Pollination deficits and contributions of pollinators in apple production: a global meta-analysis

1. Apple is one of the most widely cultivated fruit crops worldwide, and
apple yield benefits from pollination by insects. The global decline in
wild pollinator populations raises concern about the adequacy of
pollination services in apple production. 2. Here, we present a global
meta-analysis of pollination in apple. We assembled from the literature a
dataset comprising results of 48 studies across five continents on fruit
set and seed set in apple with insect pollination, artificial pollination
and pollinator exclusion, and analysed the effects of explanatory factors
such as variety and continent. 3. Fruit set was on average 41% lower with
open pollination than with artificial pollination, while seed set was 20%
lower. These pollination deficits varied across continents and cultivars.
Pollination deficits for fruit set were greatest in Asia (63%) followed by
Europe (30%), whereas pollination deficits for seed set were greatest in
Asia (47%) and South America (40%). Important differences in pollination
deficit were also identified between cultivars but these differences were
confounded with continent effects. 4. Fruit set and seed set were 71% and
62% higher, respectively, when insects had open access to flowers than
when they were artificially excluded, while results varied among
cultivars. 5. Synthesis and Applications. Globally, there are substantial
contributions of pollinators to fruit set and seed set in apple, as well
as considerable limitations in apple pollination services, particularly in
Asia, Europe and South America. Several management strategies could be
applied to reduce the pollination deficits in apple production: (1)
conserving wild bees and enhancing their abundance and diversity, (2)
using managed bees for pollination, (3) using varieties with low
pollinator dependency, and/or (4) artificial pollination. These strategies
should be tailored to the regional situation, considering the potential of
landscapes for restoring wild pollinators, the acceptability of cultivated
varieties for available pollinators, the acceptance in the market of
self-compatible varieties, and the costs of management, such as artificial
pollination, pollinator conservation, beekeeping and planting
self-compatible varieties. Conservation of wild pollinators is preferred
in regions with sufficient potential for wild pollinators as it
contributes to biodiversity conservation and improves pollination in both
crops and wild plants.