Honeybees play an indispensable role in human society. Not only do they provide us with honey and other bee products, but they also serve a crucial ecological function through pollination. However, despite their significant economic contribution, the number of managed bee colonies has seen a concerning decline in recent years, posing a serious threat to crop production. According to recent national surveys on colony losses in Canada and the United States, combined with reports from beekeepers, the winter loss rate of bee colonies ranges between 30% and 60%, a figure far beyond what beekeepers can sustain.

The primary stressors driving this large-scale decline in bee populations include environmental changes, habitat fragmentation, parasites, pathogens, and pesticide contamination. These factors often compound at both individual and colony levels, further worsening the survival conditions of bees. While the academic community has made efforts to study the effects of these stressors on bee health, progress in preventing or mitigating colony losses remains limited. One possible reason is that most colonies typically face simultaneous assaults from multiple stressors, which act synergistically rather than independently. Therefore, there is an urgent need to develop new approaches to understand how these stressors interact within bee colonies and address the challenges to bee health more effectively.

In a recent study published in Current Biology, Sarah K. French and her team from York University, Canada, used systems theory and network analysis to simulate how multiple stressors affect honey bee colonies pollinating crops across Canada. The researchers employed a network framework to identify and quantify the relationships between hundreds of potential co-occurring stressors. Throughout the experiment, bee colonies were sampled from crop fields at different times: before, during, and after crop flowering. At each time point, the researchers assessed levels of parasites, pathogens, and pesticides. They collected nurse bees, bee bread, pollen, and nectar to detect and quantify the presence of 239 agricultural chemical residues. Additionally, they measured levels of Varroa mites, Paenibacillus larvae, and nine honey bee-related viruses.

The study identified 53 different stressors in the bee colonies. Notably, pesticide residues were detected in over 97% of nectar and pollen samples from beekeeping sites. Varroa mites and P. larvae were found in 73% of samples, while five significant viruses were present in more than 55%. These viruses included black queen cell virus (BQCV), deformed wing virus type A (DWV-A), Lake Sinai virus (LSV), sacbrood virus (SBV), and deformed wing virus type B (DWV-B). In contrast, only 30% of sites tested positive for the remaining three honey bee-related viruses: acute bee paralysis virus (ABPV), chronic bee paralysis virus (CBPV), and Kashmir bee virus (KBV). Each beekeeping site faced 15 stressors, including an average of 7 pesticides, 5 viruses, and 0.3% Varroa mite infestation.

The researchers further used association network analysis to evaluate the relative importance of each stressor within the networks. They found that certain stressors consistently appeared across all time points for specific crops, and the number of stressor elements in the network was independent of the number of days since the stress experiment began. In their analysis, the researchers discussed the relative expected impact (EI) of each stressor, revealing that Varroa mites, with a lower EI compared to other stressors, are easier to manage, whereas pesticides and viruses are more challenging to address due to their strong interactions with other stressors. The number of stressors and their interactions varied significantly between different crops and regions. The study also revealed that honey bee colonies experienced multiple stressors at every observation point, and exposure to crops and the pollination period generally increased the number of stressors. These interactions among stressors could seriously harm bee colonies, such as the interaction between Varroa mites and clothianidin, which can reduce bee weight, or certain fungicides, making pesticides more toxic to bees or causing sublethal effects.

The researchers suggest that future studies focus on identifying the most influential stressors within the networks, particularly those with lethal and non-lethal effects on bee health.

Moreover, some stressors have not yet been fully recognized by beekeepers. During the pollination season, beekeepers typically manage various stressors, such as parasites and pathogens, which often have lesser effects on the network, while they may overlook the differential impact of pesticides and viruses on other stressors. Therefore, involving multiple stakeholders, including beekeepers, growers, and government regulatory bodies, is essential to explore viable risk management strategies or measures. These efforts should adjust traditional risk assessment methods to consider interactions between harmful stressors. Additionally, the stressor environment changes before, during, and after the pollination season, and the stressors encountered during pollination may persist over time, contributing to the weakening of colonies as they enter the overwintering period.

In summary, the significant variations in stressor networks within bee colonies highlight the need for tailored solutions to manage the risks faced by honey bees and their colonies across different crops and regions. Careful management of multiple stressors is required, and further exploration of the complex stress environments bees face is necessary to establish a more comprehensive framework for managing bee and colony health.

Professor Juliana Rangel of Texas A&M University commented on this study, emphasizing that it sheds light on the complexity of biological and environmental stressors affecting honey bees. She noted that stressor networks become increasingly complex when bee colonies interact with certain vital crops, and some stressors have a much more significant impact within these networks than others. Additionally, the interactions between stressors change over time. This study offers a new perspective on understanding the survival crisis of honey bees and underscores the value of systems theory and network analysis in future research to deeply understand how specific stressors affect bees. It also provides essential scientific evidence for protecting honey bee health.

https://doi.org/10.1016/j.cub.2024.03.039