Not really a pest or disease but I just found mold starting up between the cover and top board of my flow hive. It doesn’t seem to be effecting the bees but I’d rather take care of it before I start seeing dead bees everywhere. Has anyone else had this problem? How did you deal with it? I’ve seen people use vinegar but I’m a little reluctant the spray vinegar in my hive even if it’s not really in it. Thanks in advance😊
Hello and welcome to the forum. This topic has been discussed quite a bit! The mold itself isn’t a problem and the bees would probably clean it up if they are given access under the roof. I would not use vinegar or other chemicals as the mold is really more of a cosmetic problem in that location.
Make sure your roof is reasonably water resistant - how water resistant depends on your climate so maybe you’ll want to be more specific than CA for your location. Depending on your climate, you may also benefit from more sunlight on the roof to make sure things are drying out completely during the day. You will always get some moisture under there, if not from the environment then from the bees and condensation.
Ventilation under roof - many people add holes under the gables with little vent covers. For a simpler approach, you could just prop the roof up a little bit to let more air underneath.
If you search for moisture under roof, mold under roof, sealing your flow roof, ventilation under roof, etc you should find some good discussions.
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Hi Ella, just to add to what @chau06 wrote. If your roof is wrc, that timber can handle moisture quite well, however from personal experience, it doesn’t appear that hoop pine can handle prolonged periods of being wet, it tends to go rotten with time. Therefore if your roof is hoop pine, I’d suggest to do whatever is necessary to keep it dry.
cheers
Really it’s considered cosmetic there?? I thought it would eventually get into the hive and even if the bees sealed it off it would still effect their respiratory system and kill them off.
I’m in the Sacramento valley. It’s pretty dry. I completely sealed the outside of the hive before I installed the bees and it’s in the highest and driest spot in my yard. Drilling vents sounds like a pretty good bet. I’ll just need a really small bit because the last thing I want is for the bees to find their way in and start building so that I get a nasty surprise at next inspection 
thanks and I’ll check those forums too
I have the flow classic which I’m pretty sure is hoop pine.
I agree with @chau06 - the black molds (mildew) will not hurt the bees at all. It can’t even grow on them. The main mold to worry about is chalkbrood. That is very damaging for bees, but it doesn’t grow on wood, just on bee larvae and pupae. 
I would actually suggest drilling a hole at least 1" in diameter. You can then just staple a small patch of insect screen or fine mesh hardware cloth over the inside of the hole. No bees, wasps, hornets or roaches will be able to get in. Simple! 
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Double check on that Ella because hoop pine is the one that doesn’t like water. Wrc is lightweight, it has a beautiful looking grain with a beautiful smell.
I inspected a hoop pine flow hive early last year, only to discover dry rot in the crown board after only 12 months of use, with it being continually damp.
This was posted on the forum 18 months ago.
Hello all,
I’m cleaning up brood boxes from last year’s keeping ahead of my receipt of a nuc. I noticed some black mold on the bottom brood box.
Should I be concerned? Will its presence contribute to swarming (this occurred in this hive late last summer)? i.e. a bad environment for the bees.
Does it affect the honey?
Thanks Eric in Colorado…do bee do bee do
No harm in just wiping it down with a cloth but that doesn’t look too bad. The bees will clean up.
Not sure if connoisseurs/supertasters can detect earthy fungus smell in honey but I would guess no difference in the honey.
thanks…I’ll wipe the inside of the box with a “lysol” towelette and keep the box in the sun.
do bee do bee do…Eric
I’d recommend just using water or maybe dilute bleach but not anything else.
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I agree, household unscented chlorine bleach, diluted 1 part bleach to 9 parts water. Use a spray bottle to thoroughly wet the black areas, and let it sit for 20 minutes. Rinse it off with a hose, then thoroughly dry, in the sun if you can. If there is a residual smell of bleach, don’t worry about it. Bees happily drink swimming pool water and aren’t harmed by it.
thanks 1:9 bleach to water was sprayed. I’ll hose off the areas this AM.
Eric