Varroa mites

Varroa mites are parasitic mites, which require a honey bee host to survive and reproduce. The Varroa mite is only able to reproduce on honey bee brood, while only adult female Varroa mites are able to feed on adult honey bees. Therefore, the entire life cycle of Varroa mite occurs within the honey bee colony.

Female Varroa mites are more likely to lay eggs on drone brood than on worker brood (10–12 times more frequently). This is due to the drone’s longer brood cycle. For this reason inspection of drone brood provides the best chance of detecting Varroa mite infections, however, worker brood also provides an effective means of detection.

Varroa mite life cycle

Varroa mite life cycle

The Varroa mite life cycle consists of the following stages:

  • Adult female Varroa mites enter honey bee brood cells (especially drone brood) at the pre-capping stage and lay two to five eggs after the brood cell is capped.
  • 0.5 mm long eggs are laid on the bottom of the cells, on the walls, and sometimes directly on the larvae.
  • The first egg laid is a male, and subsequent eggs are female.
  • After hatching Varroa mites pass through two larval stages (called a ‘protonymph’ and a ‘deutonymph’) before developing into an adult. It takes about 5–6 days for male Varroa mites to develop and 7–8 days for female mites to develop.
  • Mating occurs in the brood cell. The male Varroa mite dies inside the cell shortly afterwards. 
  • Young female Varroa mites and the mother Varroa mites emerge from the brood cell with the emerging honey bee.
  • The daughter Varroa mites will lay eggs in other brood cells after 2 weeks. Adult female Varroa mites usually live for 2 months, but can overwinter between the sclerites (the hardened plates of the exoskeleton) of adult honey bees.

Varroa mite numbers increase slowly at first (population growth is exponential), and it may not be until the third year of infection that Varroa mite numbers are sufficiently high for the pest to be readily detected. Close inspection of brood, especially drone brood, will provide the greatest chance of detecting Varroa mite infections early.

Once on a honey bee the female mites crawl between the sclerites of the honey bees’ abdomen where it feeds on the bee’s haemolymph (the bee’s equivalent to blood). By riding on adult honey bees Varroa mites can be rapidly spread to new areas due to the swarming, robbing and drifting habits of honey bees.Only adult female Varroa mites will parasitise adult honey bees. Adult males only feed on larvae and pupae and do not leave the brood cell once they have hatched. In contrast adult female mites are very mobile and move over the combs or between adult honey bees. This behaviour means that the Varroa mite can also act as an effective virus vector allowing the transfer of viruses between individual bees. The spread of viruses is a significant impact of Varroa mites.

The life-span of Varroa mites depends on the presence of brood, and can vary from between 25 days, to around 5 months. During the summer, Varroa mites may live for 2–3 months, and if brood is present they can complete 3–4 breeding cycles. In winter, when brood is either not present or is limited, the Varroa mites over-winter on the bodies of adult bees through their phoretic life phase. An adult Varroa mite may live and feed on an adult bee for up to around 3 months. Adult female Varroa mites can live for up to 5 days without food.

 Varroa mite destructor on the head of bee nymph

hive from Varroa Mite infestions

Effect on Honey bees

Individual bees infested with Varroa during their development usually survive to emergence but may show signs of physical or physiological damage as adults. Some brood infested by Varroa may die, usually at the pupal stage of development and remain in the cell until removed by adult bees. Varroa mites feeding and reproducing on developing larvae and pupae (worker and drone brood), has a major effect on individual honey bees, as they are affecting the most sensitive life stages of the honey bee.

Firstly, the process of Varroa feeding causes the loss of haemolymph during brood development, which significantly decreases the weight of the hatching bee. The weight loss depends on the number of Varroa mites in the cell and the level of reproduction taking place, but even a single female Varroa mite in a brood cell can result in an average loss of body weight of 7 per cent for hatching worker bees, and between 11–19 per cent for hatching drone bees. This subsequently leads to an impaired flight performance. This feeding behaviour also causes reduced hypopharyngeal glands (the glands that secrete royal jelly) which affect the honey bees’ ability to feed developing brood in the hive.

Secondly, worker bees which were parasitised during their development begin their foraging life stage earlier, but also have a significantly reduced lifespan. Infected worker bees and drones also display a decreased capability of non-associated learning, prolonged absences from the colony and lower rate of return to the colony, which may be due to a reduced ability to navigate.

Lastly, a significant impact of adult Varroa mites feeding on developing brood, as well as adult honey bees is through the transmission of viruses. Varroa mites feed on the honey bee’s haemolymph during brood development, as well as on adult bees. This results in Varroa mites acting as an effective vector for numerous viruses. There is also some evidence that Varroa feeding can reduce the effectiveness of the bee’s immune system, so they are more affected by viruses in the presence of Varroa, as well as other pests such as Tracheal mite or Nosema sp. This can cause common symptoms in the honey bee population, such as deformed and shrivelled wings, legs or abdomens, as well as symptoms specific to these other pests.