Because of the nice weather in winter and spring the varroa populations have exploded. Winter treatment against varroa using oxalic acid is used during winter because that it is the only time that the colony is not expected to have any brood. A single treatment should impact all varroa as there should be no varroa protected inside caped brood cells.
But because of the mild weather this winter there is a good chance that the bees did have capped brood, most likely filled with varroa. And then there was the mild spring which gave the mites a head start in the bee season letting their population grow to new heights.
That is why this year the varroa treatments here in western Europe are especially important. Varroa can be a major cause in colony death over winter. More so than lack of food or extreme cold.
The local apiarists organisation in the Netherlands has published an article on the explosive varroa population growth. A beekeeper counted 4 times the varroa he would count in an average year. You can view a translation of the article by following this next link (in all honesty the translation isn’t all that good, but kind of readable).
I’m not sure how the varroa population is going in my hives. I’ll check the mite drop after the 5 weeks of apiguard treatment tomorrow and update this post accordingly (if the weather allows, lot of rain here lately).
Talk to you soon,