Varroa is a scourge for sure, and that's great advice! I would be cautious with the significance of your winter wash number. On the surface it looks devastating, like you didn't even bother to deal with mites. However the testing protocols for mites assume that you're collecting 300 nurse bees in an active hive where 50% or more of the mite population is currently under sealed brood. Your winter wash came from bees that had clustered together, with no brood for the mites to hide in -- so 100% of your mites were available to sample from a tightly clustered group of bees. That changes the factors on which the alcohol wash protocol is based, and thus the significance of the counts.
There's no doubt in my mind that the varroa killed the one hive, but it didn't get the other. Since you did have a series of OA treatments in the fall, I think you probably got the varroa knocked back to a borderline threshold where one hive could make it but not the other. You should do a test on your good hive and see where you're at.
The Dyce lab for pollinator studies has a great IPM guide (pdf) for beekeepers. eCornell offers a great four week online course for newer beekeepers too. I'm a bit biased on that since I help facilitate it, but I do believe that it's the most thorough training course a beginner can do online.
There's no doubt in my mind that the varroa killed the one hive, but it didn't get the other. Since you did have a series of OA treatments in the fall, I think you probably got the varroa knocked back to a borderline threshold where one hive could make it but not the other. You should do a test on your good hive and see where you're at.
The Dyce lab for pollinator studies has a great IPM guide (pdf) for beekeepers. eCornell offers a great four week online course for newer beekeepers too. I'm a bit biased on that since I help facilitate it, but I do believe that it's the most thorough training course a beginner can do online.