Nocturnal bees

Nocturnal bees

Four bee families (Andrenidae, Colletidae, Halictidae, and Apidae) contain some species that are crepuscular (these may be either the vespertine or matinal type). These bees have greatly enlarged ocelli, which are extremely sensitive to light and dark, though incapable of forming images. Many are pollinators of flowers that themselves are crepuscular, such as evening primroses, and some live in desert habitats where daytime temperatures are extremely high.

Bees were monitored weekly for 7 yr at light traps in the tropical monsoon forest of Barro Colorado Island. Fifty species and 17 genera were recorded, including nocturnal Megalopta and crepuscular Rhinetula (Halictidae) and Ptiloglossa (Colletidae). Twenty-eight native meliponines and African Apis mellifera in traps increased the known resident highly social bees to 32 species. Most of the 11,860 bees were Megalopta; these species were present throughout the year, as were Rhinetula and Ptiloglossa. Abundance of Megalopta was lowest during the dry season; this was its only consistent seasonality. Peak abundance occurred at various times during dry-to-wet-season transition and in the first half and the end of the wet season. Centris and Epicharis, large, solitary anthrophorid bees, were present in the early wet season and absent in the late wet season and most of the dry season.

 The highly social bee Trigona aff. cupira was aseasonal but had abundance peaks during all but the mid-to-late wet season. Megalopta genalis abundance and M. ecuadoria abundance were highly positively correlated within years, and weekly correlated with abundances of other bees. Pairwise correlations between years were low for each Megalopta species and for combined bees of other genera.

A singularly large catch of Megalopta genalis in 1978 corresponded to flowering of the monocarpic tree Tachigalia versicolor near light traps. Fluctuation of bee abundance at light traps probably indicates local flower availability for this species and, to a lesser extent, for the other bees. The data suggest large year-to-year variation in seasonal abundance of resources used by nocturnal bees and by bees in general, but are limited to indicating the presence or absence of most other bee species.

The yearly variances in abundance of a stingless bee species and of Megalopta in Panamá are comparable to those of eight temperate bumble bee species studied in England, but combined diurnal and crepuscular bee abundance is considerably less variable. The African honey bee arrived on Barro Colorado Island in mid-1982 and has produced no noticeable changes in abundance of other bees but has yet to reach its potential, numbers there; light-trap studies could provide perhaps the most reliable information on its impact.