Pollen grain morphology is not exclusively responsible for pollen collectability in bumble bees

Abstract

Bee-pollinated plants face a dilemma in that bees both passively transport pollen grains among conspecific flowers and actively collect pollen to feed their larvae. Therefore, mechanisms that reduce pollen collection by bees have evolved in melittophilous plants. Malvaceae pollen is uncollectable for corbiculate bees which has previously been ascribed to pollen size, spines, and pollenkitt. We analysed the influence of pollen grain properties (diameter, spine length, spine density) on the collectability of echinate (spiny) pollen by bumble bees (Bombus terrestris). Workers individually foraging on one of eight plant species from six families performed significantly less pollen foraging on plants which have large, echinate pollen grains. Nevertheless, neither pollen grain size, spine length, nor spine density prove to be an absolute disqualifier for collectability. While pollen foragers did not shift to nectar collection but seized visiting flowers with uncollectable pollen, nectar foragers performed regular foraging bouts on these plants. Pollen that is uncollectable for corbiculate bees limits pollen depletion by generalist bumble bees and probably also honey bees while maintaining them as pollinators, which is an effective solution to the pollen dilemma. As previous assumptions about the impact of pollen morphology on its collectability are disproved, potentially determining factors are discussed.

Sexual and natural selection on pollen morphology in Taraxacum

“…The interaction between spines and pollenkitt is thought to act as a mechanical defense against consumption or collection by flower visitors (Lundgren, 2009;Lunau et al, 2015; but see Konzmann et al, 2019). However, we found no evidence that pollen packed…”

“…The interaction between spines and pollenkitt is thought to act as a mechanical defense against consumption or collection by flower visitors (Lundgren, ; Lunau et al., ; but see Konzmann et al., ). However, we found no evidence that pollen packed in the corbiculae of bumblebees for larval feeding or moved into the digestive tract of flies for adult nutrition differs in spine spacing or spine length from that in donor inflorescences.…”

Pollen defenses negatively impact foraging and fitness in a generalist bee (Bombus impatiens: Apidae)

“…Yet these generalist bees spent time foraging in cucurbit flowers, presumably for nectar 37,41,49 . This foraging behaviour is important because it suggests that bees recognize cucurbit flowers as a nectar source, but distinguish that the pollen should be avoided, likely using multi-sensory cues 15,50 . This collection avoidance means that as bees fail to groom pollen attached to their body into their corbiculae, more is available for transport and pollination, and in www.nature.com/scientificreports www.nature.com/scientificreports/ fact, bumble bees have been found to deposit the most pollen on a per visit basis in cucurbit systems, as compared to squash and honey bees 37 .…”

“…buzz-pollination) to release pollen 1,9 . Finally, plants can also reduce pollen consumption directly through pollen traits that act as defenses, such as chemical compounds [10][11][12][13] , physical properties 14,15 , and even a lack of essential nutrients [16][17][18][19][20] . Strategies that reduce the size of the community of possible floral visitors can benefit plants provided that they increase the fidelity and effectiveness of the remaining pollinators 1,21 .…”

Pollination syndromes in the 21stcentury: where do we stand and where may we go?

“…Anthers preserved in lactophenol with aniline blue Advertisement (Carr et al, 2015) Pollen grain size and ornamentation Scanning electron microscopy or light microscopy (e.g. Konzmann et al, 2019) Ethanol-preserved flower material Efficiency (collectability of pollen e.g. Lunau et al, 2015;Konzmann et al, 2019) Pollen kit, viscin threads e.g.…”