1. Brood-associated strategy – VSH

The removal of infested brood, known as VSH behaviour (for Varroa sensitive hygiene), is a very effective method to reduce Varroa, which is also used by the Asian honey bee.

In the illustration below, the “normal” reproduction of Varroa is shown on the left-hand side. The queen bee lays an egg. The egg develops into a larva that is fed by a worker bee. Once the larva is almost fully grown, a Varroa enters the cell. Shortly afterwards, the cell is capped by a worker bee. Varroa can now start reproducing by laying an egg that produces a male first, and then laying subsequent eggs that give rise to females. The founding Varroa female then makes a hole in the cuticle of the bee larva and collects haemolymph (=blood) from it. The same hole is also used to feed her offspring. The newly born female Varroa then mate with the male Varroa and leave the cell once the bee pupa emerges.

Bee and Varroa illustration

 

On the right-hand side of the illustration and in the video below, what happens is shown in the case where the bees have inherited VSH behaviour. Another Varroa female enters another cell to start a new reproductive cycle. However, some of the bees can detect that a Varroa is present in the capped cell (possibly through the odour of the infested larva, or the odour of Varroa itself or its offspring – this is not clear). These worker bees open the cell containing the immature pupa. Once the cell is opened, the infested pupa is removed by a worker. Varroa therefore no longer has the opportunity to reproduce. Some of the expelled Varroa may try to enter another cell, but ultimately their lifespan is also limited.

It has been shown that this VSH behaviour is very effective. If infested brood from a non-VSH colony is transferred into a VSH colony, most Varroa (> 90%) are detected and removed. If the queen of a non-VSH colony heading a colony with a high mite load is replaced by a VSH queen, the mite number starts to decrease as soon as enough daughters of the new queen are present in the colony. Given these good results and the possibility of selecting for this behaviour (see below), we will use this behaviour in the breeding, selection and distribution of our bees (BrSD project).

2. Other brood-related strategies

• Non-reproduction of Varroa in brood: in some colonies, a relatively high number of mites enter brood cells but are unable to start reproduction. The mechanisms behind this trait are still unclear, but we can detect the outcome of this trait in our experiments because we will measure both the number of reproductive mites and the number of non-reproductive mites (BrSD project).
• Recapping: opening and then re-capping infested brood. In this case, the brood and Varroa are not removed, but apparently Varroa is disturbed in its reproduction, and Varroa daughters (offspring) have also become less fertile. This trait might in fact be a variation of VSH behaviour, but the phenomenon can be measured because the cells have a different appearance (VIMM, BrSD projects).
• Relative attraction to brood versus adult bees: it has been shown that in some bee strains, Varroa is less attracted to brood compared with other strains. Varroa then remains longer on adult bees and does not reproduce, which leads to a higher probability of grooming or damage. Detecting this trait requires a lot of work, and selection on this trait therefore seems difficult.
• Honey bee population dynamics (extended brood, swarming…): a bee colony makes many key decisions. It may decide to prepare swarming (which will create a broodless period for the new colony and for the remaining colony – a period during which Varroa cannot reproduce), it may decide to stop laying very early in the season (to have a long broodless winter period – fewer Varroa survive the winter), and so on. The extreme tendency to swarm in tropical areas is a way of countering Varroa infestations. For colonies managed by beekeepers, we can use population dynamics to create an environment that is hostile to Varroa, during which treatments are more effective or simply prevent Varroa growth in combination with the use of a bee that is already partially resistant (Bee management and integrated treatments project (BMaT)).
• Duration of the capped-brood stage: the Asian honey bee, which is smaller than the European honey bee, has a shorter capped-brood stage. This results in fewer mature Varroa at the time the bee emerges. It has been speculated that bees could be selected for a shorter duration of this capped-brood stage. However, no progress has been made in this regard (it is probably biologically difficult to achieve). However, it has been shown that using small cells (natural size) results in small bees that also have a shorter capped-brood stage, making it more difficult to produce mature Varroa in worker brood. The effect of small cells in relation to the use of Varroa-resistant bees will be investigated in the BMaT project.
• Temperature changes in the brood nest. It has been shown that Varroa reproduce faster when the temperature is low. Therefore, increasing the temperature (for example in tropical climates) will decrease the growth in Varroa numbers. The importance of this trait for overall Varroa resistance is not very clear. However, proponents of small cells argue that colonies with small cells also have frames that are closer together and therefore create a more compact and warmer brood nest, with a negative effect on Varroa populations.

 

3. Strategies associated with the period during which Varroa remains on adult bees.

When Varroa is not in brood reproducing, it lives on bees where it feeds and prepares to re-enter brood. This period is also an opportunity for bees to combat Varroa :

• Grooming (inside and outside the hive): the bee removes the mite from itself or from a nestmate. This behaviour is also observed in the Asian honey bee and is very likely of interest for selection if a selection method could be developed. Varroa is at risk when it is no longer on a bee, because it has more difficulty holding onto the frames and then risks falling. Grooming will be studied in relation to the use of small cells in the project entitled: additional traits of Varroa resistance (VaRT). Grooming is also relevant in relation to the following behaviour, the ability to damage or kill Varroa.
• The ability of bees to damage or kill Varroa: for example by tearing off a leg or damaging the mite’s shell. Different degrees of damage caused by bees have been observed in different bee strains. Measuring the amount of damage to the mite is not easy and requires special handling of colonies and careful examination of dead mites under a stereomicroscope. More research is needed before this trait can be used in a breeding and selection programme. 
• Resistance/tolerance of bees to viruses: the bee has a relatively underdeveloped immune system because it relies on its cuticle to keep viruses and bacteria outside their body. Some bee strains should resist certain viruses or bacteria better, but it can be debated whether this is a good selection criterion. Measuring and determining virus resistance are difficult and would not address the problem at its source – Varroa (because few Varroa means few viruses) – so we will not study this trait.

Checking colonies in a commercial apiary (Ralph Büchler)

4. Methods to study or select traits of Varroa resistance

The traits described form the basis for selection in a breeding programme. However, practical measurement methods are required to determine effectively whether such a trait – or at least its effect – is present in a colony or not.

Having good measurement methods is in fact the most important factor in a breeding programme. Beekeepers have a great deal of breeding knowledge. However, reliable, precise and practical measurement methods are rare and constitute a bottleneck for most breeding programmes. For this reason, improving and developing methods is a priority in the projects presented. The following paragraphs describe the most commonly used methods.

VSH behaviour: for this method, brood from a colony highly infested with Varroa is introduced into a colony to be tested, after first counting the number of Varroa in 200 brood cells. After one week, the number of Varroa is counted again in 200 brood cells. The level of reduction in infestation can be determined. This is a well-described method that can be used by an institute or a group of experienced breeders. The advantage of this method is that it is very reliable and provides direct information on VSH behaviour. The disadvantage is that highly infested brood is required (and therefore heavily infested colonies), preferably as early as possible in the year and in large quantities. This method is time-consuming, and many beekeepers do not like having such a “hotspot” of Varroa close to their bees. This method will be redesigned by the Arista Bee Research foundation to enable its use at large scale (selection of a large number of queens/colonies) and – ideally – without relying on heavily infested colonies (Varroa infestation measurement methods project (VIMM)).

Counting Varroa in brood of VSH colonies and control colonies (ABR)

 

• Determining the percentage fertility/infertility of female Varroa in brood: in this case, the colony being tested has a reasonable Varroa level (for example at the end of the season), and its brood can be used to determine the proportion of non-fertile and fertile Varroa. Fertile Varroa have offspring (sons and daughters) in the cell, whereas non-fertile Varroa have no offspring. As hygienic bees remove only brood containing fertile Varroa, a hygienic colony will have a relatively high proportion of non-fertile mites. This method can be used to develop resistant strains. For mass selection, this technique may be less suitable because one would like to take selection decisions quite early in the season (when fewer Varroa are present in worker brood), whereas a large number of Varroa is needed for a reliable measurement.
• The pin test: a small area of brood is deliberately damaged with a fine needle (or frozen with liquid nitrogen). The time and intensity with which the bees remove the damaged cells is an indication of their degree of hygienic behaviour. Bees expressing VSH behaviour also score well in the pin test. Conversely, a good score in the pin test is not a direct predictor of a good score in the VSH test. This could be explained by the fact that VSH behaviour depends on detecting Varroa in the cell as well as on the behaviour of removing brood. The pin test can be an aid in a Varroa resistance programme.

Removal of frozen brood (Renaud Lavend’Homme)

• Varroa population growth (MPG): growth is estimated over the season. Reducing Varroa population growth is the main objective of obtaining Varroa-resistant bees. It is difficult to measure infestation level directly because mites are present both on bees and in brood. However, a good indication can be obtained if several measurements are carried out on a sample of bees collected in the colony (for example by mixing and shaking 100–300 bees with powdered sugar, alcohol or detergents and then counting the Varroa that have detached) or by counting the number of dead Varroa beneath a screened bottom board (assuming a fixed relationship with the number of living Varroa in the colony). Each of the methods described raises practical issues that will be studied further and streamlined in the VIMM project.
• Studying damage to dead Varroa: damage inflicted by bees is examined on dead mites found beneath the screened bottom board of the hive. This method requires the expertise of a researcher in order to allow a reliable and consistent assessment, but it could open an additional selection pathway in the future – through active removal and physical elimination (death) of Varroa – and this could be an additional behaviour to VSH.
• The “James Bond” test – live and let die (Live and let die): stopping all treatments or reducing treatments. Colonies that survive or have the lowest Varroa infestation are kept. This method can be used if a pre-selection has already been carried out and one is convinced that some Varroa resistance is already present. It is the ultimate test, because it shows what happens when no or few treatments are carried out – it is fundamentally the ultimate test if the goal has been achieved. The advantage of the test is that colonies will probably have a high Varroa level (at least initially). Colonies are therefore tested under strong selection pressure and can demonstrate their value. This can also be the disadvantage of the method, because colonies with moderate levels of Varroa resistance (thus interesting genetic material) and other good traits (gentleness, honey production, …) may be lost for breeding and future selection.

 

5. Available sources of Varroa-resistant bees

In different parts of the world, bees with certain levels of resistance to Varroa have emerged. In isolated regions (often tropical), without beekeepers (and therefore without treatments), strong selection pressure has produced populations that have more or less adapted and developed certain levels of natural resistance to Varroa. These wild colonies may contain relevant resistant material and can provide directions for selection.

Examples include:

• Primorsky bees from eastern Russia. The United States Department of Agriculture (USDA) has selected and developed a line derived from these bees.
• Bees from the tropical islands of Antigua and Barbados (no research has started yet, but this could be an interesting project for a student researcher to identify what happened on these islands after bee populations were decimated by Varroa).

Bees in Barbados (Ireen Roskam)

• Africanised bees (race hybridisation, accidentally released in Brazil and now present in the United States).

Human selection has also demonstrated its effectiveness, with the USDA’s VSH lines as a good example. While these lines show good Varroa resistance properties, they are considered less productive compared with other commercially available lines. For this reason, these lines are used to create crosses with other lines that have traits favourable for beekeeping. In Europe as well, the first crossing attempts have started to integrate VSH into popular bee strains (such as Buckfast from Luxembourg or Belgium).

Buckfast x VSH crosses (ABR)

 

In Europe, several other initiatives have also started to select bees that are more resistant to Varroa : the Carnica AGT programme, breeding through Buckfast–Primorky combinations, Buckfast-Brandenburg, Elgon bees, the bees of Juhani Lundén, the black bee of Texel, or in France, Gotland, Tiengemeten, the selection of John Kefuss, the Waterworks Dune project, the Las Palmas project, the Duurzame Bij organisation, and more. These many projects rely on highly motivated volunteers and would be likely to benefit from scientific and financial support.

The Arista Bee Research foundation is currently establishing links with these projects and is trying to organise cooperation between these initiatives in order to ensure optimal use of resources and knowledge and, if possible, to integrate and exchange material and methods.

6. Literature review

Varroa destructor is currently considered the greatest threat to the survival of the honey bee (Rosenkranz et al. 2010). Indeed, in addition to causing direct damage to bees through repeated consumption of their fat body and through suppression of their immune system (Ramsey et al. 2019; Yang et Cox-Foster 2007), Varroa also affect bees indirectly by transmitting several pathogens, including notably deformed wing virus (Boecking et Genersch 2008). In response to the pressures caused by Varroa, several acaricidal treatments (i.e. synthetic or organic products used to kill mites) have been developed. However, the use of such chemical compounds is not without risk, as they persist in hive products (Wallner 1999) and also pose a danger to bee health (Gregorc et al. 2018). Moreover, their long-term effectiveness is also being questioned in the face of emerging resistant mite populations (Ellis et al. 2001). This is why it is essential to turn to more sustainable alternative solutions. Among these, we find bee lines expressing various levels of resistance to the parasite. On the one hand, there are bees showing the trait Suppression of Mites Reproduction (SMR), and on the other hand, bees expressing Varroa Sensitive Hygiene (VSH) behaviour. Although expression of both traits can combat Varroa effectively, there are many characteristics that differentiate them.

► Literature review

 

► open article on the internet