The evolution of nocturnal behaviour in sweat bees, Megalopta genalis and M. ecuadoria (Hymenoptera: Halictidae): an escape from competitors and enemies?

Abstract

Evolutionary transitions to dim-light foraging (predawn matinal, crepuscular, nocturnal) have occurred repeatedly in bees, and may be associated with an escape from enemies or competitors. To date, however, little information has been available to test these hypotheses. Here we provide the first detailed information on the nesting behaviour of two species of Neotropical, nocturnal sweat bees, Megalopta genalis and M. ecuadoria (Hymenoptera: Halictidae). Females are facultatively social or solitary, and construct nests in dead wood. Nocturnal foraging behaviour is bimodal. Bees began foraging after sunset ( ~ 18:30 h) and ceased foraging approximately 1 h later even though nocturnal flowers with pollen were still abundant; a second foraging bout occurred in the predawn morning, which began at ~ 04:45 h and ended around sunrise ( ~ 06:15 h) when diurnal-blooming flowers were abundant. Bees are capable of controlled flight in full light. They utilized pollen from both canopy and understory plant species, which have diurnal or nocturnal pollen anthesis. Megalopta nests are attacked by generalist predators such as ants, as well as the endoparasitic fly Melaloncha sp. nov. (Phoridae), the beetle Macrosaigon gracilis (Rhipophoridae), the parasitic wasp Lophostigma cincta (Mutillidae), and the brood parasite Megalopta byroni (Halictidae). Overall nest survivorship rates were comparable to those for diurnal relatives, but rates of cell parasitism for Megalopta ( < < 5%) were substantially lower than they are for day-flying relatives, offering some support for the hypothesis that the evolution of nocturnal behaviour enables escape from natural enemies.

Socially induced brain development in a facultatively eusocial sweat beeMegalopta genalis(Halictidae)

“…Adult females were individually marked with white paint on the thorax. Social nests were filmed with a camcorder under infrared light during the approximately 2 h a day (1 h each before sunrise and after sunset, respectively) when bees forage (Wcislo et al 2004;Kelber et al 2006). These recordings allowed us to determine which bees were queens and workers (Smith et al 2008(Smith et al , 2009).…”

“…Comparative interpretations are speculative because MB development has not been studied in other solitary bees. To avoid these problems and directly test the effect of sociality, we used a facultatively social sweat bee, M. genalis (Halictidae), which can nest either socially or solitarily (Wcislo et al 2004;Smith et al 2007Smith et al , 2009, to directly compare the brains of social and solitary individuals within the same species.…”

Survival and productivity benefits to social nesting in the sweat bee Megalopta genalis (Hymenoptera: Halictidae)

“…Multi-female nests usually show strong division of labor: in glass-topped observation nests, one bee monopolizes reproduction, while the other female(s) perform >90% of the foraging trips and feed the queen via trophallaxis (Wcislo and Gonzalez 2006). M. genalis forages nocturnally approximately 45 min after sunset and 90 min before sunrise (Kelber et al 2006; for further details of Megalopta behavior see Smith et al 2003;Arneson and Wcislo 2003;Wcislo et al 2004;Warrant et al 2004). …”

“…Megalopta excavate tunnel nests in dry, rotting sticks (dead branches or lianas) suspended in vegetation or otherwise raised above the ground (Smith et al 2003;Wcislo et al 2004). To enable observations, we cut away part of the side of the stick that formed the tunnel wall.…”

“…Glass-topped observation nest data suggest that this method is likely to count most emergences before the cell is reused and closed again (mean time from emergence of an adult to closing a reused cell is 16.6 days, with only one of 24 emergences resealed in <6 days). For the glass-topped observation nests, we included only data collected between Feburary 1 and June 1 to avoid 4y Springer biasing analyses with data from the relatively unproductive late wet season (late July-early December; Wcislo et al 2004) because none of our other groups contained data from this period. Productivity was measured as the number of new cells per week because there was no predetermined observation period as in the natural observation nests.…”