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What is causing honey bee colony collapse in the U.S.?

Honey bees are crucial pollinators of flowers, enabling the plants to form seeds and fruit, and reproduce successfully. In addition, around one-third of food eaten in by Americans comes from crop plants that are pollinated by honey bees. The collapse of bee colonies thus poses a threat to food security and is a phenomenon that concerns beekeepers across the U.S. and worldwide.

The sudden disappearance of all the worker bees from a colony, leaving a queen, a few nurse bees and the immature brood, is what characterizes colony collapse. The colony cannot survive without the worker bees, and so it will die off in time. There is currently no all-embracing explanation for why this occurs to certain bee hives and the research that has been conducted usually focuses on the effects of one stressor, such as parasites, or on a combination of stressors in one isolated region or geographical area. 

A new study, published in the journal Scientific Reports, has taken the analysis of variables, recognized as potential contributors to colony collapse, to a higher level. The researchers conducted a large-scale, multi-variable analysis using publicly accessible datasets from across the contiguous U.S. In particular, they used weather data for the years 2015–2021, and combined open data on honey bee colony status and stressors, weather data, and land use, in order to understand the relationships between the disappearance of bee colonies and the various stressors.

“Honey bees are vital pollinators for more than 100 species of crops in the United States, and the widespread loss of honey bee colonies is increasingly concerning,” said Luca Insolia, first author of the study, a visiting graduate student in the Department of Statistics at Penn State at the time of the research, and currently a postdoctoral researcher at the University of Geneva in Switzerland. 

“Some previous studies have explored several potential stressors related to colony loss in a detailed way but are limited to narrow, regional areas. The one study that we know of at the national level in the United States explored only a single potential stressor. For this study, we integrated many large datasets at different spatial and temporal resolutions and used new, sophisticated statistical methods to assess several potential stressors associated with colony collapse across the U.S.”

One of the main reasons for the lack of previous analyses at the national level is the absence of same-source or same-resolution databases. For example, weather data may be available on a daily scale, but information on colony disappearances may only be collected by apiculture groups on a quarterly basis. Because these data differ in spatio-temporal resolution, the researchers used a method of data up-scaling, rather than averaging the information to the lowest resolution available, which would mean a loss of detail.

“In order to analyze the data all together, we had to come up with a technique to match the resolution of the various data sources,” said Martina Calovi, corresponding author of the study, a postdoctoral researcher in the Department of Ecosystem Science and Management at Penn State at the time of the research, and currently an associate professor of geography at the Norwegian University of Science and Technology. 

“We could have just taken an average of all the weather measurements we had within a state, but that boils all the information we have into one number and loses a lot of information, especially about any extreme values. In addition to averaging weather data, we used an ‘upscaling’ technique to summarize the data in several different ways, which allowed us to retain more information, including about the frequency of extreme temperature and precipitation events.”

The researchers used the resulting integrated resolution-matched dataset – which they have made available for use by other researchers – alongside sophisticated statistical modeling techniques that they developed to assess the large number of potential stressors at the same time.

The research team found that several stressors impacted honey bee colony loss at the national level, including the use of pesticides nearby, frequent extreme weather events, and weather instability. Colony loss was also related to the presence of parasitic mites, Varroa destructor, which reproduce in honey bee colonies, weaken the bees, and potentially expose them to viruses. The researchers also found that losses typically occurred between January and March, probably reflecting the harsher conditions endured by bees while overwintering. 

“Our results largely reinforce what regional studies have observed and confirm that regional patterns around these stressors are actually more widespread,” said Insolia, a beekeeper himself. “These results also inform actions that beekeepers could take to help circumvent these stressors and protect their colonies, including treatments for the Varroa mites‚ especially in areas of weather instability. Beekeepers could also consider strategies to move their colonies to areas with high food availability or away from nearby pesticides or to provide supplementary food during certain seasons or months with frequent extreme weather events.”

The researchers suggest that having data about beekeeping practices, such as treating colonies for Varroa mites and recording colony losses, at a finer resolution would allow validation of their results as well as a more detailed understanding of factors that act as stressors for honey bees. 

“It would be incredibly beneficial to explore beekeeping practices at a finer scale than the state level,” said Calovi. “In many cases, beekeeping associations and other organizations collect this data, but it is not made available to researchers. We hope our study will help motivate more detailed data collection as well as efforts to share that data – including from smaller organizations such as regional beekeeper associations.”

The research team also found a strong relationship between colony loss and a broad category of beekeeping practices noted on a USDA survey as “other,” which contained everything from hives being destroyed, to food scarcity for bees, to queen failure. They noted that collecting this data in more detail and breaking up this catch-all type variable into smaller categories would improve their ability to identify links between particular stressors and colony collapses.

“A changing climate and high-profile extreme weather events like Hurricane Ian – which threatened about 15 percent of the nation’s bees that were in its path as well as their food sources – are important reminders that we urgently need to better understand the stressors that are driving honey bee colony collapse and to develop strategies to mitigate them,” said Francesca Chiaromonte, professor of statistics and the holder of the Lloyd and Dorothy Foehr Huck Chair in Statistics for the Life Sciences at Penn State and a senior member of the research team. 

“Our results highlight the role of parasitic mites, pesticide exposure, extreme weather events, and overwintering in bee colony collapse,” she concluded. “We hope that they will help inform improved beekeeping practices and direct future data collection efforts that allow us to understand the problem at finer and finer resolutions.”

By Alison Bosman, Staff Writer

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