Stigmatic Pollen Grain Deposition by Honey Bees and Bumble Bees after Single Bee Visits to Pistillate Watermelon Flowers

Abstract

As a means to assess pollinator efficiency, the number of pollen grains deposited by honey bees (Apis mellifera L.) and bumble bees (Bombus impatiens Cresson) after single visits to pistillate watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] flowers was compared. Pistillate flowers were enclosed in nylon bags prior to anthesis, and were then exposed to single visits by each bee species (RCBD: 10 flowers per bee species per replicate; 4 replicates). Pollen grains were removed from stigmata immdiately after bee visitation by use of a glycerin jelly-gelatin mixture that was mounted onto microscope slides. Light microscopy was used to count the number of pollen grains per slide. Bumble bees deposited significantly more pollen grains than did honey bees (P < 0.05). After single bee visits to flowers, bumble bees deposited an average of 481 pollen grains, while honey bees deposited an average of 119 pollen grains. Although multiple bee visits are usually required to set watermelon fruit, this work demonstrates that bumble bees are more-efficient pollinators of watermelon at the single visit level than are honey bees.