Don’t believe in varroa mites? Here is a video Charlotte Ekker Wiggins took with a macro lens inside of drone brood from a third year Italian swarm colony. She regularly removes drone brood as part of her varroa mite management.
On this particular frame, varroa were noticeable only in a 4-inch corner of the drone brood. Considering how quickly varroa mites can reproduce in drone brood, it is easy to see how just a tiny population in a corner of the comb can take over by July:
In addition to drone comb removal, Charlotte is now treating her colonies with oxalic acid vapor for 3 weeks this spring in 5-day intervals. She will treat again in July.