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.
Bees tend to nest in the same area year after year.
So if you leave a bee site undisturbed for the long term, the bee population can grow. As for food plants, bees need flowers that produce a lot of nectar and pollen. Ideally, something will be blooming all spring, summer and fall. Good bee plants should be attractive to bees, easy to grow, vigorous and noninvasive. And their bloom period shouldn't compete with the crop for pollinators.