What are Varroa Mites and why you should be vigilant

  • 5 min read

What are Varroa Mites and why you should be vigilant

Varroa destructor are a small mite that infects honeybee colonies (apis mellifera). As their name suggests, these mites are highly dangerous and destructive to your bees. They will both deplete their energy and foraging power, and ultimately lead to the colony’s death. Australia is one of the last regions in the world to experience a varroa invasion. Strong surveillance has kept varroa out for many decades. Continuing this surveillance, yourself, in your own hives will help to find and eradicate this pest before it becomes endemic.

Varroa basics

Varroa are small brown mites (around 1.5 mm in size) that infect honey bees. They feed on the fat bodies of the bee and have a severe impact on their health. While they are small, they are comparatively large to the bee. The analogy goes that for a human, its like having a parasite the size. of a rabbit attached to your ribs. If left untreated varroa will kill most hives within a season and concerningly, the hive will also become a source of varroa infection for all other hives in the area. This means varroa control and surveillance is a true team sport. Secondly, along with siphoning energy from the bee they also spread viruses and other diseases back to the honeybees. This further impacts the hives health and varroa infections are often associated with increases in Bee paralysis virus, Deformed wing and sac brood virus.

Symptoms of infestation

For early-stage infections there are often no symptoms. Surveillance should be across all hive not just those that are weaker or show symptoms. As the infestation grows you may begin to see scattered brood, crippled or crawling honey bees, increased rates of wing deformation, impaired
flight, decreasing bee numbers and a reduced bee weight, slumped larvae and sunken cappings. Parasitic mite syndrome (or PMS) is the name often given to this group of symptoms.

Life cycle and how varroa spread

The varroa mite lives in two stages. Firstly phoretic mites, these are mites attached to a bee.Secondly, reproductive stage mites. These are found inside capped brood.Phoretic mites - These are the mites that are hitched onto the bees, these represent only 20% of mites inside a hive at any time. These are exclusively pregnant females and are carried throughout the hive. They are most often found on nurse bees but also spread onto foragers. These infected foragers are how varroa can spread hive to hive. You can imagine the infective zone is anywhere that these carrier bees can move to.

Common infection sources

  • Bee Drift – Bees returning to a neighboring hive. Trials have found labelled bees in hives up to 5km away.
  • Swarms and splits – This can spread phoretic and varroa in the reproductive stage.
  • Transport or dump sites – Disorientated bees will sometimes return to the wrong hive. This can be an issue for large hive movements such as pollination or apiaries in close proximity.
  • Lost bees or left behind bees – such as when hives are moved during the day and the foragers then return to neighboring hives.
  • Shuffling brood frames - weather your balancing brood between hives or moving honey stores you may also be moving infected bees or comb.
  • Robbing – Bees that come into a hive to rob may also leave varroa behind. This Is believed to be the highest risk activity and how most varroa is spread. Aim to decrease this risk by limiting open sugar or pollen feeding and decreasing entrances on weak hives.

The linking risk factor in all these scenarios is proximity. We encourage beekeepers in an area to work together proactively. First get your varroa surveillance plan in place and then check neighboring beekeepers to ensure they are doing it also.

Once the infected bee lands in a new hive it will often make itself at home and not realise it is not in its original hive. The varroa hitchhikers can then move off the bee into the hive and begin the reproductive phase of their lifecycle. The reproductive stage starts with a pregnant female mite entering a brood cell as it is being capped. The mite is adept at hiding beneath royal jelly or underneath the pupa itself to avoid detection. It is the scent omitted by the bee larva that attracts the mite to the cell.

Once inside she will begin to feed on the pupa and the cell is capped with her inside. Over the coming days she will start by laying male offspring (eggs are laid and not fertalised) and then females, these females are referred to as nymphs. These nymphs develop and feed on the pupa until they reach sexual maturity. Keep in mind the brood cell is still closed.

The mites then interbreed among themselves. When the cell is uncapped days later the pregnant females emerge along with the new bee. The mites then move onto the phoretic stage. These pregnant female mites spend 3-4 days hitchhiking on workers (ample time to spread between hives) before going back into another reproductive stage. The male mites and adolescent mites on the other hand remain inside the cell and die. Male mites can not survive outside of a brood cell.

In a highly infected hive mites will be seen when any brood is uncapped. It looks like brown dots on the white pupa. The mites do have a preference for drone brood rather than worker brood, so checking drone brood is a good surveillance technique. More on that later. Varroa prefer drone brood as it has a longer developmental stage, so more varroa can be breed per brood cycle. On
average each mother has 1.5 daughters in worker brood and 2.5 daughters in drone brood. The mites can smell a scent difference between the two and will preferentially enter the drone brood. This sense organ is on their front legs and they also have a crude sense of light or dark and sense vibrations too as they move around the hive.

As the reproduction of mites is paired with bee brood you can see that spring, summer and autumn are the key times to look for varroa. While a queen is laying varroa can double in number every four weeks.

As an emerging invasion is unfolding at the moment in Australia, varroa must be checked for all year round and no season should be considered low risk.

Please see our current Varroa products here

Varroa Mite Series
Learn more about Varroa Mites in our special 4-part series.

Part 2:  Varroa surveillance planning

Part 3: Varroa surveillance tools and techniques

art 4: Why record keeping is crucial for varroa mite surveillance