The Varroa destructor mite may be tiny — only a millimetre or two long — but it poses a massive threat to honey bees, beekeepers and honey producers, and agricultural sectors that largely rely on bees to pollinate crops worldwide.
Australia was stripped of its title as the last inhabited continent free of varroa in June 2022, when mites were detected in a hive at the Port of Newcastle.
Since that initial detection, the mite has been on the move. It was found in hives in Victoria last August, and last month only a few kilometres from the Queensland border.
Varroa wreaks havoc by latching onto adult European honey bees (Apis mellifera) and feasting on an organ called the fat body, which is akin to our liver.
“Imagine having something the size of a side plate or dinner plate, attached to your torso, sucking out your liver,” Nadine Chapman, a bee scientist and National Varroa Management Education and Training Coordinator, says.
Varroa mites reproduce and feed upon developing baby bees, and can also spread potentially lethal honey bee pathogens such as deformed wing virus.
In a honey bee hive, most varroa mites are concentrated in the bee nurseries where they breed. (Getty Images: IgorChus)
Left untreated, a varroa mite infestation will eventually spell the end of an entire honey bee colony.
And while honey producers and beekeepers have some ways to try to keep mite levels down, the bees themselves also have a few tricks up their sleeve.
Varroa treatments
There are broadly two ways of ridding a honey bee hive of varroa mites: chemical and mechanical treatments.
Chemical techniques involve applying chemicals, such as synthetic miticides, to kill mites on adult bees.
Use chemicals too much, though, and varroa mites can quickly develop resistance to them.
“What we advocate is for an integrated pest management approach so that you’re keeping your chemical use as low as you can, if you’re using chemicals,” Dr Chapman says.
This means also using a mechanical treatment, which often involves interrupting the mite’s life cycle without using chemicals.
Mechanical treatments include using “drone combs” — commercially available wax combs with large holes that encourage the queen bee to lay eggs that will produce male bees or drones.
Varroa mites prefer to breed on baby male bees over baby females, so the drone comb — and any mites within it — can then be removed in one hit.
Another mechanical treatment is called a “brood break”, which involves temporarily removing the queen bee from her colony.
Without the egg-laying queen, no new baby bees (or baby mites) are born. This means any mites in the hive end up on adult bees, which can be treated with pesticide.
How can bees fight varroa mite?
While there is no such thing as a fully varroa-resistant honey bee, there are some populations around the world that have naturally developed defences against the parasite.
They’re mostly found on islands where varroa has killed off susceptible colonies, leaving the more resistant bees to repopulate the area.
These more resistant honey bees have a few traits that keep mite levels down:
- baby bees change the chemicals they emit at various stages of development, which affects varroa mite reproduction
- worker bees, tending to the bee nursery, are better at sniffing out and disposing of mite-infested babies
- adults are more adept at grooming and removing mites from their body.
If an adult bee can’t quite remove a varroa mite that’s latched onto them, “they can do a little dance to get one of their sisters to groom them”, Dr Chapman says.
“Then the varroa will hopefully end up at the bottom of the colony and be unable to get up, or it might suffer damage to its body or legs.”
How can breeders help bee defences?
Naturally resistant honey bees tend to be more aggressive and produce less honey, which is not ideal for honey production, especially in urban areas.
So bee breeders are trying to create colonies with some of these defences against varroa while retaining their positive qualities.
They select drones from colonies that seem to do better against varroa mite and take their sperm using a tiny syringe.
And a drone’s penis is surprisingly hefty: around a third of their entire body weight, Dr Chapman says.
“As my university professor used to say, they are penises with wings. They do no work in their life. [Insemination is] their one job.”
Semen from male bees from colonies that seem to resist varroa mite can be collected and used to inseminate queen bees. (Getty Images: christening)
Bee breeders then use the collected semen to artificially inseminate a “virgin” or unmated queen — a delicate operation conducted under a microscope.
After being anaesthetised with carbon dioxide and placed in a small harness, small hooks open the queen’s reproductive tract, into which the drone’s semen is transferred.
Queen bee artificial insemination is a delicate process conducted under a microscope. (Getty Images: Paul Starosta)
In the wild, a queen bee might mate with dozens of drones, and store all that semen to use over the rest of her life.
Artificially inseminated queens, Dr Chapman says, tend to receive semen from far fewer individuals — around eight.
And because she lays the eggs, it’s hoped the genetic instructions that give bees varroa-resistant traits will be passed on to the next generation.
Breeding varroa resistance into honey bees is a slow process, and varroa continues to spread, but Australia is lucky in a way, Dr Chapman says.
“Thinking about it relatively, we’ve got quite a number of different treatment options — both chemical and non-chemical — that we can use … No other country has had as many options when they first got varroa.”
Listen to the full episode of Lab Notes for more on how honey bees can fight the mite.
