Unfortunately, wild nonhoney bee populations are unpredictable. They're numerous in some places and scarce in others. It's hard for farmers and gardeners to depend on them as their sole source of pollination. Methods for raising most nonhoney bees are poorly worked out. So the best way to increase their numbers is to practice bee conservation and encourage growth in natural populations.
Bumble bees like to nest in thick grass, abandoned rodent nests, or abandoned furniture.
You can conserve bumble bee nesting areas by leaving idle lands undisturbed. Don't plow them or mow them regularly. You may have to mow from time to time to keep shrubs or trees from taking over. Soil-nesting bees prefer sunny patches of bare soil, soil lightly covered with leaf litter or sparse grass, or exposed dirt banks. When you find a bee-nesting site, flag the area so you can protect it.