Varroa 2
Varroa and Viruses
How do bees defend themselves from disease?
The high density populations and conditions within the bee colony (enclosed, moist, dark, sometimes poorly ventilated) are ideal for the outbreak and spread of disease. Fortunately, because bees are constantly expose to pathogenic micro-organisms, they have evolved strategies to resist infection. The cuticle of bees acts as a barrier to penetration, and immune system based defence can prevent infection of many minor pathogens. However, the recent completion of the honey bee genome sequence has shown that they have only about a third of the number of known immunologically related genes when compared to flies or mosquitoes, suggesting that bees rely less on individual immunity than most insects.
My own experience with varroa has been rather limited. I suppose like most hobby beekeepers I have taken the easy way out and followed the local practice. Each November I have treated each colony with the approved miticide and hoped for the best. However, last year my eyes were opened in two ways:

Firstly, on the chance meeting with a neighbouring beekeeper she told me that her colonies that summer had been devastated by varroa, and stated that "Bayvarol was useless". The fact that the varroa mite had become resistant to Bayvarol has been well reported but I did not think that this was apparent so close to home.

Secondly, in two of my colonies I saw quite a number of bees with deformed wings on the alighting boards. In fact in one colony the defective bees were being forcibly ejected from the hive by worker bees. Whilst I congratulated myself that my bees were showing a strong hygienic behaviour, I was forced to address the varroa problem and in the future resolved to take a more pro-active approach.
varroa mite on honey bee
How does the varroa mite affect bees?

The effect comes about in two ways:
either when the varroa mite is being carried on an adult bee, it finds the weak spot and crawls under the overlapping abdominal plates where it feeds on hemolymph (bee blood). The bee now becomes susceptible to viruses;
or as a result of the mother mite feeding her offspring on bee larvae or pupae in the brood cell. This then affects the development of the pupa into an adult; deformed wing virus (DWV) can result.
Low temperature scanning electron micrograph (LTSEM) of Varroa destructor on a honey bee host
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