While bees are incredibly resilient, they do face challenges. From robbers to Small Hive Beetle to American and European Foulbrood, Nosema, Chalkbrood and Tracheal Mites and more, it's amazing they survive so well.
But they do and that's a testimony to the way they establish and grow the colony, the grooming and nursing the workers do and their absolute determination to protect resources at all times.
And yet there's one threat that has had a dramatic effect on bees over a period of decades. It's a pervasive, ever-present threat that can often be prevented or treated, but has also shown a tremendous capacity to completely wipe out colonies. It has had an enormous economic impact and is a suspect in Colony Collapse Disorder.
It is the Varroa destructor (generally simply called the Varroa mite).
(Note: Don't assume "controlled" means it is down to you - bees can help themselves too, as we will see).
In this and the next chapter we will take a very close look at Varroa mites. First we describe the Varroa mite, how it lives and reproduces and why it presents such an incredible threat to honey bee colonies.
In the next chapter we look at how beekeepers can be vigilant about Varroa mites and what actions they can take to protect their bees. It also looks at some of the remarkable behaviors bees take on their own to combat Varroa.
What is the Varroa mite?
The Varroa mite is a small parasitic mite that can only reproduce in honey bee colonies. At around 1 to 1.5 mm, it is far larger than the Tracheal mite and can be seen with the naked eye.
The Varroa mite has a flat, crab-like body with eight legs, a shape that allows it to easily slip between segments on a bee's abdomen.
In considering the life cycle of the Varroa mite, let's start with the example of a mated female mite escaping from a previously capped cell (we will be back to this point later, to discover how that mite got there in the first place).
Her flat body allows her to hitch a ride with a passing bee. Since she is both small and flat, she can be very difficult to see, between the segments of the bee, while also almost impossible for the bee to dislodge.
The mite may potentially move from bee to bee, even attaching to a robber bee that can take her to another hive. This is one way that Varroa can spread between colonies.
Feeding off bees may sustain her for quite some time. If no brood is present, she may survive for six months or longer. If brood is present, she will typically live around four weeks.
If she gets lucky, our example mite will find an uncapped cell with a larva. She will leave her "transport bee" and nestle deep inside the cell, hiding in the food that is there for the larva. She enters the uncapped cell about a day before it is capped.
Eventually, unknowing workers will cap the cell, trapping the Varroa mite, alongside a tasty larva (stated from the perspective of the mite!).
Then she starts her work.
She will leave the mass of food and climb on the larva. She will then use her mouthparts to pierce the larva - from which point she has a feast.
The act of gorging on the captive larva reduces its immune system. From this point, the larva is highly vulnerable to many diseases and viruses. Without Varroa, the larva might normally be quite able to cope with these issues, with her strong immune system. But the Varroa bite greatly reduces her ability to do so.
Sadly, the Varroa mite is just getting started.
Three days after the female mite starts eating and building her strength, with the help of the larva, she lays her first egg. This will be a male, with 7 chromosomes. She will then go on to lay female eggs, each with 14 chromosomes, at about 24-hour intervals. The mite larvae will have six legs, eventually developing eight as an adult mite.
The female Varroa mite will lay more eggs in a drone cell than a worker cell. In a drone cell, she will average around 4 female eggs vs. 2 in a worker cell.
This is the reason that drone cells are considered accommodating to a Varroa outbreak.
Role of the male
Remember we said the first egg laid is a male? That guy mite has one role - to mate with his sisters in the cell! He doesn't even eat. He emerges from his egg, takes about 6 1/2 days to mature, mates and dies, all in the cell.
Nature....is just strange!
Role of the female
The remaining eggs laid by the mite will be female. They will feed on the larva in the same spot bitten by their mother. They take about 6 days to mature.
They will then mate with their (slightly) older brother, as above, wait for the cell to be uncapped, search for a host bee...and the cycle continues.
If you do the math from the life cycle explained above, you can see how serious this is for bees. Our story all came from a single, mated female mite. This may well be happening all over the hive.
A single female mite may go through two reproductive cycles, potentially producing 8 females (if in drone cells).
The growth in Varroa population can easily become exponential. In six months, the number of mites can rise by a factor of 10-15. But if left untreated and with the presence of brood for a year, an 800-fold increase may occur.
Impact on bees
Larvae infected by Varroa are vulnerable to many damaging viruses such as Deformed Wing Virus (DWV) and others. But being confined to a cell with Varroa is not necessarily a death sentence.
If there are three or fewer mites in the cell, there is a reasonable chance the larva will survive. It will have a radically reduced immune system and may have a much lower body weight than normal larvae. These factors will greatly reduce its lifespan - even without considering the diseases it may have picked up - but it will not die immediately.
If a larva does survive and eventually exit her cell, her troubles are just starting!
First, she probably has viruses she would otherwise have been able to repel. But assuming none of those kill her, one of her first jobs is to feed other larvae.
The glands she uses to feed larvae are less developed. But, still worse, in the act of feeding she passes on the viruses she has picked up. She can also do so when feeding the queen, who may pass the viruses on in eggs she lays.
Impact on population
Beyond this disastrous series of events, the viruses passed on to bees across the colony will shorten the average life span. Bees that will normally live to the point where they become foragers will die before that time.
Over time that means that the collective foraging of the colony as a whole is significantly reduced. This puts a tremendous strain on the colony and soon bees will die at a quicker rate than the colony can create new brood.
This is the critical point at which the overall population starts declining. If the infestation reaches that point and continues, the colony as a whole will die.
Varroa mites are one of the biggest threats to any honey bee colony. We explained why - how the mites live, eat and reproduce, as well as the dramatic series of events they can cause, as their population grows.
In the next chapter of this two-part series, we will look at detection of Varroa mites, prevention of their growth and treatment options.