Do sown flower strips boost wild pollinator abundance and pollination services in a spring-flowering crop? A case study from UK cider apple orchards

Abstract

Flower strips are widely recommended as a tool to boost insect pollinators and yield in pollinator-dependent crops. Using UK cider apple orchards (Malus domestica Borkhausen) as a model system, we assessed whether flower strips increased pollination services in orchards. Pollinator communities (visual observation) and pollination services (fruit set) were assessed at increasing distance from surrounding semi-natural habitats (0–200m) in eight orchards. In four orchards, perennial flower strips had been established and bloomed in the year before the main experiment. In a separate experiment, insect visits to apple flowers were observed to investigate possible functional mechanisms underpinning pollinator efficacy.The visit rate of wild insects to apple flowers (non-Apis bees and flies), but not that of honeybees (Apis mellifera L.), increased by 40% in flower strip orchards compared to control orchards, particularly in areas close to semi-natural habitat (<100m). Wild insect visitation was also positively related to dandelion (Taraxacum species) abundance in orchards. Fruit set in orchards was positively related to wild insect richness, and andrenid bee (Andrena species) visitation, but neither richness nor andrenid bee visit rate responded positively to flower strips. Wild bees (andrenid bees and bumblebees (Bombus species)) contacted apple stigma (95 and 100% of visits) more often than honeybees (81%), but only bumblebees moved frequently between different tree rows, an important trait for transfer of compatible pollen in apples.Our results demonstrate that flower strips enhanced overall wild insect abundance but not pollination services in cider orchards. Positive effects of ground flora on wild insect abundance in orchards suggest that flower mixtures or orchard management could be optimised for andrenid bees, the single most important pollinator taxa, by increasing the availability of early-flowering plants in orchards. Equally, wild insect richness was highest in areas close to semi-natural habitats. Therefore, whilst flower strips can boost abundance of the existing species pool, only large scale preservation of (semi-) natural habitat will maintain pollinator diversity in apple orchards.