Spillover of managed bumblebees from Mediterranean orchards during mass flowering causes minor short-term ecological impacts

Abstract

Commercial bumblebee colonies are routinely used for crop pollination in greenhouses and are increasingly introduced into orchards as well. Bumblebee spillover to natural habitats near the orchards may interfere with local wild bees and impact the pollination of non-crop plants. Concurrently, foraging in natural habitats may diversify the bumblebees’ diets and improve colony development. To evaluate these potential effects, we placed commercial Bombus terrestris colonies in blooming Rosaceae orchards, 25–125 m away from the margins. We recorded the colonies’ mass gain, population sizes, composition of stored pollen, and temperature regulation. We monitored bee activity, and seed sets of the non-crop plant Eruca sativa, along transects in a semi-natural shrubland up to 100 m away from the orchards, with managed bumblebees either present or absent. Rosaceae pollen comprised ~ 1/3 of the colonies’ pollen stores at all distances from the orchard margins. Colonies placed closest to the margins showed prolonged development, produced fewer reproductive individuals, and had poorer thermoregulation than colonies closer to the orchards’ center. Possibly, abiotic stressors inhibited the bumblebees’ development near orchard borders. Wild bees were as active during the colonies’ deployment as after their removal. E. sativa’s seed sets decreased after bumblebee removal, but similar declines also occurred near a control orchard without managed bumblebees. Altogether, we found no short-term spillover effects of managed bumblebees on nearby plant-bee communities during the orchards’ two-week flowering. The colonies’ prompt removal after blooming can reduce longer-term ecological risks associated with managed bumblebees.