The effectiveness of flower strips and hedgerows on pest control, pollination services and crop yield: a quantitative synthesis.

Floral plantings are promoted to foster ecological intensification of agriculture through provisioning of ecosystem services. However, a comprehensive assessment of the effectiveness of different floral plantings, their characteristics and consequences for crop yield is lacking. Here we quantified the impacts of flower strips and hedgerows on pest control (18 studies) and pollination services (17 studies) in adjacent crops in North America, Europe and New Zealand. Flower strips, but not hedgerows, enhanced pest control services in adjacent fields by 16% on average. However, effects on crop pollination and yield were more variable. Our synthesis identifies several important drivers of variability in effectiveness of plantings: pollination services declined exponentially with distance from plantings, and perennial and older flower strips with higher flowering plant diversity enhanced pollination more effectively. These findings provide promising pathways to optimise floral plantings to more effectively contribute to ecosystem service delivery and ecological intensification of agriculture in the future.

Effects of temporal floral resource availability and non-crop habitats on broad bean pollination.

Context: Flowering plants can enhance wild insect populations and their pollination services to crops in agricultural landscapes, especially when they flower before the focal crop. However, characterizing the temporal availability of specific floral resources is a challenge. Objectives: Developing an index for the availability of floral resources at the landscape scale according to the specific use by a pollinator. Investigating whether detailed and temporally-resolved floral resource maps predict pollination success of broad bean better than land cover maps. Methods: We mapped plant species used as pollen source by bumblebees in 24 agricultural landscapes and developed an index of floral resource availability for different times of the flowering season. To measure pollination success, patches of broad bean (Vicia faba), a plant typically pollinated by bumblebees, were exposed in the center of selected landscapes. Results: Higher floral resource availability before bean flowering led to enhanced seed set. Floral resource availability synchronous to broad bean flowering had no effect. Seed set was somewhat better explained by land cover maps than by floral resource availability, increasing with urban area and declining with the cover of arable land. Conclusions: The timing of alternative floral resource availability is important for crop pollination. The higher explanation of pollination success by land cover maps than by floral resource availability indicates that additional factors such as habitat disturbance and nesting sites play a role in pollination. Enhancing non-crop woody plants in agricultural landscapes as pollen sources may ensure higher levels of crop pollination by wild pollinators such as bumblebees. Supplementary information: The online version contains supplementary material available at 10.1007/s10980-022-01448-2.

Dominance of cropland reduces the pollen deposition from bumble bees.

Intensive agricultural landscapes can be hostile for bees due to a lack of floral and nesting resources, and due to management-related stress such as pesticide use and soil tillage. This threatens the pollination services that bees deliver to insect-pollinated crops. We studied the effects of farming intensity (organic vs. conventional, number of insecticide applications) and availability of semi-natural habitats at the field and landscape scale on pollinator visits and pollen delivery to pumpkin in Germany. We found that wild bumble bees were the key pollinators of pumpkin in terms of pollen delivery, despite fivefold higher visitation frequency of honey bees. Critically, we observed that the area of cropland had stronger effects on bees' pollen deposition than the area of seminatural habitats. Specifically, a 10% increase of the proportion of cropland reduced pollen delivery by 7%. Pumpkin provides a striking example for a key role of wild pollinators in crop pollination even at high numerical dominance of honey bees. In addition, our findings suggest that habitat conversion to agricultural land is a driver of deteriorating pollination. This underlines the importance to maintain sufficient areas of non-crop habitats in agricultural landscapes.

Sensitivity of commercial pumpkin yield to potential decline among different groups of pollinating bees.

The yield of animal-pollinated crops is threatened by bee declines, but its precise sensitivity is poorly known. We therefore determined the yield dependence of Hokkaido pumpkin in Germany on insect pollination by quantifying: (i) the relationship between pollen receipt and fruit set and (ii) the cumulative pollen deposition of each pollinator group. We found that approximately 2500 pollen grains per flower were needed to maximize fruit set. At the measured rates of flower visitation, we estimated that bumblebees (21 visits/flower lifetime, 864 grains/visit) or honeybees (123 visits, 260 grains) could individually achieve maximum crop yield, whereas halictid bees are ineffective (11 visits, 16 grains). The pollinator fauna was capable of delivering 20 times the necessary amount of pollen. We therefore estimate that pumpkin yield was not pollination-limited in our study region and that it is currently fairly resilient to single declines of honeybees or wild bumblebees.