I find the concept of bees and mites reaching a state of equilibrium an interesting one. Certainly, in my TF colonies there are always some varroa present but it's never an excessive amount - when there have been spikes in numbers they have rapidly returned to a low level. Do the bees manage the load or do the mites control their reproduction to prevent damage to the host colony ? I don't know but it's an interesting question.
How much damage the mites do to their host is also an interesting debate .. Yes, the mites will seriously damage the larvae in the cells where they reproduce .. but, as they prefer drone cells, does this have more impact on the drone population in the colony than the workers and consequently the colony as a whole is less affected ?
My colonies have free reign in the amount of drones they produce and I always see quite large percentages of drone comb .. do the bees compensate for the loss of some drone larvae by increasing the number of drone cells ?
There is evidence that high temperature/high humidity restricts the ability of varroa to reproduce; I've also noticed that in my highly insulated poly hives that the bees reproduction cycle is often one or even two days shorter than the established cycle. Do these two factors also hve an effect on the varroa's ability to thrive in my colonies ? Another question that is impossible to answer.
I also think that local conditions must play a significant part in varroa levels. The forage in my location (basically a mix of urban gardens, allotments, trees, fields, parks, railway banking) is a long season and I don't think the bees forage on a wide ranging area in order to secure season round forage - if they forage in a confined, but productive, area they may not have the opportunity to interact with other bees and become reinfested with fresh phoretic mites.
Similarly, I think that the biggest threat from varroa infestation comes from the diseases they vector - if the incidence of DWV etc, in the vicinity is low then this threat will also reduce. I don't ever see any significant levels of brood diseases in my colonies so the only threat they face is the damage the mites do in feeding off their hosts. Perhaps, with the high levels of drones in the colonies this damage is more prevalent in the drone population rather than the workers ?
There are probably other factors that influence the prevalence of varroa in a colony, local climate, being foundationless, variety of forage and of course, the bees themselves ... in a species that is managed but effectively wild it is virtually impossible to test what are the prime factors that allow some bees to maintain a balanced relationship with varroa. I would like to think I would worry about it but I don't .. it works for me and I'm grateful.
I am about to relocate two colonies from another beekeeper to my apiary, these colonies have a disastrous history of varroa loads and did not respond well to lots of treatment last year. What I intend doing is moving the colonies into my hives, treating them to ensure the mite load is low before they start living the dream in my apiary. It will be interesting to see if a couple of colonies with a really bad history of varroa continue to have a problem with varroa or whether they reflect the other colonies that I keep ...
