This AFAIK happens when the pollen of the plants they visit do not provide enough energy for them to do the collection. And yes, that is why you can sometimes find a lot of stunned or dead bumblebees under certain tree species. It is a kind of parasitism of these plants as they depend on other plants (and the bumblebees ofc.) to provide enough fuel and they are just freeloading.
Edit: but sudden cold spells can also have a similar effect as bumblebees use their wing muscles to generate some heat during cold spring days to allow them to operate when normal bees can not. This of course also takes a lot of energy.
This AFAIK happens when the pollen of the plants they visit do not provide enough energy for them to do the collection. And yes, that is why you can sometimes find a lot of stunned or dead bumblebees under certain tree species. It is a kind of parasitism of these plants as they depend on other plants (and the bumblebees ofc.) to provide enough fuel and they are just freeloading.
Edit: but sudden cold spells can also have a similar effect as bumblebees use their wing muscles to generate some heat during cold spring days to allow them to operate when normal bees can not. This of course also takes a lot of energy.