The varroa mite continues to be a source of ongoing concern, economic loss and frustration for many beekeepers. Many queen breeders, including us, are actively selecting for bees that express mite resistance as our breeding stock, with our ultimate goal being to reduce (or eliminate) the need for routine mite treatments.
Varroa in Apis cerana vs Apis mellifera
Originally the Varroa mite occurred only in Apis cerana, the Asian honey bee, which a species closely related to Apis mellifera, the European honey bee. The Asian honey bee displays several behaviors enabling it to co-exist with the Varroa mites, without much difficulty.
One could say that over time Apis cerana and the Varroa mite evolved a mature host-parasite relationship. Varroa benefits from having a place to feed and reproduce, while the Asian honey bee is most of the time able to prevent the Varroa population from becoming too high and causing problems.
Unfortunately, the European honey bee had evolved no such behaviors and were essentially without natural defense. This is why when Varroa first transferred over from Asian honey bees to European hives, colonies were decimated across the world. There are only a few places where Varroa mites are not yet found.
Although many chemical treatments were developed, there were many limitations , including the fact that the mites often became resistant. An even greater concern is that these chemicals have been found to readily contaminate honey and beeswax. Even “soft” organic treatments such as concentrated essential oils or organic acids such as formic acid, are not completely without harm to the bees.
Thus bee breeders around the world have been selecting for honey bees that not only display traits that allow them to better tolerate Varroa but which also have all of the other characteristics beekeepers desire in their bees including gentleness and productivity.
Honey Bee Resistance to Varroa
Varroa mite resistance can be separated into two type of traits: those traits that are related to the time when the Varroa is within the brood (when mites are reproducing) and those traits that are related to the time when the mite is on the bees (when mites are phoretic).
Of all brood related traits, VSH (Varroa Sensitive Hygiene), formerly called SMR (Suppressed Mite Reproduction) has been the most studied, and been shown to reduce mite burden. When a female mite enters a cell to begin reproduction, VSH bees will open the cell and interfere with that process. Sometimes the pupae itself is removed and destroyed; other times the cell will be recapped.
It appears that VSH is an autosomal recessive trait, which means that there must be at least 50% VSH genetics in a hive for there to be sufficient suppression of mite populations.
VSH seems to be due to two related traits. One group of bees uncaps the brood with mites; while another group of bees actually removes the pupae. Interestingly, in those bees expressing strong VSH, even if the mite survives, reproduction is decreased or stopped.
Generally increased hygienic behavior, such as with the Minnesota Hygienic strain, seems to reduce mite populations but not in quite the same manner as does VSH. Unlike VSH, however, increased hygienic behavior does not appear to reduce mite reproduction, though it does increase resistance to other brood diseases such as foulbrood and chalkbrood.
Other brood related behaviors that may provide resistance to Varroa may include decreased attractiveness of mite toward brood, and shorter duration of capped brood (such as when using small cell brood comb).
Other behaviors that also appear to promote mite tolerance including increased swarming and/or longer periods of broodlessness (such as with the Russian bees).
In contrast to brood related traits, behaviors that provide resistance to Varroa when on the adult bee included increased grooming behavior, increased “biting” of the Varroa mite, and increased tolerance of the bee to the mite and/or resistance to viruses spread by Varroa mites.
Selecting for Varroa Resistance
The Varroa mites’ natural host, the Asian honey bee, appears to display all of the above traits. Other subspecies of bees which were not original hosts to the Varroa mite also appear to be resistant, including the African bees.
As a queen breeder, it is important to select the stock showing the best combination of all characteristics desired, Varroa resistance being only one of them.
A problem in bee breeding is that when aggressively selecting for only a single trait, such as Varroa resistance, the bees sometimes may lose other positive characteristics. For example, even though African bees are extremely mite resistant, they obviously have other limitations!
There are several methods of selecting for mite resistant bees.
The most straight forward method is to simply breed from those colonies which have survived without treatment. Increased colony survival without treatment is the ultimate goal, but apiaries in which all mite treatments have been withheld may experience >80% mortality the first few years.
Mite counts are simple way of monitoring for varroa: just count the number of dead mites on the screened bottom board after a certain preset period of time, then breed only from colonies with the lowest counts.
The problem with mite counts is that they are non-specific and can be affected by many confounders, resulting in inconsistent or false results.
For example, if there are two colonies which have exactly the same percentage of mite infestation, if one colony is much larger than another, that may give a false impression that that col0ny has a worse level of mite infestation, when in fact they are the same.
Other factors can interfere with mite counts, including the time of year. When the bees are in the spring buildup, most of the female varroa mites will be reproducing in the brood, with low mite count on the bottom board, giving the false impression that mite population is low, when it isn’t.
A better method of evaluating the mite population is to perform a “mite roll” with alcohol, ether or even powdered sugar. Mite roll is a more accurate method of estimating the percent of phoretic mites on the adult bees, because it corrects for differences in honey bee population. However, the mite roll also has limitations, including the ones mentioned above relating to time of year, and whether most of the mites are reproducing in the brood, or on the adult bees.
Some beekeepers evaluate mite populations by scratching open capped brood (drone brood is preferable because that is what the mites prefer), and getting a general idea of how high the population of reproducing mites is. This method also has limitations, and is insensitive and nonspecific.
A method to select for increased general hygienic behavior is by objectively evaluating the bees ability to remove killed brood. A section of brood is killed by freezing or liquid nitrogen, then how much of the dead brood is removed within 24 to 48 hours is determined. The goal is >95% brood removal. Increased hygienic behavior is not specific only to varroa mite resistance, but also provides some resistance to other brood diseases.
Selecting for VSH is somewhat more challenging than selecting for generally increased hygienic behavior. Bee breeders must examine colonies for several characteristics associated with VSH including mite infertility (15-25% of mites in non-resistant colonies do not lay eggs whereas mite infertility is 80-100% in VSH colonies), high rate of uncapping of infested combs (VSH bees should uncap 90% of brood with varroa), as well as percentage overall of brood infestation rates.
In addition to selecting for brood-related mite resistance traits, some bee breeders are now also select for traits promoting resistance during the time the mites are on the adult bees, such as increased allogrooming or “mite biting” behavior. Dead mites that fall through the screen onto the bottom board are examined to determine what the prevalence of damaged or “chewed up” mites are, breeding only from those colonies that display a higher rate of bee damaged mites.
In 2015, when analyzing dead mites we found that the range of damaged and chewed mites in our breeder hives ranged from as low as 0% to as high as 14.8%, with an average of 3.1%. Interestingly, our average of about 3% is similar to the rate of this behavior which other bee breeders found at baseline.
In the future, we too will select for mite biting behavior in addition to hygienic behavior and low mite populations- as we have already been doing.
Unlike breeding for resistance to tracheal mites, which appears to be much more straightforward, varroa mite resistance is more complex and due to many variables and genetic traits. Many predict the eventual solution to varroa will not be one single characteristic but for our bees to express a combination of various behaviors and traits.
For more information:
- Arista Bee Research Foundation
- Forsman T, et al. Introductionary study for breeding Varroa resistant bee. Swedish Beekeeping Association, (2004).
- Rozsenkranz P, et al. Biology and control of Varroa destructor. Journal of Invertebrate Patholog 2009.
- Rinderer TE et al. Breeding for resistance to Varroa destructor in North America, Apidologie 41 (2010).
- Varroa Sensitive Hygiene and Mite Reproduction. USDA-ARS Baton Rouge Bee Lab.