Identification of honeybees with resistance to varroa – Dennis Ryan (spring 2007)

Much research worldwide is now devoted to identifying bees with disease resistant traits, especially bees resistant to varroa.  The mite levels can be reduced by selection practices i.e. breeding from colonies displaying evidence of hygienic behaviour rather than solely relying on chemical treatments.  Hygienic behaviour refers to the tendency of some strains of bees to identify and remove diseased and dead larvae and pupae from the brood comb and the hive much more accurately and promptly than the norm.  This hygienic behaviour is assuming increased importance, as it can greatly reduce the use of chemicals in bee management. The effective life of a chemical is limited as the mites gradually adapt to them and become resistant.  There are many traits which are naturally found in the honey bee and are known to reduce mite infestation levels, such as:

–     Grooming behaviour

–     Hygienic behaviour

–     Brood pheromone cues

–     Reduced development times

–     SMR bees (Suppressing Mite Reproduction).  Bees with the SMR trait produce pupae on which for some unknown reason varroa mites are unable or reluctant to reproduce.  The mother enters the cell, but when the bee emerges the mother emerges too but without offspring.

Identifying those characteristics and then selecting and breeding from stocks expressing them will eventually produce bees resistant to varroa.  The goal is finding the natural balance of the host-parasite relationship of varroa and Apis mellifera mellifera, which already exists in Apis cerana, the original host of varroa.  There are many methods of testing for varroa resistance, some of which can be adapted by beekeepers during colony management throughout the year:

1. Grooming Behaviour – Some bee stocks groom one another to remove varroa more thoroughly than other bees.  Also, some bees chew up any varroa they find, thus ensuring the mite cannot climb onto another passing bee.  The beekeeper investigates this grooming behaviour by examining the consequent damage to varroa mites, such as legs bitten off by the bees.  Mites are examined for damage under a microscope of X20 or X30 magnification after collecting with an artist’s paintbrush size “0”, which is moistened with water.  The mites are then mounted on a surface smeared with petroleum jelly for examination and the percentage of damaged mites calculated.  Research has shown that colonies survived without treatment when 60% of dead mites on the floorboard are damaged by the bees.  Furthermore, if a level of 45% is found selective breeding could increase this to 60% or 70% within a few generations (Ruttner 1993).

2. Hygienic Behaviour  – This consists of three tasks: (i) Finding dead or damaged brood quickly, (ii) The uncapping of dead or damaged brood, and (iii) Removing dead or damaged brood from cells.  This task, which is carried out by middle-aged worker bees, consists of detecting diseased brood and removing all such infected brood, thereby decreasing the incidence of infection.  There are a number of ways we can investigate this property in bees:

(i) Pin-killing brood at the purple eye stage, i.e. piercing 25 cell cappings with a fine pin or needle and then removing the frame 48 hours later and recording the number of sealed cells remaining of the 25 tested.  For a non-resistant strain, few cells will be uncapped and contents removed, whereas for a resistant strain, all cells will be uncapped and contents removed.

(ii) Freezing brood using liquid nitrogen.  The liquid nitrogen is poured onto a specific spot on the brood using a tin soup can open on both ends.  It freezes and kills the brood, and the amount of brood removed by the bees in a 48 hour period is recorded.  Great care must be taken when handling liquid nitrogen.  Freezing tests should be repeated and only colonies removing 95% of frozen brood for two consecutive tests should be bred from.  Before the use of liquid nitrogen became the standard method, beekeepers screened colonies for hygienic behaviour by cutting out a section of comb (2’’ X 2.5’’) containing sealed brood, freezing it for 24 hours, then placing the frozen comb section in the colony to be tested.  If the colony was hygienic, the bees would uncap and remove the freeze-killed brood within 48 hours when tested repeatedly.  Although this method of cutting comb sections out of frames is relatively messy and damages the combs, it still can be used by those who want to test their bees for hygienic conduct and who do not have access to liquid nitrogen.  In addition to the above hygienic behaviour, bees may vary in associated behaviours such as how well they polish cells and clean floorboards and frames.  Some bees are even reputed to scrub off the paint dots on queens that beekeepers have applied to mark them for easy recognition.

3. Other ways of identifying disease resistant colonies during normal routine management:

(i)         Noting any colonies free from chalk brood when it is present in other colonies in the same apiary.

(ii)        Making a quick check of mite drop on floorboards after chemical treatment at the end of the season.  Colonies that have a significantly lower mite drop should be noted; this should be obvious without counting.  The beekeeper should make allowance for any variation in factors affecting the build up of the mite population, such as the size of the colony, queen performance and the number of frames with drone comb.

Course of Action:

1. Group involvement:  For selective breeding to be effective, several hundred colonies are necessary as within GBBG or a local association.  A group can achieve more in a shorter period of time than a single beekeeper.

2. Purity of strain:  It is only in Pure Race Breeding that other characteristics, such as hygienic behaviour, can be reproduced with reliable and repeated results.

3. Fit all hives with open-mesh floors and trays to facilitate counting or collection of mites.

4. Use low efficiency treatments, such as Apiguard or oxalic acid as appropriate.  If bees are seen in the brood nest with deformed wings immediate chemical treatment should be given to prevent colony loss.

5. Look for different levels of infestation by comparing the mite drop in different hives, and then queens and drones should be raised from the selected colonies showing the possibility of some degree of resistance, i.e. colonies with a low mite drop.

6. Tests for hygienic behaviour (pin test) and grooming behaviour (damaged mites) could be carried out with the assistance of neighbouring beekeepers or in association with a research project in a school or college under laboratory conditions.

We must keep a positive look out for bees that show any sign of resistance to varroa – It is really the only long term solution.

References

1. “The Quest for Bees Resistant to Varroa” by John E. Dews, Bee Improvement, 1998, No.1

2. “Minnesota Hygienic Queen” by M. Spivak and G. Reuter, www.beeculture.com/storyCMS

3. “Hygienic Behaviour by Honey Bees from Far Eastern Russia”, www.honeybeeworld.com/misc/hygiene.htm

4. “Bee Improvement in Cornwall” by J. Kilty and R. Dewhurst, www.kilty.demon.co.uk./beekeeping/improvement.htm

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