Dawn has answered your question about white stuff in cells. Your bottom board looks pretty nasty but normal. Guess that mite or two are croaked/dead ! I try to clean the SBBoard slider once a month in the winter n every other (usually) during the active months. I learn a lot viewing the slider without opening up the hive. I’m trying to inspect every other week now n check the sliders on the other week.
I also lost several hives to varroa mites. 2 last early autumn n one more finally surcumed late winter/early spring. That’s a real bummer at best but makes me analyze my notebook entrances n data. I rather not make that mistake if possible too often.
The rest of last winter into our cool wet spring I’ve been sliding in a sheet of new white paper n pulling 24 hours later every two weeks as a mite drop check. Nothing seen yet in my two healthy surviving hives. I’m not nhieve enough to think a few mites are upstairs already but still too low of count to matter. Weather n brood should improve soon so I can administer a true (sugar-roll) mite test end of April.
I’ve got to be more watchful this season n with last season under my belt … A few changes n guessing my inspections will be more n more fruitful as I gain more experience.
I have gotten four new Nuc’s (pricy replacements) to replace the three die-outs n gaining one additional apiary colony. So learning n moving on. Lots to figure out but every new endevour has its learning curve!
Good luck n take care,