A brief intro to Varroa resistance

As I have mentioned in previous posts, the Varroa mite is a huge problem for honey bee colonies. However, bees can overtime become resistant to the mites and survive without chemical treatments. This phenomenon has been observed across a number of regions including South America, Europe and Africa.

Beekeeping in a forest in Brazil

In south America the majority of honey bees are Africanised. Africanised bees are formed from breeding European bees such as Apis mellifera ligustica (Italian honey bee) with African bees such as Apis mellifera scutellata. These bees have been rather unfairly dubbed “killer bees” as people found them to be more feisty than regular European bees. However, an upside of Africanised bees is that they appear to become resistant to mites very quickly. For example in when the Varroa mite invaded Brazil in the 1970s the managed European honey bee colonies pretty much all died out unlike the wild africanised bees which appeared to experience very few ill effects. This led to them becoming favoured by beekeepers in the region. Similarly, in south Africa the Savannah bee (Apis mellifera scutellata) and the cape bee (Apis mellifera capensis) became resistant after 6-7 and 3-5 years respectively.

A Savannah honeybee worker (Apis mellifera scutellata)

In Europe the acquisition of resistance has been much slower and is less widespread. A large factor in this, is the use of chemical control methods in Europe. Due to the cost and inaccessibility, these are not common place in areas such as South Africa and Brazil. Chemical treatment prevents natural adaptation because it removes the selective pressure (the mite). In the UK there are increasing reports of resistant colonies having developed where people have ceased chemical treatment such as in North Wales (https://vimeo.com/157019200). This is impressive but suddenly stopping treatment is not advisable because it is likely the vast majority of colonies will die. Particularly if they are not showing any resistance traits. What is better is to screen your colonies for the resistance traits and then slowly phase out the use of chemicals in the colonies showing resistance. The phasing out of chemicals needs to be accompanied by an increase in the monitoring of Varroa levels so that any surges can be treated promptly. Even resistant colonies can die if a sudden surge of mites enters the colony, for example from a nearby collapsing colony.

My supervisor, professor Stephen Martin, and I have published a booklet with the BBKA which explains about resistance and how to encourage it in your colonies.

You can purchase a copy for just £4 from the BBKA website:

https://www.bbka.org.uk/shop/bbka-special-edition-natural-varroa-resistant-honey-bees-new

How does resistance work?

To understand resistance, you first need to understand that the relationship between Varroa, DWV and honey bees.

I have covered this in other posts but to put it simply, DWV is a virus of honey bees that in cases of high infection causes deformed wings and a reduced life span. Varroa's relationship to DWV is that it acts as a vector for the virus. This means it spreads the virus from bee to bee. Without Varroa, DWV is not really much of a problem. In fact, bees have had DWV for a very long time before Varroa, we just never really noticed because it rarely caused problems. This is because DWV could only spread between bees via contaminated food or on the surface of eggs. Both of these transmission methods are not very effective at infecting bees and so the negative symptoms were rare, and colonies survived. Varroa made it a problem because it could transport the virus directly into the bees hemolymph. This method is way more effective at setting up symptomatic infections in bees. Thus, together Varroa and DWV lead to the downfall of many colonies across the world.

This little illustration I made just to show how without Varroa (top line) the virus struggles to cause a symptomatic infection. This is because when the virus is taken orally it must pass through the gut which has many immune defenses. These defenses make it very difficult for the virus to infect the pupae. Conversely the mite injects the virus straight into the hemolymph (similar to blood) which bypasses many of the immune defenses.

Similarly, if you remove DWV from the picture Varroa is not as much of a big deal, in fact off the coast of brazil there is an island called Fernando de Noronha. On this island there is a population of European honey bees that has been living there since 1984. These bees have Varroa mites but no or very little DWV and as such can survive with higher mite infestations than colonies with DWV. The lack/ low levels of virus on this island appears to be because the strains present are not ones that can be transmitted by mites. If the transmittable strains were to appear on the island, then the current scenario would likely change.

Beekeeping on the island of Fernando de Noronha, Brazil

For all the unfortunate bees that have both transmittable DWV and Varroa the solution is simple…reduce the Varroa mite population. Reducing the number of Varroa mites means there are less DWV spreaders and consequently less DWV in the hive. Which means healthier workers and a healthier colony.

But how do they manage to reduce the mite infestation themselves? Well, the answer is surprisingly simple. They remove brood that are infested with mites. This leads to less mites because when the cell is emptied the mite offspring die. Additionally, the foundress mite that originally invaded the cell will have a lower success in reproduction in the next cell she infests. Therefore, if a high enough percentage of infested cells are removed the colony can keep mite reproduction and thus mite levels low. In resistant colonies around half of the infested cells are removed on average.

Stages of brood removal, wiggly red lines indicate the scent that workers detect. (It's actually believed to be a mixture of different compounds).

It is important to note that brood removal is not a process that is unique to Varroa infestation so all bee colonies can do it. What separates a resistant colony from a susceptible colony is the ability to detect mite infested cells. There are many ideas as to why resistant bees can detect more infested cells. It could be because they have a better sense of smell or that they require a lower stimulus to trigger a response. The latter seems plausible as a recent study found that both hygienic and non-hygienic bees could detect the scent of Varroa infestation but that only the hygienic bees were triggered to do something about it. There is also a lot of work ongoing to discover how much of this can be attributed to genes and how much to the environment. Is it possible that bees could become sensitized to or learn the scents involved?At present we are not certain, but we do know one thing for sure and that is bees are capable of becoming resistant if given time and encouragement which means that honey bees likely have a very positive future ahead of them.

For the full details (with some scientific jargon) you can check out my published paper at: https://doi.org/10.1098/rspb.2021.1375