There could be many explanations for the behavior of your bumble bees. They might have run into a pesticide on a plant they visited some time earlier. They could be reaching the end of their normal life span. Was the weather cool, or has it been particularly dry? They might have a disease, such as a fungus, bacteria, mite, or other infection. Some ignorant human could have been swatting at them and caused injury. Have you seen any dead bees on the ground, especially in large numbers? Unfortunately, it's impossible to know what might be going on. It's also possible they are just fine, and a bit drunk from visiting a fermented pile of fruit. All you can do is continue to avoid pesticides yourself, and educate everyone you meet about the crucial status of the insect world.

Martha

this time of year in my area it gets down to freezing at night and if not killed they are sitting on my raspberry leaves so sluggish you can put them gently in your hand without getting stung. once the sun hits them and they warm, they're back to foraging in the raspberry flowers.