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.