Varroa Resistance

It has become clear that the Honey Bee can develop several traits (by selection) to reduce the infestation by Varroa. This has been shown in un-treated colonies in the wild and in special selection breeding programs (like Varroa Sensitive Hygiene (VSH) bees from the USDA-Baton Rouge). The traits can be categorized in those related to brood and those related to the period that the Varroa is on the bees.


Brood related traits – VSH

Removal of infected brood, Varroa Sensitive Hygiene (VSH),  is a very efficient method, also used by the Asian Honey Bee.

In the illustration below the “normal” reproduction of the Varroa is shown on the left side: The honey bee queen lays an egg. The egg develops into a larva which is being fed by a worker bee. Once the larva is almost full-grown, an adult Varroa enters the cell. Shortly after, the cell is closed by a worker bee. The Varroa can now start her reproduction by first laying an egg producing a male, and after that several eggs producing female Varroa mites. The mother-mite makes a hole in the larva for collecting hemolymfe. The same hole is also used as feeding place for her offspring. The newborn female Varroa mites mate with the male Varroa and leave the cell once the pupae has become a bee.

Honeybee and Varroa 1754 px Engl

Honeybee and Varroa Illustration – click to enlarge

On the right side of the illustration and the video below it is shown what happens in the case the bees have inherited the Varroa Sensitive Hygiene trait. A female Varroa mite will still enter the cell and start with the reproduction. However, some of the worker bees can detect that a Varroa is in the closed cell (maybe by smelling the damaged larva, the reproducing Varroa itself, or the offspring – this is not clear). This worker bee is opening the cell of the undeveloped pupae. Once the cell is opened, other worker bees remove the pupae with the Varroa. The Varroa has now not been able to reproduce. Part of these Varroa mites might try to enter a cell again, but have only a limited live-span.

It has been shown that this VSH-trait is very efficient. If infected brood from a non-VSH colony is given to a VSH-colony, most of the Varroa mites (>90%) are detected and removed. If the queen of a non-VSH colony with relative high mite numbers is replaced with a VSH queen, mite numbers start to go down once enough daughters of the new queen are becoming present in the colony. Given these good results and the ability to select for this trait (see later), we will use this trait in the Breeding, Selection and Distribution Project(BrSD).

Varroa Sensitive Hygiene: Behavioral resistance to Varroa mites by bees bred in Baton Rouge, USA. Jeffrey Harris of the USDA-Agriculture Research Service.

Other brood related traits

Non-reproduction by the mite in the brood: in some colonies a relative high number of mites will enter the brood cells, but can/will not start reproduction. The mechanisms behind this trait are not clear. But we will detect the effect of this trait in our projects as we will be both measuring the number of reproducing as well as non-reproducing mites (BrSD project).

Recapping: Opening & closing of infected brood: in this case the brood and Varroa is not removed but she apparently gets disturbed in the reproduction. Less fertile Varroa-daughters are being produced. This might be a variation on the VSH behavior and can be measured as recapped cells have a different appearance (VIMM, BrSD project).

Relative attractiveness of brood versus bees: it has been shown that in certain strains of bee, the Varroa is less attracted to the brood compared to other strains. The Varroa is then longer on the adult bee, not reproducing and has a bigger probability of being groomed or damaged. It takes a lot of measurements to detect this trait, so selection on this trait looks like to be difficult.

Bee population dynamics (prolonged brood, swarming, ..): a bee colony has many “decision-points”. It can decide to prepare for swarming (which will create a brood-less period in the swarm and in the remaining colony – a period that the Varroa cannot reproduce), it can decide to stop early with the brood (so having a long winter period without brood – less Varroa surviving the winter), and so on… The intense swarming tendency in tropical areas is one way of dealing with the Varroa infestations. In beekeeper controlled hives we can use the bee-population dynamics to create a Varroa-unfriendly environment in which for example treatments are more effective or which would suppress the Varroa growth in combination with already partly Varroa resistant stock (Bee Management & Integrated Treatment (BMaT) project).

Post-cap duration: shorter closed-cell-brood period: the Asian Honey Bee, which is smaller than the Western Honey Bee, has a shorter closed-cell-brood period. This causes a smaller number of Varroa-daughters being adult on the moment the young bee is leaving the cell. It has been speculated that one could select for a shorter post-cap period, but not much progress has been made (and is biologically most likely very difficult). However, it has been shown that the use of Small (natural) Cells will create smaller bees but also have a shorter post-cap period making it more difficult to produce mature Varroa mites in the workers brood. The effect of Small Cells in relation to Varroa Resistance will be investigated in the BMaT project.

Temperature changes in brood-nest. It has been shown that Varroa mites reproduce quicker if the temperature is relative low. So increasing the temperature (most likely easier in tropical climates) will decrease the growth of the Varroa. It is not very clear how important this trait is for the overall Varroa Resistance. However, also in this case it is argued by the supporters of the Small Cells that the Small Cell colonies also have their frames closer to each other and for that reason make a more compact and thus warmer brood nest with a favorable effect on the Varroa levels.


Adult Bee related traits

If the Varroa mite is not in the brood, reproducing, then she is living on the bees themselves – feeding and preparing to go in the brood again. This is also an opportunity for the bees to do something against the Varroa.

Grooming (inside and outside hive): the bee removes the mite from herself or from a sister bee. This is also a trait observed with the Asian Honey Bee and most likely an interesting one to select for if a good selection method can be identified. Removing the mite from the bee brings the Varroa at risk as she has difficulty holding on to the combs and can lose the contact with the bees and the colony. We will study grooming in relation to Small Cells in the Varroa Resistant Traits (VaRT) project. Grooming is also relevant in connection with the following trait, the killing or damaging of the Varroa Mite itself.

Killing or damaging of the Varroa mite by the bee: by for example biting off a leg or damaging the body of the Varroa mite. Different degrees of damage by the bees have been observed in different strains of bees. Measuring the amount of damage to the mite is not easy and requires special handling of the hives (to exclude non-bee-mite damage) and experienced microscopic investigation of the dead mites. More research is needed before this trait can be used in a breeding & selection program.

Virus resistance/tolerance of the bee: the Bee has a relative weak developed immune system as it relies on it armor to keep viruses and bacteria outside the body. In the case certain bee strains would still be better to withstand certain viruses or bacteria, one could argue that this would be a selection criteria. Measuring and determining of virus-resistance would be very difficult and would still not deal with the underlying problem – the Varroa mite – so we will not investigate this trait.

Checking colonies in a commercial apiary  (Ralph Büchler)

Checking colonies in a commercial apiary (Ralph Büchler)

Methods to study or select for Varroa Resistant traits

The described traits are the basis for selection in a breeding program. However, practical measuring methods are needed to actually determine whether such a trait – or at least the effect of the trait – is present in the colony.

Having good measurement methods is actually the most important factor in a breeding program. There is a lot of knowledge in breeding bees in large groups of beekeepers. However, reliable, precise and practical measuring methods are scarce and basically the bottleneck for many of the breeding programs. It is for that reason that improvement and development of methods has a high priority in the presented projects. In the following paragraphs the most used methods are described.

– Hygienic behavior (Varroa Sensitive Hygiene (VSH)):for this method, brood of a heavy-Varroa-infested colony is, after counting the number of Varroa ’s in 200 brood cells, introduced in a to-be-tested colony. After a week, again the number of Varroa ’s is counted in 200 other brood cells after which the degree of reduction can be determined. This is a well described method that can be used in an institute or experienced breeding group. The advantage of this method is that it gives very reliable, strong, directly related data towards the VSH trait. The disadvantage is that highly infested brood is needed (so heavily infested Varroa colonies), preferably as early as possible in the year and in high quantities. This is very labor some and not many beekeepers would like such a “Varroa-hotspot” close to their bees. This method will be further developed by the Arista Bee Research foundation to allow for large scale use (selection of large number of queens/colonies) and – ideally – without the need for heavily infested colonies – in the Varroa Infestation Measuring Methods (VIMM) project.

Counting Varroa in brood in VSH and Control colonies

Counting Varroa in brood in VSH and Control colonies (ABR)

Determine the % (un-) fertile female Varroa mites in the brood: in the case the tested colony has a reasonable level of Varroa mites (for example at the end of the season), than brood of the colony itself can be used to determine the relation between un-fertile and fertile Varroa mites. Fertile Varroa mites will have offspring (son and daughters) in the cell, whereas un-fertile Varroa mites will not have offspring. As hygienic bees only remove the brood of fertile Varroa mites, a hygienic colony will have a relative high percentage of un-fertile mites. This method can be used to test established Varroa Resistant stock. For high-volume selection it might be less suitable as one would like to make selection decisions earlier in the season (when less Varroa mites are present in the worker-brood) and in any case a high number of Varroa mites need to be present to have a reliable measurement.

– Pin-test: A small area of the brood is deliberately damaged with a small needle (or frozen with liquid nitrogen). The time and intensity in which the bees clean out the damaged cells is an indication of their level of brood-hygiene. Bees with VSH behavior also score high in the pin-test. The other way around, a good pin-test score is not directly predicting a good VSH score. This might be explained by the fact that the VSH behavior is depending on both detecting the Varroa mite in the cell as well as the follow up behavior of cleaning the cell out. So the pin-test can be a supporting measurement in a Varroa Resistant program.

Removal of frozen brood  (Renaud Lavend'Homme)

Removal of frozen brood (Renaud Lavend’Homme)

Mite Population Growth (MPG):  estimate the growth during the season. A reduced growth of the Varroa population is the main objective of Varroa Resistant bees. It is difficult to directly measure the Varroa infestation level as the Varroa mites are both on the bees as well as in the brood. But one can get a fair indication if multiple measurements are done on either sample of the bees collected from the hive (for example mixing & shaking 100-300 bees with powder sugar, alcohol or detergent and counting the Varroa mites which get disconnected from the bees) or counting the amount of dead Varroa mites on the bottom board (assuming a fixed relation to the amount of living Varroa mites in the Hive). Each of the described methods have their practical issues which will be further investigated and streamlined in the VIMM project.

Investigate damage of dead mites:  investigate damage inflicted by the bees on dead mites found on the bottom board of the hive. This method requires extended expertise of the researcher to enable reliable and consistent assessments but might open an extra line of future selection as the underlying trait – the active removal and killing of the Varroa mite by the Honey bee – could be an additive and complementary behavior in addition to the VSH-brood removal.

The “Bond – let live and let die” method: stop all treatments or reduce treatments, those colonies which survive or have the lowest infestation of Varroa are kept. This method can be used if already some pre-selection has been done and one is convinced that already some Varroa Resistance is present. It is the ultimate test as it shows what happens if no or few treatments are done – it basically is the ultimate test whether the target has achieved. The advantage of the test is that the colonies will have most likely a relative high level of Varroa mites (at least at the start). So the colonies are tested under heavy selection pressure and can show what they are worth. This can also be the disadvantage of the method as colonies with moderate levels of Varroa Resistance (so with interesting genetic material) and other good traits (gentleness, honey flow, …), might not make it and be lost for further selection and breeding.


Available leads for Varroa resistant bees

In different parts of the world, bees with certain levels of Varroa resistance have surfaced. In isolated (often tropical) areas with no beekeepers (and consequently no treatments), the heavy selection pressure has produced populations which have more or less adapted and have developed certain levels of natural Varroa resistance. These colonies from the wild can contain relevant resistant material and can give us directions for selection.

Examples are:

– Primorsky bees from Eastern Russia. The United States Department of Agriculture (USDA) has further selected and developed a line of these bees.

– Bees on tropical islands like Antigua and Barbados (no research has started yet, but it might be a nice student project to identify what happened on these islands as the natural population has recovered after being decimated by Varroa).

Bees on Barbados  (ABR)

Bees on Barbados (Ireen Roskam)

– Africanized bees (mix-breed, released by accident in Brazil and now present in South America and parts of the south of USA).

Also strong selection by human intervention has shown to be effective with the Varroa Sensitive Hygiene (VSH) line from the USDA as prime example. Whereas this line has good Varroa Resistant properties, it is considered to be less productive compared to the other commercially available lines. For this reason this line is used to create hybrid crosses (F1) with other lines which have these favorable traits. Also first attempts have started to get the VSH trait cross-bred into the more popular strains of bees (like Buckfast).

Buckfast VSH crosses  (ABR)

Buckfast VSH crosses (ABR)

In Europe several other initiatives have started to search for a more Varroa resistant bee; Carnica AGT program, Buckfast-Primorsky, Buckfast-Brandenburg, Elgon bees, Lunden Apiaries, Black Bee Texel, Black Bee France, Gotland, Tiengemeten, John Kefuss selection, Waterworks Dune project, Las Palmas project, Stichting de Duurzame Bij and more. Many of the projects are depending on highly motivated volunteers and would likely benefit from cooperation, scientific input and (financial) support.

The Arista Bee Research foundation is in the process of establishing links and cooperation with these initiatives to ensure optimal use of knowledge and resources and where possible integrate and exchange materials and methods.