Nocturnal Bees Feed on Diurnal Leftovers and Pay the Price of Day – Night Lifestyle Transition

Abstract

Bees exemplify flights under bright sunlight. A few species across bee families have evolved nocturnality, displaying remarkable adaptations to overcome limitations of their daylight-suited apposition eyes. Phase inversion to nocturnality in a minority of bees that co-exist with diurnal bees provides a unique opportunity to study ecological benefits that mediate total temporal niche shifts. While floral traits and sensory modalities associated with the evolution of classical nocturnal pollination syndromes, e.g. by bats and moths, are well-studied, nocturnality in bees represents a poorly understood, recently invaded, extreme niche. To test the competitive release hypothesis, we examine how nocturnality shapes foraging by comparing pollen loads, nest pollen, and flower visitation of sympatric nocturnal and diurnal carpenter bees. We predicted that nocturnal bees primarily use night-blooming flowers, show little/no resource overlap with diurnal species and competitive release favors night-time pollen collection for provisioning. Contrarily, we found substantial resource overlap between nocturnal and diurnal bees. Flower opening times, floral longevity and plant abundance did not define nocturnal flower use. Smaller pollen loads on nocturnal foragers suggest subsistence on resource leftovers largely from diurnal flowers. Greater pollen types/diversity on nocturnal foragers indicate lower floral constancy compared to diurnal congenerics. Reduced activity during new moon compared to full moon suggests constraints to nocturnal foraging. Invasion and sustenance within the nocturnal niche is characterized by: (i) opportunistic foraging on residual resources as indicated by smaller pollen loads, extensive utilization of day-blooming flowers and substantial overlap with diurnal bees, (ii) generalization at two levels-between and within foraging trips as indicated by lower floral constancy, (iii) reduced foraging on darker nights, indicating visual constraints despite sensitive optics. This together with smaller populations and univoltine breeding in nocturnal compared to multivoltine diurnal counterparts suggest that nocturnality imposes substantial fitness costs. In conclusion, the evolution of nocturnality in bees is accompanied by resource generalization instead of specialization. Reduced floral constancy suggests differences in foraging strategies of nocturnal and diurnal bees which merits further investigation. The relative roles of competition, floral rewards and predators should be examined to fully understand the evolution and maintenance of nocturnality in bees.

Onset of morning activity in bumblebee foragers under natural low light conditions

“…Conditions were stable and favorable so that bees could maximize food collection during the day. Those that went out earlier are likely to benefit from early morning flowers (Ewusie & Quaye, 1977;Oltmanns, 1895;Pacini & Nepi, 2007;van Doorn & van Meeteren, 2003) and residual resources leftover from the previous day (Somanathan et al, 2020). When the majority of bees started later during dawn and the morning, light levels were already high enough for taking full advantage of their visual and flight capabilities and selecting among a larger variety of flowers.…”

Nocturnal Bees as Crop Pollinators

“…Bees of Ptiloglossa also visit different nightflowering plants, but in one single brood cell a monofloral pollen load was found [20]. The Paleotropical bee species X. tranquebarica, which visits flowers throughout the night, seems to be a generalist, as it was found to feed on diurnal leftovers from 71 plant species [76].…”

Flower sharing and pollinator health: a behavioural perspective

“…However, the majority of the work on bees' individual pollen foraging preferences has been conducted in social bees, which rely on a large foraging workforce to ensure that a suitable range of nutrients is secured by the colony (reviewed by [25]). Solitary female foragers, in contrast, must also lay eggs, build the nest and collect all food for developing larvae, and as yet very little is known about how the drivers of foraging decisions differ between social and solitary bees (but see [51,52]).…”