Why is loyalty so important to bumblebees?

There are about 250 species of bumblebees worldwide. Particularly noteworthy: Each bumblebee is faithful to a single species of flower for however long it’s in bloom. That’s important for the plant, since the insects are only carrying the pollen of that plant’s own type with them, which makes for optimal conditions for reproduction and growth. But if a species of bumblebee is out of commission— as was shown in an open-air experiment— the plants end up producing one-third fewer seeds. Substitutes have little impact: Other bumblebees, which abandon their loyalty due to the freed-up flowers, end up carrying different types of pollen to different types of flowers, and nothing fruitful comes of foreign pollen.