Cues for cavity nesters: investigating relevant zeitgebers for emerging leafcutting bees, Megachile rotundata

Abstract

Photoperiod is considered the universal zeitgeber, regulating physiological processes in numerous animals. However, for animals in light-restricted habitats (e.g. burrows or cavities), thermoperiod may be a more important cue. Our study tested this hypothesis in the alfalfa leafcutting bee, , which nests in cavities and undergoes development within a brood cell. We assessed the role of environmental cues (thermoperiod and photoperiod) on the process of adult emergence by examining: (1) whether those cues direct circadian rhythms, (2) which cue is more dominant and (3) how sensitive developing bees and emergence-ready adults are to cues. Although we found that 20% of light penetrates the brood cell, and bees respond to photoperiod by synchronizing emergence, thermoperiod is the dominant cue. When presented with a conflicting zeitgeber, bees entrained to the thermophase instead of the photophase. When temperature cues were removed, we observed free-running of emergence, indicating that underlying circadian mechanisms can be synchronized by daily fluctuations in temperature. We also found that emerging bees were highly sensitive to even small increases in temperature, entraining to a ramp speed of 0.33°C h The response and sensitivity to temperature cues suggest that evolved a temperature-mediated clock to time emergence from light-restricted cavities.

Environmental history impacts gene expression during diapause development in the alfalfa leafcutting bee,Megachile rotundata

“…As such, light would be greatly restricted or totally blocked. We have recently discovered that developing M. rotundata are sensitive to light and that a limited amount of light can penetrate the leaf cell and cocoon (Bennett et al, 2018). If photoperiodism can account for a proportion of unexplained variance (75%) in the ANOVA model, this would strengthen our conclusion that the diapause transcriptome would vary between individuals based on their environmental history.…”

The circadian clock uses different environmental time cues to synchronize emergence and locomotion of the solitary bee Osmia bicornis

“…However in several hymenoptera, an interval of hours to days lies between eclosion and emergence and also here it has been demonstrated that light and/or temperature cycles cause daily rhythms of emergence, but the molecular basis of the circadian clock influencing emergence of these animals is unknown 7,8,15–18 . Solitary bees represent an exception from the rule, because they are the only insects of the above mentioned, in which light signals have been insufficient to trigger robust daily rhythms of emergence as shown for M. rotundata 8,9 .…”

“…The separation of emergence from eclosion by an interval of several months enables spring bees to emerge as soon as temperatures start rising in spring 21 . In contrast, M. rotundata , emerges shortly after eclosion 7–9 . To our knowledge, there have been no studies on the circadian timing of emergence in an insect species whose emergence is dissociated from eclosion by diapause.…”

“…To date, the only evidence for a circadian clock involved in solitary bee emergence comes from the bee Megachile rotundata 7–9 . The species M. rotundata times its emergence to temperature rise and shows daily rhythms of emergence during exposure to daily temperature cycles 7 .…”

“…Furthermore, it has been shown that a single, but strong temperature pulse can set the circadian clock in this solitary bee species and triggers daily rhythms of emergence lasting several days after the pulse 8 . Light was shown to be insufficient to trigger robust daily rhythms of emergence in M. rotundata 8,9 , although light can be an important Zeitgeber for daily rhythms in activity of honey bees and other insects, vertebrates and plants 4,10,11 and emergence of flies and wasps 12–18 .…”