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For many years bees have been threatened by the ever-present Varroa mite. With a small number of regional exceptions, this dangerous mite has caused major disruption to beekeeping around the world (Australia is one place that, to date, has largely avoided the Varroa mite).
Given the potential for the dramatic and tragic impact of Varroa with any single colony, both beekeepers and scientists have invested a great deal of time trying to better understand this mite.
This has taken many forms, from the use of treatments, equipment enhancements (such as screened bottom boards), ongoing scientific studies, the sharing of information by beekeepers and much more. Some are focused on proactive steps to keep Varroa within reasonable limits, while others attempt to address bad situations reactively. Even the temporary heating of beehives to very high internal temperatures has drawn attention as a potential way to fight back.
While these efforts continue, perhaps the most effective guard against Varroa is the behavior of our bees. In fact, bees do a remarkable job of keeping Varroa in check. Yet that may seem a strange statement, given the damage from Varroa over the years. How can we consider bees to have combated this threat successfully?
This might not always be apparent to beekeepers who have lost colonies to mites, but the effectiveness of bees in responding to Varroa should be kept in context.
Let's start with a fundamental truth - the majority of hives have at least some infestation of mites. For the new beekeeper that realization can be something of a shock. But the battle between bees and Varroa is largely a numbers game and, in many cases, bees do a wonderful job of identifying and reacting to the threat.
Providing the number of Varroa mites is kept below a certain critical point, the colony can cope very well.
As the number of Varroa increases, however, the colony becomes more stressed. At some point the aforementioned critical point is reached and, from that stage, it can be very difficult for either the bee or the beekeeper to stop the rapid decline of the colony.
In short, avoidance is far, far better than a reactive response.
So, how do bees themselves address the threat of Varroa? It's a fascinating process, all boiling down to an overall "hygiene" maintained by bees within the colony.
As all beekeepers will happily proclaim, bees exhibit extraordinary capabilities and behaviors, which they use collaboratively for the common good of the colony. These are multi-faceted and include reactions to environmental challenges, such as they clustering around the queen as temperatures drop, to managing the threat from diseases and pests. Ongoing assessment of pheromone levels is also a key aspect of the overall status of the colony, by the colony.
But it's bees' ability to recognize and combat the threat of Varroa that is one of the more remarkable aspects of life in the colony.
Let's add consider another layer of complexity to this already complex environment. Life forms adapt!
Regardless of your worldview, it is an accepted fact that life forms have a remarkable ability to react, over successive generations, to changes in their environment. When the lifespan of a living organism is measured in just a few days or weeks, then these generational changes can be observed very quickly.
This is precisely what we have seen with Varroa, particularly in relation to the use of treatments. Neither bees or Varroa mites have long lifespans. That means that "selectivity" across generations kicks in much quicker than life forms with a longer life span.
We see this in how bees have adapted to Varroa and we'll look at some very positive aspects of that below. But, sadly, we also see this in how Varroa have responded to treatments. Many treatments have been accepted and initially successful as a viable approach by beekeepers inclined to use treatments.
However, over time, their effectiveness can diminish, particularly with overuse by beekeepers. This is because the rapid-fire creation of new generations of Varroa can respond to these treatments. Effectively, tolerance to a treatment can become "bred in" to Varroa, over time.
There are many factors that effect the reproduction of Varroa within a colony. The potential for explosive growth occurs because a single female mite entering a cell can create a single male Varroa (from an unfertilized egg) and many females. One to many!
Multiply that by a large number of infested cells and you have a recipe for disaster within the colony.
As we have indicated, there's effectively a "balance of power" within the hive at any point in time. If the number of Varroa can be kept within acceptable boundaries then our bees have no problems keeping the colony healthy.
The number of Varroa across the hive, of course, is related to their ability to reproduce and reach maturity. So anything that helps reduce that number is the proverbial good thing (unless you happen to be a Varroa mite!).
Suppressed Mite Reproduction (SMR) is a recognized factor whereby the reproductive rate of Varroa is seen to be significantly reduced. After much research, a correlation was discovered between the behaviors of bees and SMR. Over time, these positive behaviors came to be referred to, in bees, as the Varroa Sensitive Hygiene (VSH) trait.
VSH bees have a higher propensity to detect and recognize cells within which Varroa exist. In such cells, worker bees open the cell and remove or eat the infested larva, which in turn causes the death of immature Varroa within the cell.
It is not yet fully understood how bees recognize the presence of Varroa within the cell. But, across the colony, VSH bees have a direct and very positive impact on lowering the number of mites within cells.
The identification of these enhanced behaviors in bees was obviously a welcome and encouraging discovery. It soon crossed the collective mind of beekeepers and scientists that perhaps there were ways to promote and amplify these behaviors.
The real breakthrough came when VSH traits were found to be heritable. The bright minds from the United States Department of Agriculture at the Honey Bee Breeding, Genetics and Physiology Laboratory in Baton Rouge soon found they could breed selectively for this trait.
The result are bees that, through their own, inbuilt behaviors, can go a long way themselves in combating the threat of Varroa. VSH is now accepted and well recognized, particularly with commercial beekeepers.
Queens from VSH stock can mate with non-VSH drones and, although genetically diluted as a trait, positive results are still observed.
Varroa are fascinating, whether we like it or not! But, happily, our bees are even more amazing and have exhibited behaviors that directly target Varroa.
With selective breeding of bees with VSH behaviors, there is hope that, in the macro level, the Varroa threat can be managed.