Copper sulfate adds to a growing list of chemicals that threaten bees when used in the form of fertilizer or pesticide.
Since at least the 1800s, copper sulfate has been used in agricultural products ranging from fertilizer to fungicide for both organic and conventional crops. However, when specifically used as a leaf fertilizer, it can be lethal to native Brazilian bees scientifically known as Friesella schrottkyi. Even sublethal exposure of copper sulfate can alter the bees' behavior.
The study, led by researchers from the Federal University of Vicosa in Brazil, examined how copper sulfate impacted the activity of adult worker bees collected from four beehives.
Researchers specifically tested two commercial leaf fertilizers commonly used in Brazil: a copper sulfate fertilizer containing 24 percent sulfur and a micronutrient mix that contained much smaller concentrations of heavy metals. Bioinsecticide spinosad, commonly used to control mosquitoes, was used as a control because it is known to be lethal for bees.
Within 72 hours, researchers found oral exposure to the copper sulfate fertilizer killed all of the test bees and was even more lethal than the spinosad control.
"This could have implications for growers who use copper sulfate as a leaf fertilizer, and as a fungicide," explained Raul Narciso Guedes, one of the study co-authors.
Researchers also found that bees exposed to copper sulfate and spinosad ate twice as much food as bees not exposed to the compounds. Furthermore, takeoff and flight activity was also much higher for workers exposed to copper sulfate.
While simple contact with copper sulfate, such as brushing on the legs, did not cause such extreme side effects, bees were still more likely to increase how much they ate. This further underscores the risks bees face when coming in contact with copper sulfate on plants.
Similar side effects have been studied in bumblebees exposed to low levels of neonicotinoid insecticide. In this case, bees exposed to insecticides collected more pollen, but it took them significantly longer than unexposed bees, perhaps because exposure alters the way bees learn and remember.
As important wild pollinators of Brazil, stingless Friesella schrottkyi are more efficient than honeybees, which is in partly why researchers were interested in learning more about the side effects of copper sulfate exposure. The recent study therefore highlights the need for alternate fertilizers.
"When considering agricultural production as a potential threat to Neotropical stingless bees, the problems likely go beyond pesticide use," the authors wrote in their study. "Leaf fertilizers seem to deserve attention and concern regarding their potential impact on native pollinators, notably Neotropical stingless bees such as F. schrottkyi."
Their study was recently published in the Journal of Economic Entomology.
