'Or perhaps its a difference in the varroa/virus complex in
those areas compared to others.  '

There is a lot of focus on the bee and the management, but a lot of the
risk is really from the environment.  Varroa mites are vectors for viruses,
and while the varroa can damage the bees, we know that bees can handle
quite high levels of mites in the absence of viruses.   We also know that
viruses are not evenly distributed over the landscape in space or in time.
We also know that they mutate over time, so the risk of death is also
variable.   It is overly simplifying a very complex system to say that
survival in absence of treatment is due to the bee - especially when the
mechanism isn't determined.

It is very hard to disentangle the role of the 'treatment-free' bee from
the associated lowering in risk when you have a bottleneck and low
immigration.  If you have a huge epidemic, and you kill off the majority of
the population, the ones that remain may be resistant, or they maybe just
didn't reach high exposure levels because of the way the disease moved
through the population. For example, emerald ash borer in trees - it move
through the population killing almost all the ash trees, but then died out
when the population couldn't support it.   Once the population rebounds
enough to support a population of the beetle, those trees will become
parasitized.

Another way to help think about vector borne viruses is to picture a dengue
epidemic.   If I live in an area with low *Aedes ageypti* (vector), or I
and my neighbors have done a lot of work to control the mosquitoes, I am at
low risk for disease.  This is similar to places where they have had a huge
die out, or do not have neighbor beekeepers, or everyone manages varroa
levels.   With dengue, if I have a lot of mosquitoes, but the virus is not
found in the area, then I have low risk of disease - we see this in Hawaii
with both varroa and dengue, where things were fine until the viruses were
introduced.   If I have viruses and vectors, at the same time, then I have
a problem.  What individuals get sick in this context depends on their
ability to manage the vector in the area, as well as their inherent
susceptibility to the viruses, as well as the dynamics of the disease
spread through the region.   A person who manages mosquitoes on their
property well may still get sick if the disease pressure is really high
(e.g. a high proportion of mosquitoes are infected with the virus, or the
neighbors are actually running a mosquito breeding program).

We see this with bees a lot - where people have worked to raise bees with
lower varroa, but are inherently dropping the varroa risk to their
operation.  They just have fewer vectors, and likely less virus, because it
wouldn't be maintained.  Then they are dropped in areas where there is high
varroa/virus, and they just don't survive.   The treatment free hives may
be surviving because there is an unknown resistant trait, or they may be
surviving because the vector/virus complex is such that they aren't at high
risk.

This is written very quickly / not cited, but I just wanted to prompt
discussion on this.


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