“Breeding for Mite Tolerance in Honey Bees”

Bee Culture (October 2004) Vol. 132 (10): 23-26

 

By

 

Malcolm T. Sanford
http://apis.shorturl.com

 

Dr. Tom Seeley’s article in the July 2004 Bee Culture is a pleasant surprise.  It seems that honey bees that are infested with Varroa mites and have not been treated with chemicals are doing just fine thank you in the Arnot Forest of Cornell University.  Well, to be more accurate, I find it pleasant, certainly, but a surprise, no.  The fact that honey bees can take care of themselves in spite of being subjected to the ravages of an introduced species, the external (outside) parasitic mite Varroa destructor, which has been responsible for the death of so many managed colonies, should astonish no one.  This has already been seen in bee populations in the wilds of Western Russia near Vladivostok and reported in Serbia with Carniolan bees (Apis mellifera carnica).  Hints of it have shown up elsewhere in Europe, which has a much longer history of mite infestation than the Americas.  Finally, it is taken for granted in Brazil, where a huge beekeeping industry is beginning to flourish in spite of universal challenge by these mites.

 

Now that what was obvious to many has been established, Dr. Seeley will bring his talents to bear on subsequent questions.  What is the mechanism for this tolerance?  Is it “avirulence,” the idea that if vertical transmission (from mother to daughter colony) is the major route of infestation, then there is a selective advantage to parasites that do not kill their host?  Or is it evolution of bee resistance or tolerance to the mite?  These are not “mutually exclusive,” according to Dr. Seeley, which means they might occur together, providing another level of complexity.  Given what I know about biology, I am placing my bet that both are involved.

 

Another introduced mite has and continues to be a problem for beekeepers in North America, especially in temperate climates.  This is the internal (inside) mite, Acarapis woodi.  A similar situation in finding tolerant honey bees to tracheal mite exists as that for Varroa, although the inside mite does not appear to be generally as damaging as the outside one.  In the best known case, a breeding program in Europe, developed by a Monk (Brother Adam), produced a population of apparently-tolerant bees.  These insects were brought to the Americas and used in Canada and New York to help beekeepers establish other lines.  Over time in most areas where tracheal mites were a problem, their virulence has dropped considerably. 

 

According to Dr. Robert Danka, the mechanism for resistance is that populations of tracheal mites are suppressed through grooming themselves (autogrooming). 1  He concludes: “It does not appear that differences in cuticle chemistry, the presence of hairs surrounding the prothoracic spiracles, or grooming among nest mates are major determinants of resistance.”  Perhaps, but there has been evidence that the transfer of mites from bee to bee may be affected by these or other mechanisms, the reason that vegetable oil patties appear to be effective for control.  Finally, resistant bees cause little or no reduction in fecundity of tracheal mites which infest them.”

 

Dr. Danka says there are three possible stock selection scenarios that allow queen breeders to cultivate tracheal mite tolerance in stocks:

 

  1. Newly emerged bees are placed in mite infested colonies and then examined for mite loads.  Mothers of those bees having fewer mites after being exposed are selected for further breeding
  2. Mite infested bees are divided among colonies.  Queens from colonies with less infestation over time are selected for further breeding.
  3. Allow natural selection to control mite tolerance by abandoning any use of chemicals and selecting from colonies that survive and thrive the best under these conditions.

 

Scenario number one above is a technique pioneered by Dr. Medhat Nasr (now Provincial Apiculturist in Alberta, Canada) while working with the Ontario Beekeepers Association.

 

The situation surrounding Varroa is more difficult than for tracheal mites, yet similar when one looks at the practical breeding methods being used.  Dr. Marla Spivak says that in general Varroa population growth is influenced by:

 

  1. Successful entry of a mated female into a brood cell containing a 5th instar larva.
  2. Successful reproduction of the mite within the cell (at least one mated female offspring).
  3. Probability that the same mite will survive to enter another brood cell.
  4. Number of complete reproductive cycles a mite completes in on season.

 

Like the situation for tracheal mites, effects of genetics and environment come into play.  Dr. Spivak divides her discussion into sections entitled:  mite environment and genetics, bee environment and genetics and current breeding programs. 2

 

The major unknowns surrounding the mite environment are its reproductive success on adult bees (the phoretic stage or “hitchhiker” statge) and on the developing pupae and larvae.  The latter is the area where currently most of the breeding is taking place.  Mite genetics certainly play a role as we now know several haplotypes of Varroa exist, some (Russian or Korean) more virulent than others (Japanese).  Where these are most common and how they interact with the bees and colony environment in different locations is also for the most part unknown.

 

The bee environment both internal (food) and external (temperature, rainfall) also may affect Varroa reproductive success.  Experiments in the tropics seem to show that bees in the lowlands and hot areas are more tolerant than those in the cooler uplands.  Honey bee genetics plays a huge role in differences among races from tongue length to size around the world.  So it should too when considering tolerance to Varroa..  Africanized bees (Apis mellifera scutellata) are known to be much more tolerant to Varroa than Europeans (Apis mellifera ligustica), but there is evidence they are less so in Mexico than in Brazil.  Evidence also exists that even pure European honey bees isolated on an island for many years off the coast of Brazil are tolerant.  Again, this may be due to the kind of mite present.

 

Africanized honey bee biology, however, appears to favor some of things thought to be responsible for tolerance:  According to Dr. Spivak, these include:

 

  1. Shorter post capping period, meaning there is less time for mites to complete reproduction.
  2. Increased grooming of both themselves (autogrooming as found in tracheal-mite-tolerance) or among bees (allogrooming).
  3. More uncapping and removal of infested larvae and pupae by adult bees, the “hygienic behavior” thought to be responsible for some populations being resistant to American foulbrood.

 

Current breeding programs do exist for Varroa control Dr. Spivak reports.  These include those that:

 

  1. Select for a single trait such as grooming or hygienic behavior.  Carniolan “Yugo” bees fit that category. 
  2. Select from a population known to be tolerant.  Most often this is from an area where there has been no treatment for a number of years and so-called “survival colonies” can be found.  This is the case for Russian bees.  Dr. Seeley’s bees in the Arnot Forest are likely candidates in the future as are other populations that might be discovered in the future.
  3. Select for suppression of mite reproduction (SMR).  This is a program pioneered by Dr. John Harbo and colleagues at the Baton Rouge Bee Laboratory. 
  4. Select for a group of characters based on colonies that survive without treatment.  The Honey Bee Improvement Program in the UK or Dr. Sue Cobey’s New World Carniolan Project fit this model.

 

Dr. Spivak concludes: “If colonies are bred from the survivors of untreated colonies, some degree of resistance in the progeny may be obtained, but it is important to understand the reasons why some colonies survive.  The most efficient breeding program should be based on selection for characteristics that have the greatest impact on reducing mite survival and reproductive success, and those characteristics should be heritable.”

 

Many questions remain, according to Dr. Spivak who says, “there are no beekeepers or researchers who have successfully bred a line of bees that is Varroa resistant or tolerant such that they can survive without treatment.”  These questions include:

 

  1. Is it realistic to attempt to breed a line of bees that never requires treatment by miticides?
  2. Is it sufficient to breed lines that survive without treatment for one or two years?

 

There is little doubt that there has been some success in the search for the most sustainable yet difficult long-range solution to the Varroa mite problem.  However, much more needs to be done if this “holy grail” in beekeeping is to be discovered.  In the meantime, Dr. Seeley will continue his inquiry to determine whether the bees or the mites or both critters are changing and adapting to one another in nature’s constant search for stability (homoestasis) among its organisms.

 

References:

 

1.  Danka, Robert, 2001.  “Resistance of Mites to Tracheal Mites,” in Mites of the Honey Bee, Hamilton, IL:  Dadant & Sons, Inc., p. 122.

2.  Spivak, Marla, 2001. “Honey Bee Resistance to Varroa Mites,” in Mites of the Honey Bee, Hamilton, IL:  Dadant & Sons, Inc., p. 206.


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