Re: How much disease is too much?
Depends on your bees. If they are non varroa resistant they will slowly get worse till they die.
A lot of folks who have presumed varroa resistant bees, do report seeing some DWV from time to time, and it clears up again.
So you need to know your bee stock, and if you wish, monitor to see if things continue to worsen. If they continue to worsen you will at some point have to decide if you will do something for the hive or let it be one of the 4 in 10 that die.
"Thinking Inside The Box"