Bees and perhaps other flying insects act as pollinators. This reader also has a grapefruit tree that isn't fruiting and wondered about the care. The soil must drain quickly. No citrus likes its roots sitting in water. Before planting, you may want to incorporate peat moss, sawdust or ground bark.
Grapefruit requires moist soil but never free-standing water.
It needs air in the soil. Established trees should be watered every other week. If newly planted, water at least twice a week. Don't let the trees get to the wilting point. Grapefruit tree roots grow near the surface of the soil as well as deeper, so mulch is beneficial.