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Where are my mason bees and why are no holes plugged?

We know it can be very disappointing when your mason bees don’t fill many holes, but we want to reassure you that you’ve still impacted solitary bee populations and helped your habitat. The success of our program starts as soon as you release solitary bees back out into the environment. Since solitary bees are such incredible pollinators, when you released them into your yard you helped create a healthier ecosystem by strengthening your trees and flowers that will provide cleaner air, stabilize soils and support other wildlife. You’ve also helped grow the local population of solitary bees, which are commonly eliminated or reduced from urban areas as they are being developed.

This year has been noticeably different than previous years in production of holes being plugged and visible mason bee activity around nesting blocks. Unfortunately, our spring weather played a huge factor with not only our mason bees, but flowering trees that bloom in early spring and rely on pollination. This unfortunate process is called phenological mismatch, which is where the time trees and plant blooms don’t match up with the emergence of pollinators. Plants receive less pollination and pollinators don’t get enough food, really not a great situation to be in. Mother nature has been very harsh on us this past year with freezes and heat waves, as well as random jumps and dips in temperature which throws off a lot of plant bloom timing. It’s a tough thing to deal with and we expect that it’ll only get worse as the years go by as our climate and weather continues to change. When it comes to all plants, native or non-native, they may adapt to survive where they’re planted, but their circadian clock will always take cues from the temperature.

Let’s do some bee investigation to try and figure out if your bees were impacted by weather or something else:

  1. When did you get your bees?
  2. Did they go out right away or did you stick them in the fridge? If fridge, how long were they in fridge and did you check for emergence while they were in there?
  3. Do you see any scent marks on the white tube? They look like mud marks, those are the males scent marks.
  4. Are you or your neighbors using any pesticides or sprays? Any sprays or chemicals will harm sensitive pollinators.
  5. Is there a clay source? If not, they’ll fly off to find sticky/muddy clay for their babies
  6. Is there a food source? If your bees emerged and there was no food, they’ll fly off in search for food.
  7. Has there been a lot of rain? Mason bees are hardy bees and can fly in drizzle or light rain, but heavy rain they’ll seek shelter and not be able to fly. This may throw them off and they may not find their way back to your nesting block and find natural holes in your habitat.
  8. Was there daytime freezing or snow after they emerged? If so, how many days? Did it damage the blooms? (They can survive just fine in cooler temps at night, just the daytime temps need to be 45+ degrees). 
  9. Was your black bee house placed South facing in the morning sun?

Bee Science:
Now let’s investigate the contents of your PVC tube where all the cocoons are. On a paper towel or piece of paper, dump out all the contents and examine the cocoons. If they all have a hole in it, then they emerged. If not, set the ones aside that haven’t opened and you can snip the tip off the cocoon to examine them (see video above)


If your black house was hung in warm morning sun, you had mud nearby, food as soon as they emerged and didn’t use pesticides or sprays, then you did nothing wrong. I know it’s fun to watch them fill holes, but you must have a great habitat where they found natural holes (a couple of mine even filled the holes on my deck umbrella… LOL!).

If you’ve determined by answering the questions above that it is not weather related, then something you can try next year is moving the location of your black house. Sometimes a different spot is closer to a food source and they prefer it better. Other times, it’s just a matter of preference and we never know year to year what mason bees will prefer. .

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Reader Interactions


  1. Franja Bryant says

    I think my bees did reasonably well this year. I ordered the small size block and 22 of the 48 holes are filled. Cocoon check showed only 4 “unhatched” cocoons. I moved the box to a sunnier southwest facing location than I have had it in previous years. There are a few scent marks on the outside of the tube, but not as many as last year. One item of concern for me is that I am seeing a number of very small (fruitfly sized) flies hovering around the holes. Could these be the Houdini flies that parasitize Mason bees?

  2. Dave Parker says

    I have found quite a few bee tunnels with spider webs in them. I use a penlight to view deep into the tunnels. I am going to get a thin, wire brush, the kind used to clean tubing or straws, and clean out the webs from the tunnels.

  3. Ruth says

    I put in morning sun , had clay nearby , when arrived waited to put out until next week in warmer weather this unfortunately was after all flowers on cherry trees still had flowers in garden
    Have scent marks but NO holes plugged . We had a lot of rain and cooler temps .

    • Christopher Gomez says

      Last year an in identified bird destroyed all of our nests, except for one that I protected from them in the garage. I don’t know how to prevent that in the future but right now the mason bees are swarming, but I’m afraid they are not building nests in the abandoned tubes. Is it possible that the death and destruction that occurred last year is a deterrent?

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One Mason bee
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