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January 2003


This is a graph I saw on Jose Villa's laboratory wall in Baton Rouge

My Comments: I'm told that the sample on the right represents the resistant and susceptible strains kept at the Baton Rouge lab.  The scientists need mites for some experiments and thus the lab keeps some bees that always have mites (S), as well as bees that seldom have mites (R).   The other samples -- to the left of the lab sample -- are from seven randomly chosen commercial queen producers in the USA.  Obviously, when it comes to tracheal mite resistance, some are consistently excellent and some are pretty bad. 

If you got queens from producer number two or six, you'd stand a good chance of losing a lot of hives to tracheal mites, but if you bought from number five, you'd seldom have problems, if ever.  Other suppliers range between the extremes.  The majority of suppliers are very good in some samples and very poor in others.   Thus we can see that buyers can't count on consistently resistant bees from most of the suppliers sampled.

See A Survey of Tracheal Mite Resistance Levels in U.S. Commercial Queen Breeder Colonies for details. 

Here is a quote: "The most striking result of this survey was the variability in levels of tracheal mite resistance among colonies of U.S. commercial breeding stock. This breeding population can be expected to yield propagated queens that range widely in quality..."

Jose Villa Responds on BEE-L: I read with great interest the dialogue generated by Allen Dick's posting of a chart that we had on the wall during the AHPA visit to the USDA, ARS Bee lab in Baton Rouge. I want to present a few thoughts to the group that has shown so much interest in the topic. Some of the researchers with experience in tracheal mites (Bob Danka, Lilia de Guzman, Medhat Nasr and myself) believe tracheal mites can be significant problems, they can be easily resolved, but they are also easily forgotten.

First of all, the plot of breeder susceptibility was published in an article in the American Bee Journal, April 2000, by Bob Danka and myself. We really had no a priori idea of what we would find. In order to get queen breeders to agree to participate in the test, we agreed to keep their identities confidential. I agree that to some it might sound counter to what a government agency should do, but USDA-ARS is not in the business of regulation or certification, but rather research and generation of information. We feel that this information should be a warning to all queen producers, and should prompt customers to start asking their suppliers what they do or don't know about the tracheal mite susceptibility of their stocks. The commercial testing service that we have assisted in getting established (Backwoods Apiaries in Shelbyville, TN) is one way that breeders could get that information, but unfortunately few have shown interest. We do not endorse, certify or guarantee the results of this small business, but we have a memorandum of understanding to assist him with information, quality control, etc. This is not an agreement of exclusivity, so any one that shows interest in learning the process, establishing a similar commercial service, etc. will receive equal treatment.

We can say the following about resistant stocks without breaking the confidentiality agreement on the test mentioned above. Recent imports from the Old World (British Buckfast, 1990, Yugoslavian bees, 1989) and far-Eastern Russian (1997 and following years) are consistently very resistant to tracheal mites. 'New World' stocks from programs that have actively selected for tracheal mite resistance in the field or using short tests also have resistance (Ontario Bee Breeders-Mehat Nasr's program, Steve Park). We have no direct experience in the field with queens from Ohio Queen Breeders, or from Sue Cobey's New World Carniolan, or from Marla Spivak's Minnesota Hygienics, but given that they actually look for tracheal mites (and cull colonies) they report low winter levels in their stocks (see their web-pages).

It is also important to remember that tracheal mite resistance is a trait that is not exclusively possessed by any stock, race, bee color, etc. We have seen very dark bees show high susceptibility, and very yellow bees be highly resistant and viceversa. It appears as if the main mechanism of resistance is improved self-grooming by young workers of migrating female mites. Susceptible bees have the behavior, so with some selection it can easily be improved. We have not narrowed it down perfectly, but the trait seems to be governed by one or a few dominant genes. With a very simple genetic model, assuming that homozygous resistant, homozygous susceptible or heterozygote queens mate to varying combinations of haploid resistant or susceptible drones, it is easy to explain how a breeder that has a mixture of genes in his operation could produce colonies that span the range from highly susceptible to highly resistant. What is beautifully simple, or simply beautiful, about this system is that with a little bit of information and selection the bad genes can be culled out fairly quickly.

Given the variability of climates and tracheal mite resistance levels in stocks it is not surprising that beekeeper experience in the field is a real patchwork. As Medhat says, tracheal mites are not as serious in the South. First of all, the hot summers make the levels of infestation drop in most susceptible colonies during that period. Second, even if levels of mites in susceptible colonies increase during southern winters, most colonies survive. What could be really incidious in the South is that colonies could be experiencing a chronic loss of performance year round. Consider Medhat's dramatic findings that infested colonies cannot thermoregulate or use oxygen at the required levels during really low winter temperatures. The same level of oxygen consumption by clusters during cold spells, is also required for individual forager flight, specially when loaded with nectar or pollen. What happens to forager performance in colonies with 10, 20, 40, 80% worker infestation?

Sorry for the length of this post, but it summarizes some of the thoughts a group of researchers have developed through time. This is not an official document, just my personal opinions on the matter to a group of people that have shown interest in something that I think should not be ignored.

Jose Villa

March 2010

My Comments: Jose sent me this from his 2008 presentation at the ABRC meeting in Sacramento.  Seven years later, not much has changed, except that the range of variation seems to have converged towards being more susceptible and the resistant samples have disappeared!  2002 seems to have been the year that people paid attention to breeding for tracheal mite resistance.  Since then, things have been on the slide.

Villa, J.D. EVALUATIONS OF RESISTANCE TO TRACHEAL MITES IN U. S. BEES Honey bees in the United States have been exposed to tracheal mites for more than two decades, but problems still are found when certain factors coincide. To maximize our efficacy in finding infested colonies for research, we sample unselected colonies in the winter in Louisiana and neighboring states. It is common to find between a quarter and a third of colonies from unselected sources with infestations above the economic threshold of 20%, especially in colonies moved from northern states for the winter.

During the last ten years we have evaluated the genetic resistance of colonies from eleven sources using bioassays of newly emerged workers exposed to tracheal mites in infested colonies (Gary & Page, 1987, Exp. Appl. Acarol. 3: 291-305). In 1999, the resistance of colonies from eight commercial sources (n = 6-19 colonies per source) was compared to colonies known to be resistant or susceptible (Danka & Villa 2000, Am. Bee J. 140: 405-407). Using these standard colonies, a resistance index can be used to place each colony on a common scale from 0 to 1 (similar to resistant or susceptible standards, respectively). Five out of eight commercial sources showed mean susceptibilities above 0.4 and a high amount of variability between colonies (Danka & Villa 2000, cited above, results summarized in Figure). In subsequent years, colonies from three of those sources (n = 9-63 per source per year), plus colonies from three additional sources (n = 5-88 per source per year) were evaluated with the same procedures. The levels of resistance in four of the six sources appear to have deteriorated or not improved through time, with their most recent mean resistance indices being higher than 0.5 (Figure). Beekeepers should be aware of the fact that uniformly high resistance to tracheal mites is not found in all commercial sources. Active selection for resistance by breeders and purchasing of resistant breeding material by queen producers is recommended to solve these persistent problems with tracheal mites.

Figure Mean tracheal mite resistance indices of colonies from 11 sources evaluated through time. Infestations of young workers exposed in highly infested colonies are scaled to infestations of similarly treated workers from standard resistant (R) and susceptible (S) colonies. Colonies from eight commercial sources were evaluated in 1999 and are indicated with solid symbols. Colonies from three of those sources (●, ■ or ▲) plus three additional ones (○, □ or ∆) were tested in subsequent years and mean indices obtained in different years are indicated.

See also A Survey of Tracheal Mite Resistance Levels in U.S. Commercial Queen Breeder Colonies

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