Bumble Bees are a controversial topic here. Some agricultural groups were trying to get them introduced but they ran into a wall of opposition. Being a large insect they cause havoc with flowers that have evolved with small pollinators. They WILL collect the nectar and pollen, but generally damage the flower in the process so that there's no reproduction/seeds. If they're already part of the natural system there's no problem. But introducing them could be the start of a bigger problem.
I always thought beans did pretty well on their own. But there are some crops that Bumblebees are good for. It's apparently something to do with the 'vibratory'(?) way they go about their job. But there are many pollinators around, it's a matter of attracting them to the garden. Planting lots of flowers ensures the pollinators'll have a good range of options and keep hanging around in larger numbers. There are many species (even genus) of bees of all sizes. Wasps do a lot of pollinating. And quite a few other insects will do it as well.
Here when the mangos are flowering there's a sudden flush of untold billions of flowers without much time for pollinators to build up numbers. Some of the mango orchardists hang animal carcases in the orchards which attracts flies and other carrion appreciative flying insects. These also visit the flowers for the nectar. A bit of an extraordinary approach to ensuring pollination, but desperate times call for desperate measures.
A lot of insects need a 'fix' of carbohydrates, and a 'fix' of protein. They get the carbohydrate from the nectar of flowers. How they get their protein varies. Flies will get it from carrion, etc. Wasps will mostly get it from eating other insects. March flies get it from sucking blood from animals (and people). Bees and some wasps get it from pollen. There is one Trigona species bee that gets it from carrion. But the common point is the nectar, and the pollination that occurs as a result.
Now I'm sure you won't want to hang old dead carcases around your beans, flowers would be far more appealing ;O)
I completely agree with tropicbreeze -- only introduce them if they are native to where you live. Introducing non-native insects can have a devastating effect to the native flora and fauna and can wipe out entire species of natives. Most countries forbid non-native species of insects from being imported and entering the country. In the US, importing non-native species is a federal offense and millions are spent each year to prevent non-native insects from entering the country. Why do you want to introduce bumble bees, for what purpose? If you are looking to produce honey, you want honey bees, not the vicious bumble bee!
Whoa! I do not want to introduce bumble bees! I just want to find out if I can help the population of the bees that are already here! They do not attack me.
And I know that Australia has a disaster going on with the introduced Cane Toad, an amazing creature that can grow to five pounds or more, is poisonous, and is eating everything it can get in it's mouth.
Lionfish were released into the Atlantic and Caribbean by Aquarists who would buy a three inch baby that grew to a foot long in one year. Too bad the pet shops weren't required to buy back the big fish. This is an unseen disaster, the fish are changing the character of the sea, eating everything, and reproducing freely, having no predators. Oh, well, I won't get up on that soapbox.
Ah, well, it's a terrible thing that restrictions and regulations weren't in place back in the year 1350.