Warm, wet weather conditions and changing climate negatively influence the nectar intake and nutritional health of honey bees, but maintaining large tracts of grassy natural habitat with flowering plants around apiaries may help to mitigate the detrimental effects of climate, according to a new study by an international team of researchers.
The pollination provided by bees and other insects is critical for crop production and ecosystem health, but some wild and managed pollinator populations have declined significantly around the world, the researchers noted. A major driver of these declines is loss of habitat and flowering plants - the food sources for pollinators - likely due to climate change and shifts in land use.
"However, the effects of land cover can be specific to a particular location," said lead author Gabriela Quinlan, postdoctoral scholar in entomology in Penn State's College of Agricultural Sciences.
"Previous research looking at the influence of land cover, weather and climate on bee health found a greater impact from weather and climate," she said. "But no studies have monitored colonies over sufficient time and space to determine how climate and weather interact with landscape quality to affect honey bees' ability to gather nectar in North America."
A variety of factors affect the availability of flowering plants across locations and time periods, Quinlan explained, so researchers needed a standard, low-effort method for surveying honey bee colonies' intake of food resources. They found that the use of automated hive scales can provide colony-level measurements of hive performance by monitoring changes in honey bee colony weight within and across days and seasons.
"Honey bee colonies must collect and store foraged pollen and nectar to support a large population and continuous brood-rearing over the growing season," she said. "So, changes in colony weight are closely tied to changes in food availability over time. This colony weight data may be a proxy for estimating landscape-level flowering resources."
To estimate the relative importance of climate, weather and land cover on flowering plant resources, the researchers combined hive scale data from several honey bee studies spanning five years, 162 apiary locations and 644 colonies in seven states - Michigan, Minnesota, North Dakota, Ohio, Pennsylvania, South Dakota and Wisconsin. They focused on the period of late summer nectar flow in July and August, a critical time when colonies must collect sufficient resources to survive the winter.
For each apiary, the researchers obtained temperature and precipitation data and quantified the total areas of various land covers within a 2-kilometer radius, which is the average foraging distance that honey bees travel. Land covers were categorized as grass crops, woody-herbaceous crops, grassy-herbaceous natural land, woody natural land or developed land.
The team performed a statistical analysis to model the maximum weight gain of colonies across the region, taking into account land cover, weather and climate variables.
The researchers, who published their results recently in Environmental Research Letters, found that colonies gained an average of almost 20 kilograms (nearly 44 pounds) in July and August and that colonies in wet and warm climates had the lowest weight gain compared to colonies in cool, dry climates.
"There was a positive correlation between the proportion of grassy-herbaceous natural land around the colonies and greater colony weight gain, indicating that this type of land cover can help moderate the detrimental effects of warm and wet climates," said study co-author Christina Grozinger, Publius Vergilius Maro Professor of Entomology and director of Penn State's Center for Pollinator Research.
"Our data indicated that grassy-herbaceous land can enhance colony weight gain by up to 3.5 kilograms across climates," she said. "On the other hand, woody natural land was negatively correlated with rates of colony weight gain."
For colonies in warm and wet climates to gain weight comparable to the worst-performing colonies in cool and dry climates, grassy-herbaceous land must account for more than 25% of the land area within 2 kilometers of the apiary, Grozinger noted.
The researchers said that long-term monitoring across climactic zones using hive scales could offer a wealth of information on which climates support the most productive floral communities for bees and how the ranges of these plants may shift as the climate continues to change.
"Based on our findings, better models could be developed using climate and weather data to predict outcomes for honey bee colonies ahead of the growing season to help support beekeeper decision making," Quinlan said. "Land managers could also improve floral resources for managed and wild bees by preserving grassy-herbaceous land covers and planting climate-resilient mixes of flowering species across the northcentral United States."
Other researchers on the project were Douglas Sponsler, University of Würzburg, Germany; Hannah Gaines-Day and Claudio Gratton, University of Wisconsin-Madison; Harper McMinn-Sauder and Reed Johnson, Ohio State University; Clint Otto, U.S. Geological Survey, Northern Prairie Wildlife Research Center, Jamestown, North Dakota; Autumn Smart, University of Nebraska-Lincoln; Théotime Colin, University of Sydney, Australia; and Rufus Isaacs and Meghan Milbrath, Michigan State University.
The U.S. Department of Agriculture's National Institute of Food and Agriculture; the Foundation for Food and Agricultural Research; the Wisconsin Department of Agriculture, Trade and Consumer Protection; the USDA Farm Service Agency; the USDA Natural Resources Conservation Service; the Roger and Barbara Hoopingarner Endowed Graduate Fellowship in Entomology at Michigan State University; and the North American Pollinator Protection Campaign's Honey Bee Health Improvement Grant Program supported this research.
