234 Foraging Pattern Comparisons between Two Commercial Pollinators on Field-grown Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai

Abstract

The need for alternative [non-honey bee (Apis mellifera L.)] pollinators continues to increase as the number of problems facing the American beekeeping industry increase. One readily available alternative pollinator source is commercially produced bumblebee (Bombus spp.) colonies. From 1997 to 1999, three studies were conducted to compare the pollination efficacy of bumblebee and honeybee pollinators on field-grown watermelon. The experiments documented 1) bee activity periods (the onset and termination of foraging behavior in association with watermelon anthesis and duration), 2) floral visitation rates (number of flowers visited per unit time by individual foragers), and 3) stigmatic pollen deposition (number of pollen grains deposited on stigmata during single bee visits to pistillate watermelon flowers over the course of anthesis). Bumblebees outperformed honeybees in all three comparative experiments. Bombus foragers initiated foraging activity 30 to 60 min before the appearance of the first honey bee foragers. Both bee types continued to forage throughout anthesis once appearing in the field. Individual bumblebees consistently visited two or more times the number of flowers per min than did honeybees (P < 0.0001) throughout the day excluding the initial 30 to 60 min when honeybees did not forage. The number of pollen grains deposited in an initial visit to stigmata by Bombus foragers was consistently greater than for honeybees (P < 0.001). For both bee types, pollen deposition was influenced by time of day, peaking at 0900 hr and then declining until 1200 HR, when the flowers closed. Both foraging rates and pollen deposition favored bumblebees over honeybees regardless of time of day.