Current Issue

Honey bees contribute to income generation for beekeepers and play a crucial role as key pollinators in food production and ecosystem conservation (Lee et al., 2010; Jeong et al., 2016). However, honey bee health is threatened by various factors, including climate change, pests, diseases, pesticides, and nutritional deficiencies (vanEngelsdorp et al., 2009; Jung and Lee, 2018). Recently, the mortality of overwintering honey bees has been reported in South Korea (Jung and Bae, 2022; Kim, 2022; Kim and Lee, 2022; Lee et al., 2022). Such colony losses directly cause economic damage to beekeepers and, subsequently, reduce bee products and agricultural productivity due to a lack of pollination (Kim, 2022). The mortality of overwintering honey bee colonies in South Korea has been linked to various factors, including mite-related issues such as high-density infestations, pesticide resistance, and misuse of mite control agents. Abnormal weather conditions, including unusually low autumn temperatures and unseasonably high winter temperatures, have also been suggested as significant contributors (Jung and Bae, 2022; Kim, 2022; Kim and Lee, 2022; Lee et al., 2022). The overwintering failure rate in South Korea was reported to be approximately 17.4% (8.5-24.9%) from 2011 to 2013, but this percentage doubled to about 36% (22.6-67.7%) in 2022 (Jeong et al., 2016; Kang et al., 2024). In 2022, the primary observation in failed colonies was not the presence of large numbers of dead bees but rather empty hives with no bees inside (Kang et al., 2024). This finding suggests that honey bee flight activity may occur during the overwintering period.

The ecology, physiology, and activity of honey bees are temperature-sensitive, and their behavior changes in response to temperature fluctuations (Jung and Bae, 2022). The supercooling point of honey bees is -8℃ (Qin et al., 2019; Muhammad and Jung, 2021), and their minimum activity temperature has been reported to be between 6-7℃ (Southwick and Heldmaier, 1987; Eskov and Toboev, 2011). Honey bees overwinter by clustering together to regulate heat, making weather conditions during the overwintering period critically important for environmental stability (Kim et al., 2021; Jung and Bae, 2022). Due to ongoing global warming in South Korea, weather variability has increased, and abnormal temperatures were recorded during the winter of 2021, coinciding with reported overwintering failures (Jung and Bae, 2022). When temperatures rise during overwintering, exposure to sunlight can cause significant temperature fluctuations inside and outside the colony. Some of these fluctuations may reach temperatures that allow bees to become active, potentially leading them to venture outside and not return.

Therefore, this study aimed to investigate the threshold temperature for honey bee flight activity during the overwintering period and evaluate the potential of shading as a strategy to minimize temperature fluctuations. The study sought to test hypotheses that above certain threshold temperature, honey bee would show external flight activities and increase as temperature increase, and shading would decrease the temperature variability resulting in less flight activities and food consumption during winter. By monitoring honey bee flight activity and temperature profiles, the research aimed to understand how environmental conditions, particularly temperature variations, affect honey bee behavior during winter and to explore shading as a potential method to mitigate these impacts, ultimately contributing to reducing overwintering mortality in honey bee colonies.

The temperatures in the shaded house were higher than those in the rain-cover house, but the differences were not statistically significant. The flight activity of overwintering honey bees predominantly occurred in colonies placed in the rain-cover house, whereas colonies in the shaded house exhibited only a single flight event, which occurred at a temperature of 22.5℃. Honey bee flight activity showed a positive correlation with temperature, and the food consumption of overwintering colonies was highest in the open field environment.

In this study, the lowest temperature at which honey bee flight activity was observed was 9.3℃. Based on the temperatures during the first investigation period, the frequency of temperatures exceeding the honey bee activity threshold of 9.3℃, as identified in this study, was 317 instances in the shaded house and 113 instances in the rain-cover house. However, honey bee flight activity predominantly occurred in the rain-cover house. Previous studies have reported that the activity level of honey bees in mango orchards is more strongly influenced by changes in light intensity than by temperature (Park et al., 2014). Additionally, the foraging activity of honey bees has been shown to exhibit a positive correlation with light intensity (takeoff: r=0.577-0.849; landing: r=0.201-0.803), though the correlation is slightly lower than that with temperature (Choi, 2015). Since solar radiation varies seasonally (Kim et al., 2009), further research is needed to investigate the solar radiation conditions under which honey bee flight activity occurs during the overwintering period.

This study found no significant difference between the number of bees exiting and entering the hive. The presence of empty hives with no bees inside has been reported as a primary symptom of honey bee colony mortality during the overwintering period in South Korea (Kang et al., 2024). Although this study attempted to identify the patterns of mortality symptoms through partial observations conducted every 2 hours from 10 a.m. to 4 p.m., it was unsuccessful. Therefore, a comprehensive investigation covering the entire overwintering period is necessary to discern patterns of these symptoms. Furthermore, research is required to elucidate the underlying causes of colony mortality and the conditions under which it occurs.

The food consumption of honey bee colonies varied according to overwintering environments, with colonies in the open field showing the highest consumption. By the end of the overwintering period, 55.6% of the colonies in the open field had already initiated brood rearing, which may explain the increased food consumption (rain-cover house: 35.3%; shaded house: 17.4%). Honey bees form clusters to survive the winter; however, if the external temperature exceeds 12℃ for an extended period, the queen may initiate egg-laying (Winston, 1987; Jung and Bae, 2022). When brood rearing occurs during the overwintering period, winter bees undergo physiological changes, transitioning to a state similar to summer worker bees, such as nurse or forager bees (Doke et al., 2015; Jung and Bae, 2022). Additionally, these workers experience a reduced lifespan due to increased energy demand for heat production, feeding, and brood care activities (Page et al., 2001). Therefore, strategies to prevent brood rearing during the overwintering period are essential for effective colony management.

This study aimed to determine the threshold temperature for honey bee flight activities during winter and to assess the influences of shading on the bee hives mitigating the extreme temperature variability. The threshold temperature was determined to be 9.3℃, with flight activity increasing as temperatures rose. Shading was found to effectively reduce honey bee flight activity. Food consumption was higher in colonies located in open field or rain-cover condition than the shading conditions. This finding must be confirmed by repeated measurements from various circumstances, However, here we propose season-long and nationwide monitoring of honey bee flight activity relative to the temperature and overwintering environments to better figure out the cause and result relationship of winter losses from climate change.