Farmers and other land managers should learn to identify the nesting sites of ground-dwelling bees so the habitat of these wild pollinators can be protected, an Oregon State University research paper has said.
The study says US farmers who want to attract wild bees as alternative crop pollinators to introduced honey bee colonies are faced with a major challenge – so little is known about the habitat requirements of the country’s wild bee species that it’s difficult to come up with a detailed and meaningful bee conservation strategy for farms.
There are around 5,000 native bee species in North America and many are in decline due to loss of foraging habitat, industrial-scale farming and over-use of pesticides. The rusty patched bumble bee (Bombus affinis) previously occurred throughout the eastern regions of the US and south east Canada, but its populations have declined by around 91 per cent over the past two decades. It’s now listed as an endangered species in 48 states.
So little is known about these native bees – including how to provide suitable nesting areas that won’t be destroyed by agricultural and urban development.
“The soil habitat preferences for many wild bees are understudied, yet 70% of wild bee species nest in soil,” the Oregon State University study says.
“This obstacle presents an opportunity for soil scientists to partner with entomologists and growers to identify soil properties and nesting site features associated with ground‐dwelling bee activity in agricultural settings.”
The paper’s lead author, crop and soil scientist Rebecca Lybrand, says the research team looked at physical and chemical properties of soils collected from bee and wasp nesting sites in the Willamette Valley of western Oregon. The study included seven farm sites and one recreational park.
“We focused on ground‐dwelling bees, predominantly native bees, with sand wasps included as indirect pollinators,” Lybrand says.
“Soils from 17 bee and sand wasp nests were analyzed for pH, particle size distribution, and carbon and nitrogen content. We visually confirmed that eight of the nesting bees were sweat bees from the Halictidae family.”
The researchers distributed a flyer to local farmers to help them identify potential evidence of ground‐nesting bees. The signs included small emergence holes in the soil and small mounds of soil – known as tumuli – around these holes, which indicated the nests were active.
“We surveyed for bee activity by monitoring transects established along the edges and rows of agricultural fields to ensure that we did not disrupt crops or farming activities,” the study says.
The researchers found that instead of nesting in sandy, well-drained soils, the Oregon bees preferred soils “with unexpectedly high proportions of silt and clay”. Active nesting sites were found in patches of bare ground, with little vegetation, organic matter or rock cover.
“One of our observations confirmed that active emergence holes remained open throughout the year.They didn’t swell shut during the wetter, cooler seasons – despite having clay in the soils that might cause shrinking and swelling,” Lybrand says.
The study detected lipids in soil nest linings which suggested they may provide a type of waterproofing for the bees. It also confirmed previous studies in 2005 and 2014 that observed ground‐dwelling bees “prefer to nest in bare‐ground soils with minimal vegetation cover, likely due to the need for warm temperatures to enhance larval development”.
“Because the large majority of wild bee species nest in the soil, studies about how to best attract them to farms are important,” says Lybrand.
“Soil scientists and entomologists can partner with growers to identify soil habitats that support and attract more of these pollinators to agricultural lands. Improving our understanding of the connections between agriculture and the soils that bees, crops, and living organisms rely on to survive is important. Our research also provided a framework for studying ground-nesting organisms – an area of soil science that is underrepresented.”
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