From www.bumblebee.org, this and other great info can be found under the "Help Bees" section:
If you find a grounded bumblebee early in the year, just at the start of the first warmer days, then it is probably a queen. She may have been caught out in a sudden shower or a cold spell. If the temperature of the thorax falls below 30 oC the bumblebee cannot take off (see temperature regulation). The best thing you can do it pick her up using a piece of paper or card, put her somewhere warmer, and feed her. When she has warmed and fed she will most likely fly off. You can feed her using a 30/70 mixture of honey and water in a pipette or eye dropper, or just a drop of this on a suitable surface within her reach, but be careful not to wet her hair or get her sticky. By saving a queen you may have saved an entire nest. If the weather is really unsuitable for letting her go, or if it is getting dark, you can keep her for a day or so if you are willing to feed her.
A grounded bee found at the height or end of summer is another matter. Look at the wings. If they are ragged round the edges (see the photographs of wings) then you have either an old queen or an old worker. There is little you can do as really it is their time to die, however you could take them in and feed them if you wish, but let them go if they start to fly. If the wings are fairly intact then you have probably got a male that is either cold or has been so busy patrolling that he forgot to drink. As above you can take him somewhere warm and feed him, then let him go.
Bumbles in general are attracted to yellow, purple, and blue flowers, especially those with tubular flowers. "Double" varieties of most flowers will not contain pollen so are of no use to bees. Foxglove and Heather are used as shelter when rain arrives suddenly. The following flowers are great bumble attractors:
- Blueberry Bush bells
- Blooms of the Nightshade Family
- Clover (yup, that white clover in your lawn) :)
*Many flowers are great multi-species bee attractors in general, such as those found in 'cottage gardens', natives, berry and fruit blooms, high pollen sunflowers, and herbs.*
A food source is not the only requisite for hosting bumbles, they also require a good spot to nest. Bees tunnel into bare dirt to lay eggs and hibernate, so mulch of any sort makes a landscape unsuitable (this is more problematic in urban settings). The base of stones and hedges are frequent nesting sites so be careful not to disturb these areas whenever possible. Curious about how to sex a bee to see if it can sting? Check out an older post on the subject of bee bits here.