Identifying Varroa Resistant Bees

Identifying bees that can manage their own varroa populations

The initial phase of the project used biotechnical methods (eg; queen frame trapping) to replace chemical treatments.  These are very effective at reducing varroa loads ahead of the winter bees being laid (see graph above) and winter colony survival is excellent.  It felt an important step to overcome the pervasive fear in beekeeping that: “If you don’t treat your bees for varroa, they will die”.  The bees certainly seemed fresher in the spring and research from Dr Ralph Buchler demonstrated that spring colonies were significantly larger using queen frame traps rather than treating (ref: National Honey Show 2019).

The project, which was planned in 2017 after meeting Dr Ralph Buchler at Gormanston that year, started for the 2018 season with a group of like-minded beekeepers.  No miticide treatments were employed after 2017.

We mapped out the project without knowing the time frames between different phases and tentatively estimated a 5 year time frame.

Identifying varroa resistant traits in honeybees

The second phase of the project focused on identifying the characteristics of bees where colonies already manged their own varroa populations.  What actions were the bees taking?  What could we learn from them?

Thankfully, these are questions that many of the world’s leading bee scientists are focused on.  There are adapted and unmanaged (for varroa) stocks of Apis mellifera (our western honeybee) in the UK, Continental Europe and the Americas.  Barbara Locke summarised many of these in her research paper “Natural Varroa mite-surviving Apis mellifera honeybee populations”

Working from bee scientist research on how adapted bees had become resistant to varroa, and together with our own observations, we formulated a proxy for beekeepers to identify bees that were managing their own mite populations.

Uncapping worker brood cells is a key trait, where nurse bees seek to interrupt the reproduction of the mites.  The uncapped cells are easy to identify by the beekeeper and have a small rim of wax around the cell circumference.  Usually, the pupa is at the white-to-pink eyed stage of development (pic below), which occurs 4-6 days after the cell is capped.  Badly infected pupa are chewed out by the nurse bees, leaving a tell-tail sign of white pupa bits on an inspection board under the open mesh floor (see pic above).   

The graph below illustrates the high level of uncapping and recapping in “Mite Resistant” colonies versus “Mite Susceptible” colonies (referred to in this research as “Surviving” or “Susceptible”).  Who would have thought all this uncapping/recapping activity was going unnoticed and not understood?

Recapped cells are harder to identify by the bee keeper; often they are slightly “domed” and sometimes a change in colour is noticeable where the “bee made” hole in the cell has been repaired.

Uncapping in early spring is a strong indicator of a mite resistant trait

Spring is a key time for untreated colonies to demonstrate their good health through strong brood development and foraging.  Early spring also provides clues to the beekeeper about which colonies prioritise brood hygiene.  Signs of white pupa bits on the insert board in February demonstrate uncapping and chewing-out of infected pupae, where over-wintering varroa sought to reproduce in the early batches of worker brood.  If bees are focused on varroa control at an early stage of the season, they will continue this behaviour throughout the season.  The genetics to prioritise brood hygiene are in place.

The graph below shows such a colony.  Evidence of uncapping was seen in February 2020 (and again in February 2021) and despite the build-up in brood through the spring (10 frames with brood in May 2020), where varroa have the opportunity to reproduce, the bees controlled their mite population.

“Mite Resistant” versus “Mite Susceptible” colonies

There is a vast difference in varroa populations between “Mite Resistant” and “Mite Susceptible” colonies.  The graph below illustrates this, where Hive 8 has poor “varroa-centric” genetics (queen & drones) which need changing.  As an aside, the old approach would be to treat the colony and keep these genetics in place, which perpetuated the varroa problem.

All of these colonies were in single National brood boxes in the same apiary.

Proactively breeding from your best varroa resistant genetics quickly improves the apiary’s resilience to mites.  It also makes a valuable contribution to the local drone congregation areas.

Mite mutilation

Studies of varroa resistant colonies show that the bees “possess multiple behavioural mechanisms for supressing the mite populations…..” (Tom Seeley – Lives of Bees)

This multi-faceted defence of their brood area includes mite mutilation, which is seen in some of our colonies.  The pictures below show missing legs & antennae and chunks taken out of the mite’s carapace (nb; the insert board is sealed to stop access by any other insects).   Killing a foundress mite, before she has the opportunity to reproduce, is very efficient by the bees.

“Breeding” protocol

Not so much breeding, but identifying traits of varroa resistance from bees that have already demonstrated uncapping and low mite loads.  We also like to see good bee health, evidenced by strong foraging and brood development in the spring, plus gentle temperament.  

A key test is to over-winter the colonies without any beekeeper reduction of varroa.  Importantly, this introduces an element of natural selection, which stops the cycle of breeding from bees that are susceptible to mites – that’s what we have been doing for some 30 years since varroa arrived in the UK.  There is an element of judgement here for the beekeeper looking at uncapping and mite loads. A cautious approach is sensible, starting with just some of your colonies.  It is also worth over-wintering some nucs as back-up.

The slide below illustrates an apiary set-up to evaluate and breed varroa resistant bees, including nuc back-ups to produce surplus bees for any losses, replace underperformers or to sell.

Outcomes for colonies

We found that using biotechnical techniques (eg; queen frame trap) to reduce varroa loads in mid-July to mid-August, ahead of the winter bees being laid, worked very well (see earlier written up results), resulting in a route to chemical free beekeeping. 

How would the bees fare without any varroa reduction by the beekeeper?  After selecting 37 colonies with uncapping evident and lower mite loads, a proportion of our hives of the “lead breeding” group went into winter.

We were very pleased with the outcome.  A loss rate of under half of those in the South East, most of which are treated bees.  Importantly, we now had a strong genetic base from which to expand our own stocks and throughout the region.

Colony outcomes will be published as we proceed through the project.

Other observations included:-

  • These were all our own local queens
  • Colonies / apiaries had raised their own queens for a minimum of 3 years
  • There were no (or low) genetic disturbances from queens imported into the area
  • Colonies were spread out over c.50sq miles (see map below)
  • Only 3 out of 8 apiaries shared any local “mating areas”, so most apiaries were unconnected
  • Temperament was good
  • Honey yields were consistent with previous years (allowing for weather)

Background adaption

It is difficult not to conclude that a level of background adaption between our honeybees and varroa had occurred.  Perhaps this is no surprise in the almost 30 years since Varroa destructor arrived in the UK.  Other 10 year + resistant colonies  (eg; Arnot Forest, Gotland Island, Gwynedd region in north west Wales, Swindon Conservation group, Avignon etc….),  indicate varroa resistance occurred in a period of up to 10-15 years.


We are grateful for the mentorship of Dr Ralph Buchler through this project.  We would also like to acknowledge contributions from Professor Stephen Martin (University of Salford), Dr Jeffrey Harris (Mississippi State University and ex-USDA –> helped discover VSH), and learnings / inspiration from Professor Tom Seeley.  Thank you for the scientific underpin to our field work.