Oilseed rape and pollinators: the impact of variety on resource availability and pollination resilience

Abstract

Mass-flowering crops help support the nutritional demands of insect pollinators in agricultural environments. With an estimated 70% of crops experiencing increased yields through animal pollination, recent declines in pollinator abundance and diversity have severe consequences to global food production. Oilseed rape (Brassica napus) is the most abundant oilseed crop in Europe and experiences enhanced yield from insect pollination. Subject to intensive commercial breeding programmes, growers face continuous annual variety selection, with new varieties offering increased yields and more favourable agronomic characteristics. At a critical time for pollinators, little is known about the effects that variety selection may have on resource provisioning. This thesis examines the impact of pollination on oilseed rape and the inter-dependence between pollinators and growers, with an emphasis on variety type and the breeding systems used to produce them. The value of oilseed rape to the insect community was studied. Insect visitor surveys were undertaken in fields of conventional and hybrid varieties of oilseed rape, comparing the abundance and species composition between the field centre and crop edge, adjacent to semi-natural habitat. Overall, insects were more abundant and diverse at the edge of the crop than the field centre. While conventionally recognised pollinators (e.g. bees) were scarce during flowering, bumblebees were most abundant, particularly in the crop centre, whereas solitary bees favoured the crop edge. However, Diptera abundance was high, suggesting that their contribution to oilseed rape pollination in Scotland is more significant than that of bees. Conversely, the contribution of insect pollination to oilseed rape yield was estimated through pollinator exclusion experiments. Insect pollination increased seed set by 23% and seed weight per pod by 29%. Evidence of resource allocation was found, where plants with flowers subject to pollen limitation redirected resources to other parts of the plant. Increased pollinator abundance did not have a positive effect on the proportional contribution of pollinators for any of the yield metrics measured. To measure the effect of pollination on plant development and reproduction, glasshouse experiments, comparing wind- and insect-simulated pollination against a control were undertaken. The addition of supplementary pollination had significant effects on vegetative and reproductive metrics. Both wind- and insect-simulated pollination produced shorter plants, a reduced flowering period and the number of flowers produced per plant. Although plants receiving supplementary pollination produced lighter individual seeds, they produced a greater number of seeds per pod. In combination with increased fruit set, this resulted in a greater overall seed weight per plant. The prediction of floral resource availability (i.e. nectar and pollen) using oilseed rape agronomic characteristics was also investigated. Multiple regression analysis and predictive modelling were used to conclude that agronomic traits influence nectar sugar content and pollen quantity in oilseed rape. Contrary to the expectation that developing varieties with desirable traits for growers may come at a cost to floral resources, the opposite was found. Varieties with a higher tolerance to stressful environmental factors, particularly those found during winter, offered more nectar sugar per flower. The opposite was found for pollen, where early maturity, a desired trait for growers, had a negative effect on pollen quantity. Statistical analysis also highlighted the influence of short-term climatic changes on the sugar content of nectar. Conclusions indicate that the inter-relationship between oilseed rape and pollinators is complex but has the potential to be mutually beneficial. The floral rewards offered by oilseed rape attract a plethora of insect pollinators during a period of resource scarcity. In return, pollinators have a significant effect on plant development and seed production. Furthermore, by making considered varietal choices, oilseed rape growers can increase the potential to financially benefit from this mutualistic relationship by exploiting this valuable ecosystem service.