Pollination services of managed common eastern bumble bees (Bombus impatiens) in seedless watermelon (Citrullus lanatus)

Common eastern bumble bees: A) at the entrance of a commercial colony and 
B) visiting a watermelon flower.

By Mateos-Fierro et al.

Pollinator-dependent crops rely on pollinating insects for successful pollination and thereby achieving commercial yields. Seedless watermelon (Citrullus lanatus) is particularly dependent on insect pollination, as pollinators must transfer pollen from pollen-donor plants. While pollen transfer can be provided for free by wild pollinators, these gratis pollination services are typically insufficient. As a result, growers use managed pollinators. The western honey bee (Apis mellifera) has historically been the only managed species employed, but recently, growers have also supplemented with bumble bees, such as the common eastern bumble bee (Bombus impatiens). However, adding a second managed pollinator entails a greater investment, which may not pay off, particularly since the role of managed bumble bees in seedless watermelon production is uncertain.

In this study, we investigated the pollination contribution of managed bumble bees by combining field observations and experiments. Field observations were conducted in commercial seedless watermelon fields; all fields were stocked with honey bees, but only some were also supplemented with bumble bees. We recorded the number of flower visits and the behaviour of different pollinator groups. Additionally, we conducted a powder tracing experiment in which we sprayed fluorescent powder into the bumble bee colonies so that they would leave traces on flowers. Finally, we analysed pollen collected from bumble bees and their colonies to explore what percentage of the pollen belonged to watermelon.

Our field observations revealed that bumble bees were infrequent visitors to watermelon flowers. Supplementing fields with managed bumble bees did not increase watermelon visits across the entire field, but visits increased locally, within 10 m around their colonies. Additionally, managed bumble bee presence in the fields influenced other pollinators’ behaviours, but this differed across pollinator groups. Small solitary bees were, in general, more affected. The powder tracing experiment indicated little managed bumble bee activity, with only 1.5% of flowers showing powder traces, whereas the pollen analysis revealed that watermelon pollen made up only 2.9% of the pollen collected by bumble bees.

These data suggest that stocking fields with managed bumble bees may not increase visits to watermelon flowers and therefore may not be important contributors to watermelon pollination. However, their direct contribution to yield and whether an increase in stocking rate can increase visits to flowers have yet to be explored.

Read the scientific article in JPE