Floral resource wastage: Most nectar produced by the mass-flowering crop oilseed rape (Brassica napus) is uncollected by flower-visiting insects.

Abstract

Cultivation of the mass-flowering crop oilseed rape (OSR), Brassica napus, can provide insects with super-abundant nectar and pollen while in bloom. Several authors have suggested breeding cultivars to produce more abundant nectar and pollen to help mitigate insect decline. However, in Britain most, 95%, OSR blooms in spring (March-May), which has been suggested to be a period of nectar surplus and reduced exploitative competition. Therefore, a large proportion of floral resources produced by OSR during this period may be uncollected. Although there has been extensive work examining OSR nectar and pollen production, no study, to our knowledge, has measured this in relation to the demand by the flower-visiting insects. Here we quantified the percentage of nectar produced by spring blooming OSR which was uncollected in four OSR fields per year over 2 years. This was achieved by measuring the nectar in both insect accessible and inaccessible (i.e. mesh-covered) flowers. We also quantified uncollected pollen in flowers at the beginning and the end of anthesis using a haemocytometer. Most of the nectar (69%) and a fifth of pollen (19%) was uncollected in spring blooming OSR. Based on the estimates of nectar production and observed number of insects, nectar supply per insect was estimated at 2204 μL nectar insect-1 h-1, which exceeds potential collection rates by flower-visiting insects. Given the majority of B. napus is spring blooming, breeding cultivars of OSR which produce more nectar, while not being detrimental to flower-visiting insects, may be of little conservation benefit.