Australia was, until mid-2022 when Varroa Destructor was discovered in NSW, the only honey producing country in the world that did not have the Varroa Mite. It is the most serious pest of honeybees across the world. It is capable of killing hives of honey bees if they are not treated or managed properly.
What is a Varroa Mite/Varroa Destructor?
Varroa mite is an external parasitic mite which will kill honey bee colonies. As you can see by the images the mite attaches itself to the honey bee as well as the brood. They especially like Drone Brood as that suits their life cycle. Images are enlarged for you to see it, but normally the mite would be the size of a pin head on a bee.
The European Honey Bee (Apis mellifera) is managed by beekeepers and hobbyists alike, providing not only honey but maybe more importantly pollination services across the world. While these amazing bees go about their work collecting pollen and nectar they also pollinate our fruit, vegetables and nuts. The human race cannot survive without honey bees, so it is hugely important we look after our bees and therefore make sure the Varroa Mite is either eradicated, which may not be possible, or managed. So it is highly critical that our beekeepers and hobbyists know and understand the Varroa Mites Life Cycle and understand how to manage it.
Vigilance by Government and industry had, to this point, kept Australia free of this and other parasitic mites. However it is currently in NSW and hopefully it will be eradicated but this has not happened as this article is written.
The female Varroa mite has a hard oval shaped reddish brown body (1.1mm x 1.6mm about) and can be quite nimble when not in a cell. The female will attach, travel and feed on an adult bee, and in this way can move, by bee drift or robbing, from colony to colony. After around 5 to 10 days feeding on the bees haemolymph (blood) and their protein rich internal tissues, the adult female mite will enter an occupied brood cell, about 1 day prior to capping in worker cells and 2 days prior in a drone cell.
Varroa have a strong preference to lay in drone brood. The prime reason for this is that drone brood spend considerably longer developing in the capped cell, thus providing the mite a longer period to develop.
In brood-less conditions the mite will simply spend a longer time on a host bee. This disrupts its cycle but the mite is still there. The mite can live for 100 days without brood.
The female mite, will enter the occupied brood cell, and submerge in the larval brood food at the bottom of the cell. When the cell is capped she will leave the brood food, and begin, after puncturing the abdomen of the developing bee, feeding on the haemolymph (blood) and tissues in the abdomen or rear of the developing prepupa (developing bee).
In doing this the mite leaves a faecal feeding sign on the developing bee. Within 3 days of the cell’s capping, the female will lay 4 to 6 eggs, the first being a male, and the rest female. After hatching, and going through their development stages, and while feeding on the prepupa, the young Varroa mate, in the cell, and are then ready to emerge with the developed bee. The male, and any immature or unmated female mites, remain and die in the cell. The fertile females, and sometimes the original female, are then able to move on and repeat the process in further cells.
Effects of Varroa on brood
- reduced weight of bees on emergence
- failure to emerge or have deformities of wings and abdomens
- reduced drone survival
- Varroa is a carrier for several bee viruses and infections, and pass these to the bees when they feed on the bees and the brood.
Effects of Varroa on workers
- decline in lifespan and vigour
- increased deformities and vulnerability to virus and disease
- decline in percentage of bees able to return to the colony
- reduced hypo pharyngeal gland ability (royal jelly production)
Effects of Varroa on drones
reduced emergence percentages
decline in lifespan
Effects on colonies
- reduced population of adult bees
- vulnerability due to reduced immune response
- rapid decline in strength, especially in autumn
How Varroa Spreads
- Bees robbing an infected hive and taking the mites and their infected viral loads back to their own hives.
- Foraging bees with Varroa infect other foraging bees.
- The Varroa mite can come off the foraging bee onto a flower, wait on that flower for another bee to visit that flower, and hitch a ride on that bee.
- Interchange of hive components from a hive with mites to a mite free hive
- The transport of hives
- They cannot spread through honey
A number of measures are practised across the globe to control Varroa. Various countries have registered a range of chemicals, able to be used legally and under strict conditions, to control Varroa. As these vary, country to country, the government regulatory body should be contacted regarding any chemical use. Chemical resistance developing in the mites, is a major concern with this approach, and needs to be considered.
Australian Beekeepers are encouraged to carry out four methods of detection, they are:
- Sugar Shake does not involve killing bees: Sugar-shaking.pdf (beeaware.org.au)
- Alcohol wash which does involve killing a very small number of bees: Alcohol-washing.pdf (beeaware.org.au)
- Drone uncapping involves killing a small amount of drone brood:
- Colony Examination does not involve killing bees: PDF. Colony-examination.pdf (beeaware.org.au)
- Sticky Mat extermination is only used for official surveillance programs, such as the National Bee Pest Surveillance Program: Sticky-mat-examination.pdf (beeaware.org.au)
Non chemical controls such as brood removal, trapping, hive splitting and mesh bottom boards have also been utilised successfully.
There is also an increasing effort to breed honey bees with a tolerance, or an ability to co-exist with the mite. This may be done by using traits employed by Apis Cerana, and the focus on breeding from more resistant / tolerant mellifera stock. However this will take some time.
Varroa destructor (and Varroa jacobsoni) evolved originally, as a parasitic mite of Apis cerana (Asian honey bee), but during the 1960’s it was discovered to be able to swap hosts and parasitise Apis mellifera (European honey bee). Having little resistance, feral mellifera colonies suffered catastrophic losses, and managed colonies needed extra ongoing management techniques to be able to survive.