Hmmm.... my strong backyard colony today had lots, maybe 15 or more, drone crawlers. They were young and fuzzy, wings intact and apparently healthy except they were clearly crawlers. I could only find one worker in the grass. This colony has been treatment-free for a couple years, though I haven't yet done a varroa count this spring.
I'm guessing tracheal, but why the crawler drones and not workers?