Fruit Set and Single Visit Stigma Pollen Deposition by Managed Bumble Bees and Wild Bees in Citrullus lanatus (Cucurbitales: Cucurbitaceae)

Abstract

Pollinators provide essential services for watermelon, Citrullus lanatus (Thunb.; Cucurbitales: Cucurbitaceae). Managed bumble bees, Bombus impatiens (Cresson; Hymenoptera: Apidae), have been shown to be a useful watermelon pollinator in some areas. However, the exact contribution bumble bees make to watermelon pollination and how their contribution compares to that of other bees is unclear. We used large cages (5.4 × 2.5 × 2.4 m) to confine bumble bee hives to watermelon plants and compared fruit set in those cages to cages containing watermelons but no pollinators, and to open areas of field next to cages (allows all pollinators). We also collected data on single visit pollen deposition onto watermelon stigmas by managed bumble bees, honey bees, and wild bees. Overall, more fruit formed within the open cages than in cages of the other two treatment groups. B. impatiens and Melissodes spp. deposited the most pollen onto watermelon stigmas per visit, but all bee species observed visiting watermelon flowers were capable of depositing ample pollen to watermelon stigmas. Although B. impatiens did deposit large quantities of pollen to stigmas, they were not common within the field (i.e., outside the cages) as they were readily drawn to flowering plants outside of the watermelon field. Overall, bumble bees can successfully pollinate watermelon, but may be useful in greenhouses or high tunnels where watermelon flowers have no competition from other flowering plants that could draw bumble bees away from watermelon.

Floral bagging differentially affects handling behaviours and single‐visit pollen deposition by honey bees and native bees

“…Our results support this reasoning. In contrast, Campbell et al (2018) report much higher values for honey bee and native bee single-visit pollen deposition than even those we documented on flowers with accumulated nectar. These differences between datasets suggest caution must be exerted when comparing the relative magnitude of pollen deposition values across experiments, as this can be influenced by numerous study-specific factors.…”

“…Although we found only honey bee single-visit pollen deposition was prone to overestimation with floral bagging, it is probable that other bee pollinators may share this tendency. We suspect that bumble bees, which are important pollinators of watermelon (Stanghellini et al, 2002;Campbell et al, 2018), are likely prone to this overestimation just like honey bees. Nectar supplementation experiments have documented increased pollen deposition and floral visit duration time by bumble bees that visited flowers with enriched levels of nectar (Thomson & Plowright, 1980;Thomson, 1986;Cresswell, 1999;Ollerton et al, 2007).…”

Supplemental forage ameliorates the negative impact of insecticides on bumblebees in a pollinator-dependent crop

“…However, it is unlikely a behavioural result of ‘forcing’ bees to forage on the insecticide-treated crop owing to a lack of alternatives, because clover presence did not affect the visitation rate on watermelon flowers. Watermelon is not a preferred resource for bumblebees in the field [ 32 , 33 , 39 ], but in our experiment, they readily visited these flowers even when surrounded by clover, which is considered a high-quality forage whose flowers are open and accessible during the same time of day as watermelon (i.e. temporal coincidence in bloom times).…”

“…This means that bumblebees in our arenas were at least five times higher than the recommended stocking rate for optimal pollination. Furthermore, watermelon requires relatively few visits from bumblebees for successful pollination [ 25 , 32 , 33 ]. As a result, strong insecticide-mediated reductions in crop foraging appeared to offset the fact that efficient pollinators were present at such high densities.…”

“…The focal crop used in this system was watermelon, which is highly reliant on managed bees for pollination, requiring at least 12 bumblebee visits per flower for optimal yield [ 25 , 32 , 33 ]. We used a diploid pollenizer variety, AcePlus, to optimize flower availability within the limited foraging space.…”