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Natural Defense Against Varroa

Posted July 9, 2024

Introduction

After Varroa jumped the species barrier around the 1950s, from its native host Apis cerana (Asian honeybee) onto A. mellifera, it spread globally along with deformed wing virus (DWV). Currently only Australia and a few small, isolated islands are free of both DWV and Varroa. Since then, research and experimentation continue to try to identify and select for the honeybee genetic traits in defense against Varroa. Breeding successes have been limited and consistent performance of purchased queens are elusive for the consumer. The holy grail of breeding for tolerance and/or hygienic defense is still not achieved in managed colonies, but many studies are turning to wild and survivor colonies for answers.

It’s fair to say that resistant colonies have been identified that exemplify defensive traits, but breeding program trying to take advantage of these are quickly lost or they simply don’t meet expectations. It is also questionable if controlled mating (a topic for another blog) can be consistently done and in the long run beneficial for honeybee genetic diversity. It’s possible that what might be considered a “natural” solution could be both the most consistent and evolutionarily beneficial defense against any intruding pathogen or parasite into the honeybee community – brood breaks. It’s a natural result of their evolutionary swarming behavior. Maybe we just need to let bees be bees.

Varroa Destructor

The Varroa destructor is an ectoparasitic mite living on the exterior of its host and feeding primarily on the bee’s fat body tissue verses the previously assumed hemolymph (bee “blood”) like a tick feeding on blood. This recent finding (2018) highlights the need for continued research and is a possible reason for the failure of some previous attempts at developing effectively targeted treatment strategies. Furthermore, it provides some explanation for the diverse array of debilitating pathologies associated with the mite that were unexplained by hemolymph removal alone. While feeding on the bee, the mite damages it by infecting it with several lethal honeybee viruses. DWV is the most prevalent honeybee virus worldwide.

The Varroa mite’s life cycle comprises two phases: (1) a reproductive phase inside the brood cells and (2) a dispersal (spreading) phase (often called “phoretic” in a broader sense) on adult honeybees. Life expectancy for the mites varies from 27 days when they have access to brood to 5 months when females live on adult bees in the winter cluster.

This mite is of particular importance as it is currently considered the largest threat to apiculture worldwide and inflicts more damage and higher commercial costs than all other known apicultural diseases. This mite’s natural host is the Asian hive bee, Apis cerana, but damage to Asian honeybee colonies is now rarely experienced since a stable host-parasite relationship has been established over a long evolutionary scale. In these colonies, the mite’s reproduction is restricted to drone brood and when mites try to enter worker brood cells, the infested pupa along with the mites are removed by the hygienic behavior of adult bees. Also, adult bees with grooming behavior capture and kill the phoretic mites (attached to adult bees) in the colony.

European honeybees have behavioral defenses like the Asian hive bee such as grooming and hygienic behavior, but they are typically less pronounced and variable between races. The specific removal of mite-infested brood has also been termed Varroa -sensitive hygienic (VSH) behavior. Both hygienic behavior and VSH behavior remove dead or diseased brood, as well as mite-infested brood, but the latter is more effective toward mite infestation. The distinction between VSH behavior and regular hygienic behavior may be in the detection stimulus of the adult bees which for VSH seems to be indirect effects of mite infestation such as pupal virus levels or faults in pupal development. The big difference between the Asian and European bee species is that the mite can reproduce in worker brood cells of European honeybees. This results in an exponential mite population growth that can lead to managed colony death typically within a few years if mite population control is not practiced by beekeepers.

Evolution and Development

Every aspect of an animal’s phenotype, both physical and biochemical traits, derives from interactions between its genotype (genetics) and its environment. If resistant behavior in a colony of honeybees against the parasitic Varroa mite occurs in a stock of bees, it is, therefore, unlikely to be an entirely predictable, simple, genetically based response, but rather to involve perhaps a series of interactions between bees and mites. It can also be expected to vary between species, subspecies, apiary conditions and handling regimes, and such variation makes its investigation difficult.

The near-globally distributed Varroa mite has formed a lethal association with Deformed Wing Virus (DWV), a once rare and benign RNA virus. In concert, the two have killed millions of wild and managed colonies, particularly across the Northern Hemisphere, forcing the need for regular acaricide application (chemical used to kill mites) to ensure colony survival. However, despite the short association (in evolutionary terms), a small but increasing number of A. mellifera populations across the globe have been surviving many years without any mite control methods. This long-term survival, or Varroa resistance, is consistently associated with the same suite of traits (recapping, brood removal and reduced mite reproduction) irrespective of location.

As A. mellifera was completely naive to the mite, Varroa typically increased uncontrollably, which, coupled with a new viral transmission route (during mite feeding), led to the catastrophic collapse of both managed and feral populations across the globe. As a result, particularly in the Northern Hemisphere, the constant use of acaricides is necessary for beekeeping to survive. However, while acaricides help reduce the Varroa and DWV burden, they also remove the selective evolutionary pressure from A. mellifera hampering any adaptation to the parasite.

In the presence of DMV and absence of treatment, A. mellifera populations can gradually develop Varroa resistance, typically after an initial period of colony losses. Resistance is the ability of a population to survive long term without any treatment for Varroa within a given environment. Resistance is not viewed as a fixed trait but the product of adaptive traits and adaptation to the local environment in terms of the surrounding managed and feral colonies.

Studied Varroa resistant honeybee populations across seven countries have developed the same traits to control the mite. These are: (a) brood removal, in which Varro infested pupae are removed; (b) recapping, where holes are created allowing direct access to the pupa and then resealed; and (c) mite infertility, where female mites are unable to produce viable (mated) female offspring.

Long-Term Adaptations

Research regarding Varroa resistance especially since Dr Seeley’s groundbreaking studies of the wild honeybee at Cornell University have begun to consider the differences between managed and unmanaged colonies including wild colonies when possible. Starting in the 1980’s it’s been documented that the Africanized honeybee population in Brazil have remained stable and without reports of increased mite infestation rates. This suggests that mite resistance in this population is (a) based on host factors rather than parasitic virulence and (b-see above) probably owing to a combination of traits additively reducing the mite population growth rather than a single trait alone, such as reduced mite fertility.

It should be noted that the Africanized bees of Brazil are genetically identical to their ancestral African race, A. m. scutellata , due to genotypic qualities. Therefore, the mite resistance of Africanized honeybees in both Brazil and Africa could be explained by shared pre-existing genetic elements of parasitic resistance. Even though Varroa mites are extremely common in South Africa, infestation rates never exceed 4 mites/100 bees. Virus infections have also been detected at low levels in South African bees but do not seem to affect the health status of these colonies, and DWV was notably absent. DWV has been reported in Brazil along with other viruses, but negative effects of virus infections are not experienced there either. This suggested that their ability to survive is due to something other than just genetic host resistant mechanisms.

Besides active defensive behaviors, additional characteristics of the Africanized honeybees that may in combination support low mite population growth include higher rates of absconding, migratory swarming, faster colony development, and like wild honeybees generally smaller colonies. Further, a reduced bee developmental time and reduced comb cell size could reduce the ability of mother mites to produce viable mated female offspring before the adult bee emerges from the cell.

Regarding the European honeybee A. mellifera in the USA, the longest known association of this honeybee and Varroa mites is from far eastern Russia (Primorsky), where from the mid-1800s contact between the Asian honeybee, A. cerana, population edge and introduced European honeybee, A. mellifera, colonies lead to the Varroa mite’s host switch to the European honeybee. Pairwise investigations with local mite susceptible honeybees in the USA demonstrated that Russian honeybees had a slower mite population growth, increased hygienic behavior and grooming behavior, had less attractive brood for Varroa mite infestation, and had reduced mite reproductive success including high infertility rates of around 50 %.

Studies have also shown that the long-term survival of honeybees with unmanaged mite infestation was due to host traits rather than reduced mite virulence and suggested that host adaptations had occurred through natural selection in the population. For instance, reduced colony size and brood amounts may be an adaptive strategy to limit mite reproductive opportunities and slow the mite population growth, especially considering the attractiveness of drone brood for mite reproduction.

Selection Pressure and Apiculture Practices

Host resistance is defined as the ability of the host to reduce the fitness of the parasite, while host tolerance is defined as the ability of the host to reduce the effect of the parasite. Studies are now demonstrating that mite resistance is possible for the European honeybee, A. mellifera, around the world and that there are multiple genetic adaptive routes to achieving a sustainable mite resistance. In these resistant populations, there seems to be a variety of mite resistant traits that additively contribute to reducing the mite population growth within the colony, as opposed to a single super trait.

The resistant honeybee populations have evolved mite resistance as they are able, in yet unknown ways, to reduce the mite’s reproductive success. Simulation modeling of A. cerana colony dynamics has suggested that the lack of mite reproduction and limited available drone brood was sufficient enough to explain the mite resistance. The A. mellifera honeybee populations with reduced mite reproductive success may have unique ways of achieving this specific mite-resistant mechanism that could include changes in brood volatiles, adult VSH behavior selectively removing reproducing mites, or even both mechanisms combined. Reduced colony size is also an interesting mite resistant parameter expressed in the honeybee populations in Brazil, South Africa, Gotland (Sweden), and in the Arnot Forest (USA, NY) but not in Russia or the Island of Fernando de Noronha (Brazil). A reduced colony size and reduced brood production (specifically drone brood production) means limited opportunities for mite reproduction and is a very important mite-resistant characteristic of the Asian hive bee. A noteworthy observation is that small colony size seems to be a common trait of populations with wild honeybees (such as Brazil, South Africa, and the Arnot Forest) or with less intensified colony management (as on Gotland). Importantly, all the mite-resistant populations have experienced a general lack of, or less intensified, apicultural management. The apicultural industry is drastically threatened by catastrophic colony losses due to the spread of honeybee diseases and parasites, especially the Varroa mite. Ironically, the spread of these diseases in apiculture is facilitated through intensified management practices.

Co-evolutionary processes such as natural selection that led to a stable host-parasite relationship as seen with the Asian hive bee have been hindered for the European honeybee host since apicultural practices remove the mite and consequently the selective pressure required for such an adaptive process to occur. On top of that, pesticides administered to colonies by beekeepers to treat against mite infestation can actually cause more damage to bee health. Adaptations by the mite towards reduced virulence depend on the available transmission routes within the honeybee population, which can be altered by apiculture. Vertical transmission from mother to daughter leads to reduced mite virulence adaptations, while horizontal transmission between colonies leads to increased mite virulence. Modern apicultural practices actually favor parasitic transmission routes that select for higher mite virulence, mainly by preventing swarming, crowding colonies in high density apiaries, and by exchanging hive equipment between diseased or dead colonies.

Interestingly, wild honeybees in Brazil and Africa experiencing natural mite infestation selection pressure may pass heritable adaptive resistance to managed colonies that could contribute to the stability of the population. This constant selection pressure may be critically necessary even though the African honeybees in Brazil and Africa have a somewhat genetic pre-disposition for mite resistance.

Nest Size

The persistence of wild colonies with the onslaught of Varroa is aided by their habits of nesting in small cavities and swarming frequently. By the end of the second summer of the study, the colonies living in small hives had a mean Varroa infestation rate of adult bees that was only about one third of that found in the colonies living in large hives. Moreover, while none of the small-hive colonies showed signs of disease, the large-hive colonies showed symptoms of high infection with the DWV, which is closely associated with a high infestation of Varroa. It seems clear that the colonies living in large hives were more susceptible to Varroa than colonies living in the small hives; the higher Varroa infestation rates impaired their health and survival. In contrast, small-hive colonies with relatively low Varroa infestation rates, did not show symptoms of high DWV infections, and had better survival.

Investigations show that populations of wild colonies are able to survive without Varroa treatments whereas managed colonies rarely persist for more than two to three years without being treated for Varroa. Results suggest that the small size of the nesting cavities of wild colonies is helping them persist, despite having infestations of Varroa. This greater swarming in the small-hive colonies, which evidently arose from the more crowded conditions in their smaller hives, meant that more of the small-hive colonies experienced mid-summer breaks in their brood rearing. Because a swarming event exports about 35% of a colony’s Varroa and temporarily deprives Varroa of the pupal brood it needs for its reproduction, it seems likely that having a higher rate of swarming helps wild colonies limit their Varroa infestations and thereby survive and reproduce well enough to maintain a population of wild colonies in a region. It is also possible that nesting in a small cavity helps the bees avoid high Varroa infestation rates because colonies with small nests possess fewer cells of brood and thereby provide Varroa with fewer opportunities for reproduction.

Splits

Studies suggest that splitting colonies—a practice in which the queen and a portion of the adult bees and brood are removed from a colony and placed in another hive to produce an additional colony, meanwhile the original colony rears a replacement queen—might be an effective way to reduce mite populations in large colonies managed for honey production. The splitting of a colony results in a broodless period in the colony, and this may limit the infestation rate of Varroa in managed colonies in the same way that swarming evidently does in wild colonies.

A brood break in the colony can significantly impact the number of available brood cells for mite reproduction. This break can be accomplished by caging or removing the queen from the colony for approximately 3 weeks. During that time, all of the brood hatches, so the mites are forced out of the cells and onto adult bees. In addition, adult bees increase grooming behavior in the absence of brood which can help decrease mite numbers in the colony. If a brood break is properly timed, it has the potential to ease the stress of a dearth period—less consumption of colony resources—while providing the colony with a young queen for overwintering.

Results also show that recapping frequency and mite non-reproduction increased during the interruption of egg laying. Inhibiting the honeybee queen from egg laying for durations which are comparable to naturally occurring brood breaks can significantly reduce the probability for mite reproduction. Brood interruptions mainly affected the proportion of infertile mother mites. The duration of interruption and the time elapsed afterwards additionally affected the occurrence of reproductive failure. Hence, the reproduction of mites was affected by brood breaks immediately and in the long run.

What can we do?

There is an urgent need for sustainable solutions to the threat of Varroa mites for the economic viability of apiculture and agriculture, as well as for honeybee health. One potential avenue is by breeding genetically inheritable adapted mite resistant traits from Varroa resistant populations such as behavioral defenses or reduced mite reproductive success. However, these populations also emphasize the influence that apiculture has on the development of infections in honeybee colonies, and consequently, by example suggest that the most effective solution for sustainably improving honeybee health would come from adopting better management practices.

Accustomed management techniques must be revised. Regular and uniform treatments of bee populations with acaricides are in opposition to field selection for resistance. To support the spread of more resistant honeybee stock, beekeepers need to monitor, identify, and exclude highly susceptible colonies from further propagation. Preference of shorter brood rearing periods, acceptance of temporary breaks in brood rearing and complete brood removal once a season through splits are some tools beekeepers can use to lower the population growth of Varroa and thus to reduce their dependence on the use of acaricides which mask the advantages of mite resistant stock. Monitoring and selective splitting of varroa resistant colonies is shown to be possible and is the principle that all resistant studies are based on. You can do the same work within your own apiary as successfully as any university.

Referenced Materials

Robbing Stress

Harbo Assay, Varroa Sensitive Hygiene Testing

Mold, should I be concerned?

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