Antibiotics used on food crops affect bumblebees

An antibiotic sprayed on orchards to fight bacterial diseases slows bumblebees’ cognition and reduces their foraging efficiency, a lab study has found. Proceedings of the Royal Society B published the findings of scientists from Emory University and the University of Washington.

The research focused on streptomycin, an antibiotic that has been increasingly used in US agriculture over the past decade.

“No one has looked at the potential impacts on pollinators of widespread spraying of antibiotics in agriculture, despite their widespread use,” says Laura Avila, co-lead author of the paper and post-doctoral fellow in the Department of Biology. of Emory.

The current study was based on laboratory experiments using an upper limit dietary exposure of streptomycin for bumblebees. It is not known if wild bumblebees are affected by agricultural spraying of streptomycin or if they are exposed to the concentration tested in the field.

“This article is a first step in understanding whether the use of streptomycin on food crops can harm pollinators that benefit agriculture,” says Berry Brosi, lead author of the article. Brosi began work as a faculty member in Emory’s Department of Environmental Science and currently works at the University of Washington.

Funded by a grant from the United States Department of Agriculture, the researchers will now conduct field studies where streptomycin is sprayed on orchards. If a negative impact is found on bumblebees, researchers hope to provide evidence to support recommendations for methods and policies that might better serve farmers.

“The production of our food, the livelihoods of farmers and the health of pollinators are all linked,” says Brosi. “It is critically important to find ways to maintain agricultural production while maintaining the ecosystem services – including pollination – that a biodiverse ecosystem provides.”

Based on established evidence, researchers hypothesize that the negative impact of streptomycin on bumblebees observed in laboratory experiments may be due to disruption of the insect microbiome.

“We know that antibiotics can deplete beneficial microbes, as well as pathogens,” says Avila. “That’s true whether the consumers of antibiotics are people, other animals, or insects.”

Avila is a member of the lab of Nicole Gerardo, Emory Professor of Biology and an entomologist who studies the co-evolution of insect-microbe systems.

Over the past decade, the spraying of American crops with antibiotics has increased exponentially as farmers battle an increase in bacterial plant infections. “Fire blight” can blacken the blossoms and shoots of apple and pear trees, making them appear scorched by fire, and can also kill entire trees. “Citrus greening”, also known as “yellow dragon disease”, makes citrus fruits green, bitter and unusable and has devastated millions of acres of crops in the United States and abroad.

“I saw how hard it is to make a living growing crops, how expensive and difficult it can be to control diseases and pests,” says Avila, who grew up in a coffee-growing region of Costa Rica.

Largely untouched forests bordered his family farm. “The diversity around us fascinated me,” says Avila. “I decided to become a bee biologist because I wanted to understand how the natural environment can influence agricultural production and vice versa.”

Seventy-five percent 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 such as birds and bats . And yet, many species of pollinating insects, especially bees, are threatened with extinction.

Previous studies have shown that the antibiotic tetracycline, used to treat pathogens in managed bee hives, can alter the gut microbiome of insects and indirectly increase pathogen susceptibility and mortality. Exposure to high concentrations of oxytetracycline was also found to have a similar effect on the gut microbiome of bumblebees, decreasing their immunity to pathogens. And exposure to high doses of tetracycline has been shown to affect learning in bees, while oxytetracycline slows the onset of foraging in managed colonies.

For this article, the researchers conducted laboratory experiments with managed bumblebees, Bombus impatiens, to test the effects of upper limit dietary exposure to streptomycin. Half of the bees were fed plain sucrose, or sugar water, to simulate nectar. The remaining bees were fed sucrose dosed with streptomycin.

After two days of this diet, the bees were given strips of cardboard of different colors – one yellow and the other blue. One color was saturated with clear water and the other was saturated with sucrose. In a series of training trials, each bee was given a single colored band until it touched it with its antennae or proboscis.

The researchers measured the number of trials it took for a bee to show a preference for color bands saturated with sucrose. Streptomycin-fed bees often required about three times as many trials to establish the association, compared to other bees. Antibiotic-treated bees were also more likely to display avoidance behavior towards either stimuli.

Bees that passed a training threshold underwent a short-term memory test five minutes later. Each bee received both cardboard strips simultaneously and was allowed to select one. The rate at which bees dosed with streptomycin selected the sucrose reward was about 55%, while untreated bees selected sucrose at a rate of nearly 87%.

To assess foraging ability, trials were conducted in a foraging chamber containing an experimental set of artificial flowers that dispensed sucrose or plain water. The flowers were blue or yellow but were identical in size and shape. Each bee was equipped with a tiny, ultra-light radio frequency identification “backpack” to monitor its movements among the artificial flowers, which were each fitted with a short-range antenna and tracking system.

Computer-analyzed results showed that bees exposed to antibiotics visited significantly fewer sucrose-rewarding flowers compared to control bees.

In the spring, Avila and Brosi will initiate field studies to determine if widespread streptomycin spraying affects bumblebees in pear orchards.

“I was surprised at the strength of the effect we found of streptomycin on bumblebees in lab experiments,” Brosi said. “This makes it imperative to know if we see similar effects in an agricultural setting.”

Timing of antibiotic application, amount applied, and possible alternatives to using an antibiotic may be potential mitigation methods if field research identifies harmful effects on bumblebees from agricultural spraying. of streptomycin, the researchers note.

Co-authors on the current study include Elizabeth Dunne, who did the work as an Emory environmental science major and has since graduated; and David Hofmann, former postdoctoral fellow in Emory’s Department of Physics.


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