Citrus flowers are predominantly pollinated by honey bees in California. Other insects such as wasps, flies, thrips and mites may visit and occasionally transfer pollen from one citrus flower to another, but are not the principal pollinators. After pollen reaches the stigma of the female organ, the pollen swells and forms a pollen tube which grows down through the female organ. This pollen tube delivers the two enclosed sperm cells to the egg cell and central cell within the ovule deep within the female organ where fertilization occurs. Fertilization is the fusion of the one sperm cell with the egg, and the other sperm cell with the central cell nuclei.
Fertilization completes the sexual process, and as a result produces a seed from a fertilized ovule.
The seed and the tree that develops from it are genetically different from either of the two parents from which it derived. Pollination, fertilization and the resulting seeds that are formed by these events, set the stage for a whole suite of other events which are essential for a portion of the female organ of the flower to develop and mature into a fruit. It is the variations in this sexual process that are responsible for the production of seedless citrus fruit. There are four main variations that occur among citrus cultivars. One of these variations is parthenocarpy , or the ability to produce fruit without stimulus of sexual fertilization and therefore without seeds.