Phenological imbalance in the supply and demand of floral resources: Half the pollen and nectar produced by the main autumn food source, Hedera helix, is uncollected by insects

Abstract

Declines in floral resources, pollen and nectar, are considered one cause of pollinator decline. However, the supply and demand of floral resources can vary temporally. In Britain, autumn has been suggested as a period of high floral resource availability due to the flowering of ivy (Hedera helix), a common native plant, combined with fewer insects active during this season. Here, we directly quantified the proportion of pollen and nectar produced by ivy, the primary autumn food source, which is uncollected by the flower‐visiting insect community. We quantified the proportion of nectar produced but uncollected by comparing the mass of nectar sugar accumulated in insect‐accessible versus inaccessible ivy flowers and by surveying the presence of wasted, crystalised, nectar on flowers. Pollen wastage was quantified by comparing pollen counts on anthers at the start of anthesis versus anthers dropped from ivy flowers. Approximately, half the floral resources, 59% nectar and 44% pollen, were uncollected by the flower‐visiting insect community in autumn. As ivy flowers supply most of the available nectar and pollen in autumn, our results show that a large proportion of all floral resources are wasted in autumn. Our results are the first to show that a season can be characterised by a large surplus of floral resources relative to collection by flower‐visiting insects. These results demonstrate the importance of considering seasonal variation in floral resources in the conservation of bees and other flower‐visiting insects.

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

“…( 2016 ) reported that summer has the greatest nectar supply, most insects reach peak activity in summer (Balfour et al., 2018 ), resulting in low per‐capita nectar supply and greater exploitative competition (Timberlake et al., 2019 ; Wignall et al., 2020 ). Additionally, approximately half of pollen and nectar produced by the main autumn (September–November) food source in Britain, ivy ( Hedera helix ), are uncollected due to lower insect activity and high floral resource availability (Harris et al., 2023 ). This highlights that improving floral resources may not be as simple as ‘more is better’, particularly during periods, such as spring, where flower‐visiting insect numbers may be lower.…”

“…Nectar can be defined as ‘wasted’ if it is left unconsumed by the flower‐visiting insect community, which will result in it being reabsorbed by the plant or discarded with the wilted flowers (Burquez & Corbet, 1991 ; Gill, 1978 ; Harris et al., 2023 ). To directly quantify the volume of nectar wasted, nectar accumulation was quantified and compared in flowers which were either accessible or inaccessible to insects.…”

“…Gauze bags are recommended for measuring nectar production rate because it has minimal effect on the contained microclimate and therefore has little effect on nectar production (Corbet, 2003). Greater demand by the flower-visiting insect community and more frequent insect visitation will cause lower nectar accumulation in accessible versus inaccessible flowers (Gill, 1978;Harris et al, 2023;Page & Williams, 2023).…”

“…For example, although Baude et al (2016) reported that summer has the greatest nectar supply, most insects reach peak activity in summer (Balfour et al, 2018), resulting in low per-capita nectar supply and greater exploitative competition (Timberlake et al, 2019;Wignall et al, 2020). Additionally, approximately half of pollen and nectar produced by the main autumn (September-November) food source in Britain, ivy (Hedera helix), are uncollected due to lower insect activity and high floral resource availability (Harris et al, 2023). This highlights that improving floral resources may not be as simple as result that is qualitatively consistent with a lack of food limitation (Elliott, 2009), further work is needed to explore the fitness consequences of temporal gaps or surpluses in pollen and nectar supply, particularly acquiring a better understanding of the specific requirements of target insect species and how their needs change throughout their life cycle (Schellhorn et al, 2015).…”

Quantifying the production of plant pollen at the farm scale

“…It should be noted that although we can quantify the supply of pollen through the year, our understanding of pollen demand by bees and other insects is highly limited, making it difficult to infer anything about the limitation of this resource at different times of year (Sponsler et al ., 2023). In the absence of detailed data on pollinator nutritional requirements, other approaches can be used to assess resource limitation including foraging assays (Sponsler et al ., 2020) and pollen depletion studies (Harris et al ., 2023).…”