Brief Notes on Breeding for Resistance to Varroa


The use of powerful miticides, i.e. pyrethroids in the UK and also organophosphates in other countries, is now seen as providing at best, a short-term palliative in dealing with the varroa problem.

At worst, their sole use inhibits any natural resistance that may be present in the bees. Enlightened scientific thinking suggests that the long-term solution will be more rapidly achieved if the bees are continuously exposed to varroa.

The more prudent beekeeper, wishing to avoid financial loss will achieve this, as I do, by using "soft" chemicals that keep the infestation below the lethal level. This requires good husbandry and judgement by the beekeeper and might result in some loss of colonies.

My experiences have been reported in Bee Improvement Magazine and an edited and updated version of these is due to appear in The Beekeepers Quarterly No. 76.

The more adventurous beekeeper may feel inclined to follow the methods of Franz Lips, though this could lead to heavy losses. Franz Lips lived at Worms in southern Germany, kept the native or near native black bee A. m. mellifera without treatments and claimed to have selected for resistance to varroa.

His hives were fitted with wooden floors that could be slid out to examine for debris. His initial selection was from those that always had the cleanest floors, reasoning that his bees, by grooming or house cleaning, would remove varroa from the hive.

Queens were raised in their own colonies and put into mating nucs made up from the same colonies, Lips believing that this contact with their own bees should be maintained. These nucs built up to fill a brood box by autumn. In the early years he lost about half of them during the winter through varroa, but the losses reduced considerably after a few years of selection.

Weak colonies were united together to take their luck and produce honey if they survived. If the strong colonies survived another winter the best were used as queen breeder colonies when the process was repeated.

The breeder colonies that survived another winter were used as drone colonies. He said that he never found varroa on his floors except for an occasional empty shell in the winter.

There is further evidence that some untreated bees can survive. This was found in the USA with the "SMR" bees and in the bees imported from the Pacific coast of Russia.

It is reported in the American Bee Journal, April 2003, that feral colonies have once again become established in several parts of France. Studies found that the bees were not necessarily resistant to varroa, but that they and the varroa had most probably co-evolved in a way that ensured the survival of both species.

A suggested course of action:

  1. Work with your own bees and, if possible, with those of your beekeeping neighbours.
  2. Fit all hives with mesh floors and trays with an easily cleaned white surface (e.g. Fablon) marked into 4 cm squares to facilitate counting or collection of mites.
  3. Use low efficiency treatments, e.g. thymol crystals or lactic acid as appropriate, to maintain a population of varroa in the hives at all times, but below the lethal limit. The lethal limit will vary from hive to hive depending upon the level of resistance/susceptibility, but if bees are seen in the brood nest with deformed wings, immediate chemical treatment should be given. This is, of course, a risky strategy and could lead to colony loss.
  4. Look for different levels of infestation by comparing the mite drop in different hives, but make allowance for any influencing factors, e.g. if other things are equal, hives with large brood nests will have more mites than hives with smaller brood nests, hives with large areas of drone brood will have more mites than those with less drone brood and hives that have a brood break in summer (as with swarming) may have less mites than hives that do not.
  5. More difficult, but possible, collect mites to examine for damage under a X20 or X30 microscope. If no microscope is available a local school or college may be able to help and might also be interested in getting involved in the studies.
  6. Tests for hygienic behaviour (removal of diseased or dead brood) can be made by piercing a small patch of sealed brood (about 25 cells) at the purple eye stage with a fine needle and checking for percentage of cleared cells after 48 hours. Alternatively, a small patch of similar brood can be cut out, frozen overnight, thawed out and replaced in the comb.
  7. If significant variation is found between colonies, raise drones and queens from selected hives early in the following year in an attempt to get apiary vicinity mating in the cooler air conditions at that time. (Read Beowulf Cooper).

Items 5 and 6 are not essential, but add to the information about variations between colonies and may assist in the selection process by identifying the two most useful patterns of behaviour associated with resistance.

An area with mature deciduous woodland (wild swarms), or let alone beekeepers who do not use chemical treatments, may have bees with higher levels of resistance to varroa than those places where all the beekeepers have relied on pyrethroids.

These notes have been written primarily for beekeepers that keep native or near native honeybees. They are less likely to be of use to those who live in a locality where the honeybee population is genetically destabilised by imports of foreign bees.

John E Dews - 10th January 2004

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