Poor nutrition and pesticide use negatively impact bees on their own, but new research shows when these two factors work together, they cause even higher rates of bee mortality.
“Most studies, including risk assessment procedures, typically look at effects of stressors by testing one factor at a time, mostly because it’s easier to do and easier to interpret,” said Simone Tosi, a postdoctoral fellow at the University of California San Diego and lead author of the recent study published in Proceedings of the Royal Society B. “However, bees are exposed to multiple environmental stressors at the same time.”
The two stressors of poor nutrition and pesticides are particularly prevalent on intensive agricultural land, Tosi said.
In the study, Tosi and his colleagues tested the effects of two common neonicotinoids — clothianidin and thiamethoxam — on honeybees in the lab in combination with the effects of poor nutrition.
“We were very surprised,” Tosi said. “We discovered that when we exposed bees to the two stressors together, survival was reduced up to 50 percent. We discovered the effects of pesticides was extremely amplified by nutritional stress.”
The researchers also showed that food consumption and carbohydrates stored in the bees’ bodies were reduced when bees experienced both poor nutrition and pesticide exposure. The stored sugar levels rapidly declined after exposure to the pesticide only, but the effect on food consumption appeared only when both factors were present.
“Without poor nutrition, low doses of pesticides didn’t have any significant impact,” he said. “This is why pesticide effects are often underestimated when the combined stressors are not taken in consideration.”
Tosi has a few ideas in regards to how the two stressors work together. One hypothesis is that pesticides alter the bees’ metabolism, prompting them to seek more food for energy — food that might not be available. Other studies also show that pesticides alter the behavior of bees such as their flight ability and thermoregulation abilities, Tosi said.
“Pesticides could have a sublethal effect,” he said. “They don’t make them die, but they change the behavior or physiology and ways to consume energy.”
Tosi suggests refining current pesticide risk assessments by including the combined risks of poor nutrition and pesticides. “It’s extremely important to consider nutritional stress in assessments,” he said.