Varroa destructor

It is the fear scuttling about in every beekeeper’s heart. It is the ruination of livelihoods, and the grief of families. It is disease. It is death. It has, in the last fifty years, come at a hair’s-breadth away from world domination.

It is Varroa destructor, the mite of bee evil.

There is no greater threat to honey bee colonies than varroa. Even nosema comes second-place (just). Reddish-brown and crab-like, this ectoparasitic hive invader was not too long ago a relatively innocuous critter, supping on the fluids of the Asian honeybee (Apis cerana) in its native territory of the east. In the mid-20th century however, returning west on the backs of honey bees originally imported to Hong Kong, the mite stole across the oceans through trade shipment routes. Across Europe, Africa, India and – first reported in Maryland in 1979, and, after an eight-year lull, Wisconsin – the United States, the mite not only spread at an alarming rate. It was found to have an incredibly pernicious affinity with the honey bee (Apis mellifera) – mankind’s sweetest friend.

Sowing devastation when it first hit, scientists have also begun pointing the finger at varroa as leading culprit behind the mysterious colony collapse disorder (CCD), the cause of mass panic about a decade ago (though which, just as mysteriously, now seems to have vanished as a major threat). Feral bees – often overlooked for their work in adding to the bloom and bottom line of agriculture – have suffered even worse than their managed counterparts in the battle against the mite.

Currently, V. destructor is present everywhere honey bees are found except Australia and some regions about central and the horn of Africa. Though hypervigilant quarantine measures are upheld by their respective governments, it appears the die is cast: whether by accident or ill intent, these mite-free strongholds will eventually fall too.

Why is it so deadly?

It isn’t the mite itself which is so bad actually. This is despite how gigantic it is relative to the size of its host – sitting somewhere between 1-1.77mm long and 1.5-1.99mm wide for females, and about half that for males. Scaled to a human, it’d be like having a monkey on your back, endlessly jabbing at your ribs. True, you can check off some direct fatalities among bee larvae as the result of mite overfeeding. But it’s the plethora of viruses the mite is vector to that’s the real killer. You can think of it as something like the new rat to bees’ own Black Death.

Of the score or so honey bee viruses so far identified, most have been found to be associated with the varroa mite – from Kashmir bee virus to deformed wing virus. Puncturing the bees’ abdomen to gorge on hemolymph (the insect equivalent of blood), the mite delivers this toxic cocktail directly into the host’s circulatory system. They’re “basically dirty hypodermic needles“ says University of Maryland researcher Kirsten Traynor.

The result? A huge magnification of disease about the colony, a heightened vulnerability from open and unhealing wounds, and an overall shortening of colony lifespan.

The hive meets its end when there are not enough bees remaining to make it through the winter. Part of this has to do with timing – with varroa numbers peaking just as honey bees begin their natural decline in late summer, the hive gets the suckerpunch of pestilence when they’re already down. Full collapse can take anywhere between 7 months to several years.

It’s a tragic enough story, but it doesn’t end there. When a colony is on its last legs, the remnant bees – their corrupted bodies playing havoc with their built-in navigational controls – will often drift into other hives, mistaking them as their own. Under this cruel irony, the mite outlives the death of its host and erstwhile home, spreading new waves of sickness elsewhere.

Anatomy of Varroa destructor

Unlike its Asian cousin, the honey bee hadn’t been locked in an evolutionary arms race with the mite for thousands of years. Subsequently, when the mite appeared on the scene, the species was caught unawares – lacking in all the necessary defence mechanisms (structural, behavioural, or immunological) that could’ve helped it.

On the flipside, as Apis cerana and Apis mellifera are not so different anatomically, the mite was to its new host a perfect match. With its curved, ovoid body, it can guilefully slip between the abdominal clefts of the adult bee, while its coarse patchwork underside of hairy bristles (otherwise known as ventral setae) makes it ideal for clinging. It also wears a thick protein coat of sclerotin (military-grade arthropod armour), which frustrates bees’ attempts to remove it, and shields it from incidental grooming-associated trauma. Its most stealth-like characteristic however is its perfume cloak of invisibility – having a cuticle shot through with a “chemical pattern similar to that of the bees’, possibly allowing it to escape notice”.

Lifecycle of the mite

There are two stages to the life-cycle of the varroa mite: the phoretic phase and the reproductive phase.

1. The phoretic phase

The phoretic phase concerns only the female varroa mite. Like the female honey bee is the hive’s most proactive hive member, so the female varroa is its most destructive. In both species, the males keep largely to themselves until the queen sounds the booty call, their singular function being the transferal of sperm. For varroa, the male mite never leaves the brood chamber it was born in, raised in, and reproduces in – making the cell a kind of cradle, nursery and sex dungeon all at once.

The female mite travels further. Feeding on adult drones and worker bees alike, it hops between hosts in the cramped interior of the hive. When it comes to journeying beyond hive limits, the mite follows several routes. These include:

  • Bee drift – where foraging bees return to the wrong hive
  • Robbing– with bees from stronger colonies often pillaging weaker ones
  • Swarming – whether induced or spontaneous
  • Colony movement – typical for commercial pollination services

2. The reproductive phase

After a queen bee has ‘oviposited’ an egg in a brood cell, a three-day window exists before the cell is capped with wax. It is into this window that a female varroa must jump. At the opportune moment, she will unlatch itself from her host’s back and crawl down the cell wall into the chamber, where she assumes the new title of ‘foundress’.

Once inside, the foundress submerges herself in the thick pool of brood breakfast; a preparatory feast laid at the base of the chamber by worker bees. To escape detection, she will erect a little hollow tube above the surface to breathe through (just like James Bond in the Dr. No river scene!). As soon as the cell is capped, the larvae begins working its way through its nutrient-rich platter, and in doing so, frees the mite. The mite then mounts the larvae, and begins to suck.

After a few days, the foundress lays her first egg. Unfertilized, through a process known as parthenogenesis, it hatches male. Foundress and son will then mate – the latter modifying his mouthparts to create a MacGyver-style penis, down which he chutes sperm packages to his mother’s sex opening near the base of her third pair of legs. Every 25-30 hours following, the foundress pops out another egg, which – having been fertilized – will hatch into a lady mite. On average, 1.2 viable mature female offspring emerge from every worker cell invaded. However, this increases to 2.2 in drone cells, correlated to the longer time in which drone brood develops. Unsurprisingly, foundresses will more often be found in drone cells.

Though some larvae do not survive this initial assault, the majority will emerge from their cells after pupation as adults. Categorically however, they will emerge weak, unable to escape their parasite-defined life.

Conclusion: Small, but mite-y

Varroa is one of the deadliest, most widespread agents of disease threatening insect-kind today. A small caveat though now, so we don’t fall prey to the trappings of alarmism: if you spot a small red splodge on the back of one of your bees, no need to sound the klaxon. The mite is so rampant, all hives are bound to contain at least a few. It’s when their number blows out of proportion that you’ve got a situation on your hands.

On that note, stay tuned for next week’s post, where we’ll be talking best prevention, treatment and control options for beekeepers in the fight against varroa.

By Kate Prendergast