Scientists at the University of Washington and Emory University report that an antibiotic sprayed on orchard crops to combat bacterial diseases slows the cognition of bumblebees and reduces their foraging efficiency.
The study, published in the Proceedings of the Royal Society B, focused on streptomycin, an antibiotic used increasingly in U.S. agriculture during the past decade.
“No one has examined the potential impacts on pollinators of broadcast spraying of antibiotics in agriculture, despite their widespread use,” said lead author Laura Avila, a postdoctoral researcher at Emory University.
A wild bumblebee visiting a blossom. Image credit: Emory University
The team tested how the common eastern bumblebee, Bombus impatiens, responds when its food is dosed with streptomycin. They fed an experimental group of bees a diet of sugar water dosed with streptomycin, while a control group received sugar water.
Research has not established the level of streptomycin wild bumblebees receive when they forage crops sprayed with the antibiotic. But the dose that the team tested — 200 parts per million — is likely the highest that bees receive through their diet of floral nectar, based on estimates of the concentration of streptomycin sprayed on crops and the frequency of spraying.
“This paper is a first step towards understanding whether the use of antibiotics on food crops may be taking a toll on pollinators that benefit agriculture,” said senior author Berry Brosi, who began this study while a faculty member at Emory University and has been an associate professor of biology at the UW since 2020.
After two days on this diet, the team presented bees with two construction paper strips — one yellow and one blue. One color was saturated with plain water, and the other was saturated with sugar water. The team measured the number of trials it took for an individual bee to show preference for the colored strip saturated with sugar water. Bees fed streptomycin required roughly three times as many trials to make the association compared to the control group. Streptomycin-fed bees were also more likely to display avoidance behavior toward either strip of paper.
In another experiment, bees that passed a training threshold were given a short-term memory test five minutes later: They presented each bee with both paper strips simultaneously and allowed it to select one. Control bees chose the sugar-saturated strip 87% of the time. For streptomycin-fed bees, the success rate dropped to 55%.
A third experiment tested foraging abilities in a chamber with two types of artificial flowers — blue and yellow — that randomly dispensed either sugar water or plain water. Each bee was outfitted with a tiny, ultra-lightweight radio frequency identifier “backpack” to monitor its movements among the artificial flowers. The team found that antibiotic-exposed bees visited fewer flowers that dispensed sugar water relative to the control bees.
Based on evidence from other studies, the researchers hypothesize that the negative impact of streptomycin on bumblebees may be due to the disruption of the insects’ microbiomes.
“We know that antibiotics can deplete beneficial microbes, along with pathogens,” said Avila. “That’s true whether the consumers of the antibiotics are people, other animals or insects.”
Other studies have shown that the antibiotic tetracycline can alter the gut microbiome of the insects and indirectly increase susceptibility to pathogens and mortality. High doses of the antibiotic oxytetracycline can also disrupt the bumblebee gut microbiome, decreasing their immunity to pathogens. Exposure to high doses of tetracycline have been found to affect honeybee learning, while oxytetracycline slows the onset of foraging in managed colonies.
But during the past decade, the spraying of antibiotics on U.S. crops has increased exponentially as farmers battle a rise in plant bacterial infections. These include “fire blight,” which can blacken the blossoms of apple and pear trees and kill them, as well as “citrus greening,” which turns citrus fruits green and bitter and has devastated millions of acres of crops throughout the United States and abroad.
Approximately 75% of the world’s food crops depend on pollination by at least one of more than 100,000 species of pollinators, including 20,000 species of bees, as well as other insects and vertebrates like birds and bats. Many of the insect pollinator species, particularly bees, face risks of population declines.
“Production of our food, farmer livelihoods and the health of pollinators are all tied together,” said Brosi. “It’s critically important to find ways to maintain agricultural production while also conserving the ecosystem services — including pollination — that a biodiverse ecosystem provides.”
The team will next conduct field studies on experimental fruit orchards sprayed with streptomycin. If they find a detrimental impact on bumblebees, the researchers hope to provide evidence to support recommendations for methods and policies that may better serve farmers.
Source: University of Washington