Abstract
The foraging activity of diurnal bees often relies on flower availability, light intensity and temperature. We do not know how nocturnal bees, which fly at night and twilight, cope with these factors, especially as light levels vary considerably from night to day and from night to night due to moon phase and cloud cover. Given that bee apposition compound eyes function at their limits in dim light, we expect a strong dependence of foraging activity on light intensity in nocturnal bees. Besides being limited by minimum light levels to forage, nocturnal bees should also avoid foraging at brighter intensities, which bring increased competition with other bees. We investigated how five factors (light intensity, flower availability, temperature, humidity, and wind) affect flower visitation by Neotropical nocturnal bees in cambuci (Campomanesia phaea, Myrtaceae). We counted visits per minute over 30 nights in 33 cambuci trees. Light intensity was the main variable explaining flower visitation of nocturnal bees, which peaked at intermediate light levels occurring 25 min before sunrise. The minimum light intensity threshold to visit flowers was 0.00024 cd/m2. Our results highlight the dependence of these nocturnal insects on adequate light levels to explore resources.
Highlights
Being able to navigate in dim light seems to be a major challenge faced by bees when foraging at n ight[4]
Considering the limitations in their ability to see at the dimmest nocturnal light levels, and their intense competition with a massive number of diurnal bees that start to arrive at flowers around sunrise when light levels are bright enough, we should expect a higher activity of nocturnal bees during the mid-twilight period, prior to the arrival of diurnal bees, when light intensities are at intermediate levels
The foraging activity of nocturnal bees in cambuci flowers during morning twilight can be predicted mainly by light intensity, and other putative environmental factors do not contribute as much to these predictions
Summary
Being able to navigate in dim light seems to be a major challenge faced by bees when foraging at n ight[4]. Nocturnal bees are likely to be more affected by light levels than by other environmental factors, because of the extremely low intensities of light at night, and because of its immense daily variation, especially during twilight periods. Considering the limitations in their ability to see at the dimmest nocturnal light levels, and their intense competition with a massive number of diurnal bees that start to arrive at flowers around sunrise when light levels are bright enough, we should expect a higher activity of nocturnal bees during the mid-twilight period, prior to the arrival of diurnal bees, when light intensities are at intermediate levels. We investigate how light intensity affects the foraging activity of nocturnal bees on cambuci flowers. We discuss how this dependence can be explained by ecological requirements and evolutionary processes
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