Overhead sprinkler irrigation affects pollinators and pollination in pumpkin (Cucurbita maxima)

Cucurbits are monoecious plants and most of them depend on pollinators for fruit set. However, studies suggest that pumpkin and squash fields are pollination deficient. Pollinator limitation, suboptimal flower sex ratio, and exploitative competition between invasive ants and bees are a few major suggested reasons for this. Pumpkin flowers have a corolla cup and secrete nectar throughout anthesis, which make them attractive to bees. In south India, honey bee (Apis cerana) is the major pollinator of pumpkin flowers. Like pollinators, water is also a limiting factor for growth and production of flowers in pumpkin. It has been suggested to use overhead sprinklers to irrigate and fertilize pumpkin fields. We hypothesized that the sprinkler affects the visitation characteristics of foraging bees in flowers and pollination, and so, we performed an experiment to test this. For this reason, we recorded pollinator visits on staminate and pistillate flowers for 15 min and poured water in the flower cups and examined the water retention time and the visitation characteristics of pollinators in both staminate and pistillate flowers, and fruit set in pistillate flowers. Water remained inside the staminate and pistillate flower cups until the flower senescence. Honey bees entered and foraged nectar/pollen in flowers during the waterless condition. In water-filled flowers, honey bees hovered around or landed only on the corolla and did not forage floral resources or contact the reproductive parts. As a consequence, none of the water-filled flowers developed fruits. Therefore, the overhead sprinkler irrigation, despite cover a large area effectively, may not be an appropriate irrigation method for pumpkin during flowering period.