Managed and wild bees contribute to alfalfa (Medicago sativa) pollination

Abstract

Biotic pollination is an essential ecosystem service for agricultural production and is reflected in the high number of crops that depend on insect pollination in order to produce profitable yields. Alfalfa (Medicago sativa) is a crop whose flowers need to be visited by a bee to be pollinated for seed production, making it highly pollinator-dependent. Two managed bee species are currently used to pollinate this crop: the alfalfa leafcutting bee (ALCB), one of the most efficient alfalfa pollinators, and the honey bee (HB), whose efficiencies could be highly variable among production sites. Besides, there are many other wild bee species (WBs) that are effective pollinators of alfalfa, but little attention has been placed on them, especially in Argentina, where alfalfa seed production is deficient. Here, we evaluate the contribution of both managed species and WBs on alfalfa pollination services in one of the most important alfalfa seed productive regions of Argentina. During the span of two years, we calculated pollen limitation in different pollinator managed scenarios: with and without ALCBs (ALCB+ and ALCB−, respectively), and we also evaluated the relationship between pollen limitation and all bee species visitation rates. Our results show that the ALCB is a very effective pollinator of alfalfa, since ALCB+ lots have less pollen limitation (32%) compared to ALCB− lots. In contrast, HBs seem to have a detrimental effect in pollination service in ALCB+ lots and a positive effect in ALCB− lots. This differential effect could be due to differences in foraging behavior caused by competition between HBs and ALCBs. Finally, in spite of their low abundances, we found that an increase in WB visitation rates substantially reduces pollen limitation. In spite of ALCBs being a good alfalfa pollinators, the difficulty and cost of managing them hinder its widespread use. The use of WBs seems to be a good alternative, and practices that improve their abundance and diversity should be implemented to improve alfalfa pollination service.