Common ground-nesting bees appear to be growing smaller in heavily farmed landscapes, according to a new study. Intensive agriculture, therefore, adds to a growing list of threats facing crop pollinators, including mites, pesticides and loss of habitat.
Researchers from Cornell University found that female ground nesting bees, also known as Andrena nasonii, living in intensively farmed areas may not be able to find enough pollen sources to sufficiently feed their offspring.
It is believed that this link between intensive agriculture and the size of A. nasonii bees has important implications for how farmers might be able to diversify these landscapes to benefit the bees, which are considered vital crop pollinators.
"Once we knew there was an effect of agriculture on the size of the bees, we took a random sample of our largest bees from our natural habitats and smallest bees from these agricultural intensive habitats, and looked at how much pollen the female bees were carrying on their bodies," explained Heather Connelly, co-author of the study.
A. nasonii bees are thought to be the most important pollinators of strawberries. Researchers captured bees visiting strawberries in established experimental agricultural plots set up across a number of different landscapes. They then measured the length of each bee's head and thorax - the space between the wings.
"Small bees had a 40 percent smaller pollen load than large bees did, so potentially these small bees might actually be less able to provision their own offspring and we might end up seeing some cascading negative effects on the population of bees there," Connelly said.
Satellite data was used to characterize agricultural intensity by analyzing the radius from the center of each plot at scales of one kilometer and 760 meters.
"At both these scales, we tended to have smaller bees in landscapes with lots of agriculture around," Connelly explained, adding that published equations were used to calculate average foraging distance of the bees.
Pesticides used to protect crops from damaging insects are thought to be especially toxic for bees, even in low doses. In another recent study, pesticide exposure was found to disrupt the foraging behavior in bees, altering the insects' flower choices, learning, and ability to extract nectar and pollen.
Based on body size, Connelly and her team concluded that larger bees can forage twice as far as their smaller counterparts.
The study was published in the March 4 issue of the journal PLOS ONE.
