Impacts of honey bee density on crop yield: A meta-analysis

ORIANNE ROLLIN; LUCAS ALEJANDRO GARIBALDI

Congreso; Eurbee 8: 8th Congress of Apidology; 2018

There is increasing recognition that pollination deficits are limiting crop yields worldwide. However, management strat-egies for optimal pollination are still unclear for most crops. Current management focuses on providing high densitiesof honey bees, but recommended densities are highly variable, even within single crops and varieties.We performed an extensive literature search to record honey-bee densities (colony density and/or flower visitation rates)and crop productivity (fruit set, seed set, fruit weight, and/or yield). Effect sizes represented the difference in crop produc-tivity between the two most extreme levels of honey-bee densities.Surprisingly, out of 795 reviewed studies, only 22 analyzed the effect of at least two levels of honey-bee densities oncrop productivity (reporting 60 effect sizes in total). Thus, most recommendations for crop pollination management arenot based on proper experimental designs.We found that both colony density and visitation rates increased all the productivity variables. However, effects werenon-linear for visitation rates, suggesting that there is an optimum (mean of 8-10 visits per flower) beyond which morehoney bees are not beneficial (or even detrimental) for crop productivity.Effect sizes for visitation rates were greater than those for colony densities, suggesting that visitation rates are a moredirect measure of the pollination process. Data on the relation between colony density and visitation rates are lacking.Interestingly, effect sizes of visitation rates were greater for crops with separate sexes than those with hermaphroditeflowers; therefore, the benefits from honey-bee pollination varies according to the crop biology.Synthesis and applications. Current practices for crop pollination assume that more honey bees are always better forcrop yield. However, our analyses suggest that there is an optimum of honey-bee densities. Despite the importance ofhoney bees and pollinator-dependent crops worldwide, there is a lack of studies designed for finding such an optimumlevel of crop pollination. Our analyses further suggest that visitation rates could be used as a proxy to guide manage-ment recommendations such as colony density and spatial arrangement.