My sense of it is that as long as they haven't emerged, the frige is a good play.

I have a situation going here that is somewhat similar. During that same warm period, I put out about a 1/3 of my bees (they've been in the frige all winter.) No sooner were they out then the 10 day forecast predicted an artic blast with temperature in the low teens for several days.

I was pretty sure if they managed to emerge in the next day or two, they'd be killed by the cold. So I put them back into the frige. I don't know if there's a period (kind of a proto-emergence) when re-chilling them would do them in, but I figured I better take the chance.

I also imagine that this cold snap will wipe out the first wave of blossoms on the Nanking Cherries--they were just starting--which is what prompted my setting out the bees in the first place.

Hopefully someone else will chime in with more advice.

Don

Putting them back in the refrig should be fine. Their emergence can be delayed for up to a month past their natural cycle without serious problems. Even emerged bees can be held in the refrig for a few days without harm. Mason bees are exposed to fluculating weather in their natural habitat and have adapted to it.

A bigger cause for concern is the availability of pollen and nectar sources. On any warm day after they have emerged, they will be out looking for food. If there is none in your area, they will fly off to find some. Once they find food in another area, they probably won't come back. It's a good idea to have something else in your area that blooms a little before your fruit trees. That way, even if they emerge early they will stay in the area. Mason bees seem to have a strong preference for fruit trees, so you don't need to worry about other flowers distracting the bees from the fruit trees once they start to bloom. Some of the native plant blossoms are not as sensitive to cold temperatures as the crop type fruit trees. Willows come to mind for my area, but each area is different. Have a look around to see what is blooming now in your area. If you can get some of these plants started in your area, it will make the timing of emergence a lot easier and decrease the danger of dispersal due to lack of food.

Thanks for your help. I'm reassured that it will be OK. It's nice to have enough bees to set out a second batch if one needs to.
Tom