Why did all my mason bees die in their cocoons? - Ask Extension
This is my third season with mason bees. The first two seasons, I didn't harvest my cocoons and saw lots of mason bee activity in the Spring. This F...
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Why did all my mason bees die in their cocoons? #865639
Asked April 23, 2024, 4:52 PM EDT
This is my third season with mason bees. The first two seasons, I didn't harvest my cocoons and saw lots of mason bee activity in the Spring. This Fall/Winter was the first time I harvested, cleaned, and cold stored my cocoons. I put half my cocoons outside 3/17/24 and I put the rest outside 3/31/24. The bees were very slow to emerge. The majority of the cocoons were still unopened as of 4/18/24. No bees have emerged in the last 5 days and it has been reaching a high of 55-70+ each day. There hasn't been any scratching noises or movement in the cocoons either. I decided to cut open a few cocoons to see if they had mono wasp larva, but just found dead bees. I continued to open cocoons. Out of 65 cocoons I opened, only 3 had mono wasps. The rest were dead bees that didn't look to have anything wrong with them. I am trying to figure out what I did wrong. Why did over 80% of my bees die in their cocoons?
- Cleaning: I followed directions I found on OSU extension (1 tbsp bleach to 8 cups water), didn't leave cocoons for more than 2 minutes in bleach solution
- Storing: Fridge crisper drawer in small cardboard box with holes. Included damp paper towel. Even with the damp paper towel, the humidity was really low (I didn't realize until I bought a thermometer with humidity reader). So the bees were stored at only 30-40% humidity for a few weeks before I modified the crisper drawer to keep at 80%+ humidity. Could this be what killed them?
- Releasing: when no bees emerged after a couple days of putting out the cocoons, I opened the attic so the cocoons would get more warmth since the temps were 55-60ish until recently. Could direct sunlight be what killed them?
I am really trying to target how this happened so I can avoid in the future.
Multnomah County Oregon
Expert Response
Hi Beth,
I am so sorry to hear about this. It looks like you did everything correctly, but maybe we can drill down a little. How were the nest blocks stored from June until you cleaned them? When you cut open the cocoons are the bees soft for desiccated and brittle? What date (approximately) did you clean the cocoons and move them into cold storage? Were the cocoons excessively moldy?
Best,
Andony
I am so sorry to hear about this. It looks like you did everything correctly, but maybe we can drill down a little. How were the nest blocks stored from June until you cleaned them? When you cut open the cocoons are the bees soft for desiccated and brittle? What date (approximately) did you clean the cocoons and move them into cold storage? Were the cocoons excessively moldy?
Best,
Andony
Hi Andony,
The nest blocks were stored in my basement in front of a south facing window from July to about October when I moved them to my garage. Maybe they weren’t warm enough? The temp was probably 70-75.
I harvested the cocoons over a couple weeks (end of November). They were in the garage until they were all harvested and then I cleaned them all at the beginning of December and put them in my fridge crisper drawer. The cocoons weren’t moldy at all.
When I cut open the cocoons only a couple bees seemed old/brittle. But the majority were soft/pliable.
Maybe it had something to do with the pollen/nectar sources around my yard? Perhaps they didn’t have enough fat stores to make it to spring.
Thank you for helping me trouble shoot. I really want to do what I can to prevent this from happening again. I am devastated that I inadvertently killed so many bees.
Beth
Hi Beth,
They clearly died after spinning a cocoon, but I just wanted to check that they were not stored somewhere excessively hot over summer - which clearly wasn't the case. I doubt it's the nectar and pollen sources. It is clearly something else that happened. I have reached out to some experts and I will get back to you when I get a response.
Best,
Andony
They clearly died after spinning a cocoon, but I just wanted to check that they were not stored somewhere excessively hot over summer - which clearly wasn't the case. I doubt it's the nectar and pollen sources. It is clearly something else that happened. I have reached out to some experts and I will get back to you when I get a response.
Best,
Andony
Hi Beth,
I checked in with two Master Gardeners who are more knowledgable than I am, Ron Spendal (Washington Co) and Rich Little (Linn Co.).
Both thought that leaving the nest blocks by the window during the summer may have caused them to elevate their metabolic rate, resulting in a depletion of a fat reserve. As Rich noted, even though the room temperature might have been adequate, the direct sunlight may have resulted in much higher temperatures in the block. Also, as Ron notes, 70-75F might be okay for the summer, but when it is that warm in the fall, the bees will start to work through their fat reserves. The bees should cool along with ambient temperatures as the fall approaches. Rich also thought raising the humidity for the cocoons to 70% is helpful, but the damage of the heat is likely the overriding issue.
The take home message is that when you bring your blocks inside in June, keep them in a dark place that more-or-less tracks ambient temperatures. Ron observed that your cleaning time seemed reasonable, but perhaps look for ways to increase the humidity for your storage (as Rich suggests).
Hopefully this helps with your bees in 2025.
Best,
Andony
I checked in with two Master Gardeners who are more knowledgable than I am, Ron Spendal (Washington Co) and Rich Little (Linn Co.).
Both thought that leaving the nest blocks by the window during the summer may have caused them to elevate their metabolic rate, resulting in a depletion of a fat reserve. As Rich noted, even though the room temperature might have been adequate, the direct sunlight may have resulted in much higher temperatures in the block. Also, as Ron notes, 70-75F might be okay for the summer, but when it is that warm in the fall, the bees will start to work through their fat reserves. The bees should cool along with ambient temperatures as the fall approaches. Rich also thought raising the humidity for the cocoons to 70% is helpful, but the damage of the heat is likely the overriding issue.
The take home message is that when you bring your blocks inside in June, keep them in a dark place that more-or-less tracks ambient temperatures. Ron observed that your cleaning time seemed reasonable, but perhaps look for ways to increase the humidity for your storage (as Rich suggests).
Hopefully this helps with your bees in 2025.
Best,
Andony
Also, we had a response from the USDA Pollinating Insects Research Unit (aka the Bee:
"High temperature during adult cocoon stage was likely the culprit. It is advised that cocoons go into cool temperatures between 30 -60 days after becoming adults. The Osmia crew here at the bee lab, prior to me, found that 45 days was the sweet spot. Prior to 30 days they didn’t have enough time to develop properly and after 60 days they used up to many fat bodies and this resulted in large mortality. This can be very hard to gauge for anyone who doesn't have an x-ray to look inside the cocoon. Here are some better guidelines to show when to check for your bees being adults. We found that egg to adult usually takes 80 – 100 days, with the warmer the temperature the shorter the development period. Which means in hot years cocoon needs to be moved to cold storage sooner. For our bees in Utah we usually release in early to mid-May and put them 40F on Oct 1. Anyone can also check this and cut a few cocoons open to see if they are adults".
"High temperature during adult cocoon stage was likely the culprit. It is advised that cocoons go into cool temperatures between 30 -60 days after becoming adults. The Osmia crew here at the bee lab, prior to me, found that 45 days was the sweet spot. Prior to 30 days they didn’t have enough time to develop properly and after 60 days they used up to many fat bodies and this resulted in large mortality. This can be very hard to gauge for anyone who doesn't have an x-ray to look inside the cocoon. Here are some better guidelines to show when to check for your bees being adults. We found that egg to adult usually takes 80 – 100 days, with the warmer the temperature the shorter the development period. Which means in hot years cocoon needs to be moved to cold storage sooner. For our bees in Utah we usually release in early to mid-May and put them 40F on Oct 1. Anyone can also check this and cut a few cocoons open to see if they are adults".
Andony,
Thank you so much for your help. The responses from the bee experts and the response below are all great information and advice. I now have a better game plan to ensure I am taking appropriate steps for healthy bee emergence in the future.
Best regards,
