Amitraz is normally a very effective treatment but it is a slow treatment needing at least 2-3 brood cycles. That’s why it needs 6-10 weeks to work. As you used it for the first time in normal circumstances it would have worked over time. You also need to move the strips and scrape them half way through the treatment.
Your original infestation may have exploded as a result of one of your colonies robbing another. Then if colony 1 levels were high at the time of treatment, they may have absconded and moved into one of your lesser infected colonies. Added to that colonies load, bearing in mind varroa can double in population in as little as 3 weeks, there would have been very rapid explosion of mites and as the treatment is slow it may have been less effective. By this point, your all important winter bees will have been emerged, and if infected with the Deformed wing virus, have led to a quick collapse.
The most common loss of hives especially among early years beekeepers is varroa, so pls don’t beat yourself up. You did lots of checks and treated your bees.
I’d also check the layout of your apiary and make sure entrance directions are all different, not in a line. Probably wouldn’t have made that much difference in this case, but can help with drifting of bees between hives in future.
This experience will now help your future beekeeping and with Poots help you can start again with queens that are fitter and suit your environment.
Elaine
