Nectar secretion of black locust and its impact on honey bee foraging under variable weather conditions

The black locust (Robinia pseudoacacia L.) tree is a highly valuable species for both forestry and apiculture, despite its invasive spread and ecological impacts on grasslands and steppe areas. Under favourable weather conditions, it can produce high honey yields and represents a primary source of income for beekeepers in Hungary. In recent years, yields have become more unpredictable due to early warm springs followed by sudden frost events. To investigate the effects of current environmental conditions and the weekly dynamics, 24-hour nectar measurements were conducted over two years, paired with honey bee foraging data. We have compared the nectar weight, sugar concentration and sugar value in open flowers along with the hourly beehive weight and meteorological data. Our results showed that nectar secretion started at the bud stage and the nectar weight in open flowers averaged 1.9 ± 1.7 mg per flower, with a mean sugar concentration of 41 ± 18%. Optimal conditions for nectar secretion were observed at air temperatures between 15–25 °C and relative humidity levels of 60–80%. Nectar weight increased on cloudy, wetter days, while peak values occurred on subsequent days, accompanied by a decrease in sugar concentration. Honey bees were more active at temperatures above 20 °C and relative humidity below 40%. 

We found that meteorological variables, mainly air temperature, have a high impact on nectar secretion and consequently on foraging activity as well. Our results may contribute to a better understanding of weather-driven effects on nectar secretion and their impact on pollinators, especially in the context of climate change.