The utility and use of solitary bees for pollination in greenhouses and other covered growing systems: A review

Globally, the area of protected cropping has grown exponentially in the past 25 years. Concomitantly, the number of greenhouse crop species has increased, and a substantial proportion of these rely on, or benefit from, bee pollination. Both in crops grown outside and under cover, diversity in pollinating species could create resilience in this important service. Here we review, for the first time, the utility and use of solitary bee species for crop pollination in protected cropping environments. An increasing number of bee species, including solitary bees, are managed for pollination of crops grown outside, and our review shows that there is no shortage of solitary species able to pollinate crops grown under protective covers. Despite this, only four solitary species are used on a small commercial scale, and only in orchard crops, while pollination in greenhouses worldwide is delivered by a few, often introduced, bumble bee species. We argue that there are two impediments to the use of solitary bees for pollination in protective cropping environments: (a) Bumble bees satisfy most current greenhouse requirements, and (b) There are intrinsic difficulties in relation to husbandry and reliable large-scale supply of solitary bees, in particular when they are required year-round. We explore the possibility to overcome the second impediment by selecting species with suitable life-history traits or by deploying multiple species. Structured Abstract Purpose To review the utility and use of solitary bee species for pollination of crops grown under covers, and to investigate what could be done to enhance the use of solitary bees in such environments. Basic Procedures Using a systematic literature search, we review the solitary bee species that have been experimentally tested for their efficacy as pollinators of greenhouse and protected orchard crops. Studies included measured efficacy as increased seed/fruit set, fruit quality, pollination deposition or, where appropriate, buzz pollination activity, relative to controls. We also searched the literature for evidence of use of solitary bees in the pollination of crops under cover. Main findings The pollination efficacy of 45 solitary bee species had been tested in 94 bee-crop species combinations, with 87% of tests yielding positive outcomes. Representatives of different bee families differed in their efficacy for the pollination of different crop species. Despite their utility, only four species are used commercially in orchards, and none in greenhouses, because(a) bumble bees satisfy most current requirements; (b) there are intrinsic difficulties in relation to husbandry and reliable year-round supplies of solitary bees in sufficient numbers. New and important aspects We provide a global review and juxtapose utility and use of solitary bees. We also explore, for the first time, the difficulties posed by life-history traits on the deployment of solitary bees for the pollination of greenhouse crops and discuss whether and how these could be overcome.