extremes, and beekeeper preference.  There is no one silver bullet for Varroa, so we’ve had to research our options, and make some decisions about where to start.

Although we had hoped that our feral-source colonies, like Salvia, would prove to be more resistant to Varroa than commercial stock, and that we may have been able to avoid treating the hives altogether, the reality is in our apiary those hives seem to be the source of the majority of the mites…and the mites are reproducing at an astonishing rate!

Even colonies that have survived previous winters without intervention, can ultimately succumb to heavy Varroa infestations

For treatment, the use of synthetic pesticides, like organophosphates, wasn’t even a consideration for us.  After studying our alternative options, and discussing treatment experiences with some more seasoned beekeepers, we’ve chosen to treat the Saliva hive with Thymol.  Thymol is an essential oil, and considered safer to use during the hot summer months than organic acid (formic acid, or oxalic acid) treatments.  Thymol is derived from the oil of Thyme plants, and has been shown to be effective at reducing mite populations within brood cells, but the formulation we’ve chosen is somewhat challenging to obtain here, and won’t arrive for another two-three weeks.

Thymol is an essential oil derived from the Thyme plant, that has proven to be effective at decreasing mite populations in the hive

In the life-cycle of Varroa, that’s too long, as the mite population could more than double in just a few short weeks.  In the interim, to buy a little time, we’re also using an adjunct method called drone trapping, or drone brood removal, until our Thymol strips arrive.

If you watched the video in the last post, and understand the life-cycle of the Varroa mite, you now know that mite populations grow exponentially during the brood rearing period.

In a normal thriving hive, drones, male bees, are produced purely for the purpose of mating with newly emerging Queens, to serve to increase the local genetic diversity of area bee colonies.  As such, drones are important, but in weakening hives, especially with high mite burdens, they can also prove to be something of a liability.

Drones (yellow arrow) don't perform any critical tasks within the hive

They don’t forage for food, but they do consume significant quantities of hive food stores.  Other than reproducing with the Queens (which is always fatal for the drone), they serve little, if any purpose within the hive…unless you’re a Varroa mite, then you find drones to be inordinately useful. 

Varroa clearly favor drone cells for reproduction over worker cells

Varroa mites prefer to reproduce, and their reproductive rates are significantly higher, inside drone cells, than worker brood cells.  Drone brood cells are larger, and drones take longer to mature, than worker brood, and the net result is that more mites are produced per cell in drone versus worker cells.  As an example, it’s been suggested that assuming a 5% population of drones in a hive, more mites can be produced within 50 drone cells, than inside 1,000 worker cells.[1]  As alarming as this sounds, drone brood cells can be used to the beekeeper’s advantage for Varroa population control, through drone sampling and removal.

Sampling a few drone pupa for the presence of mites can help to determine whether nor not entire drone frames should be removed

Drone sampling and removal alone may not be sufficient to reduce Varroa to desired levels, but it is an IPM control method that can help to achieve some level of mite control within the colony.  The logic of this method is that by reducing the amount of drone brood produced in a colony, the rate of Varroa population growth is also reduced.

By removing drones before they hatch, mites are also removed from the hive

Although we can’t tell a Queen where to lay drones, we can encourage her by providing conditions within the hive that are more suitable for drone rearing. 

The same day we began our mite counts, we placed ‘drone frames’ inside the brood nest of each colony.  Drone frames come in various forms, and can be pre-made plastic frames, wooden frames fitted with wax drone-cell foundation, which is slightly larger than worker cell foundation, or modified wooden frames like the The Oliver Drone Trap frame. 

As we’re using all medium hive bodies, simply placing an empty medium frame toward the edge of the brood nest is proving to be sufficient to encourage the workers to draw drone cell comb, and encourage the Queens to concentrate drone production in that area of the brood nest. 

The bees will most likely draw foundationless drone comb on this empty frame (it helps to mark the top bar of the frame so you can find it again!)

The first week the bees were busily drawing out the comb.

After a week, the bees in the Salvia hive had nearly completely drawn out the empty drone frame

This weekend, four weeks after placing the drone frames, we checked for capped drone brood within each colony.  Finding the drone frames was easy, as we’d marked the top bars in advance. 

By marking the frames, it makes it quick and easy to locate the drone frame in the brood nest

Timing for removal of drone frames is absolutely critical, and drone frames should never be used in hives with high mite burdens if there is any chance they cannot be removed before the completion of the next brood cycle, otherwise mite reproduction is encouraged.  The goal is to remove the frames after the cells are capped, but before any drones emerge.

This drone frame from the Lavender hive is filled with capped drone brood

Some beekeepers will simply remove the drone frames and replace them with new ones, discarding the drones on those frames in every hive every few weeks, regardless as to the number of mites observed.  The downside of this method is that although drones have limited purpose, consistent culling of drones reduces the genetic diversity within an apiary.  After all, the purpose of drones is to mate with Queens.

Here there are likely sufficient feral colonies in the surrounding woodlands, that removal of drones in our hives would be of little consequence, but if a colony in our apiary has minimal mite levels, we’d prefer to select for, and preserve those drones that are less affected by Varroa!

To decide whether or not to remove the frames, a sample of drone pupae in the ‘pink eye’ stage are removed from the capped cells to ascertain level of mite infestation in the drone brood.  Recommendations are to sample at least 50-100 drones, selecting a few at a time from various locations on the frame. 

When the first sample reveals this many mites, the drone frame should probably be pulled

Our colony with the lowest population, the Chamomile hive, actually placed worker brood on the drone frame, there wasn’t a drone cell in sight.  As such, we returned that frame to the brood nest, as this colony needs all the worker brood it can produce.

The Lavender and Rosemary hives both drew drone comb on their drone frames, and had moderate mite levels within the cells.  We felt levels were high enough (greater than 30% of pupae had mites) and elected to pull their drone frames. 

The very first sample of drones from the Lavender hive turned up Varroa mites

The Salvia hive however was completely overwhelmed.  The mite drop count suggested a significant mite burden in the Salvia hive, but still didn’t prepare us for what we found.  Drone brood sampling in Salvia revealed the horrific truth of what’s really going on inside this colony.

This piece of comb was crawling with Varroa mites

Too many mites to count, with more than 60% of the cells containing mites.  Far worse than we could have imagined based on a simple passive mite drop count.

Varroa mite infested brood

There is a very real likelihood of this strong hive collapsing by late fall with this severe a Varroa burden.

Varroa Mites

This colony is already showing evidence of secondary viral infections, including deformed wing virus, and it’s no wonder why.

Numerous mites were visible under the cell walls

It seemed as if almost every cell was seething with mites, in various stages of development.

This cell contained both mature, and immature mites (yellow arrows)

Some cells had more than 6 mites entombed with each pupa.  Opening any cells sent mites scurrying across the cappings in all directions.  It was sickening to see the magnitude of infestation in this colony.  All this has built up in just three short months since this colony was originally hived.

See how many mites are trapped in the cell with this pupa?

Once the cells are opened, the mites immediately begin to scurry away

Despite seeing the obvious, we’re also trying to train ourselves to recognize some of the more subtle signs of Varroa problems within a colony.   Spotty brood patterns are suggestive of infestation, but by the time they are easily recognized, especially by new beekeepers, the problem is usually already severe.

Notice on Salvia’s drone frame that numerous cell cappings appear to have been chewed away?

Cells with ragged edges are where workers have removed drone pupae

The edges of cells have a ragged appearance where a cap has been mostly removed, and those cells are now empty.  This is likely due to hygenic behavior of worker bees within the hive, who are removing damaged, sick, or dead pupa from the cells.

See the holes in these cappings? 

Holes in cappings over sunken cells

These may be the result of mites emerging from the cells, or workers starting to chew the cappings away to remove affected or dead pupa.  Either way, it’s not normal in a healthy hive.   The sunken cells contained dead pupa that had not yet been removed.

Here we even found mites that appear trapped in the cappings over the brood cells.

Mites trapped in cappings

The level of mite infestation in our Salvia hive is almost enough to give us nightmares, but with appropriate management, we’re optimistic that we can still help this colony survive, and hopefully thrive.

The cap over this drone pupa has been mostly removed

Culling drones in the Salvia hive is not something we want to do, but by doing so, at least until this hive is treated, we hope to prevent the Varroa burden for the worker population from escalating too rapidly within the colony.  Continued selective culling after treatment may help us to minimize the need of additional treatments, but it’s obvious to us that various methods of control may need to be employed in our apiary for effective Varroa mite control.

The mites on this pupa can vector viral diseases, and decrease the lifespan of individual bees

In the spring, any surviving hives will be split to propagate the colonies that are doing well, and hopefully help to select for healthier bees over time.

With the promise of high-protein tasty treats, our hens are more than happy to participate in our IPM Varroa control program

In the meantime though, while we wait for the Thymol to arrive, our spoiled hens seem to approve of our interim method of Varroa control, and are more than willing to step up to do their part…


[1] D. Wilkinson and G. C. Smith.   Modeling the Efficiency of Sampling and Trapping Varroa Destructor in the Drone Brood of Honey bees (Apis mellifera)  in Apicultural Research: American Bee Journal. March 2002.  p. 209-212.