September 25, 2018

expert reaction to a study on the impact of glyphosate on honey bees

Research published in PNAS shows that glysphosate (a common herbicide) increases the susceptibility of bees to certain pathogens when ingested.

Dr Andres Arce, evolutionary ecologist at Imperial College London, said:

 “Motta et al investigated whether dietary exposure to glyphosate, a commonly used herbicide thought to have a low toxicity in animals, could alter the bacterial gut microbiome of honeybees. The bacterial microbiome plays a number of roles in keeping bees healthy, such as by helping them resist disease and process nutrients, so any pesticide induced alterations to the microbiome could indirectly affect bee health.

“The study demonstrates that the bacterial microbiota in honeybees can be altered by exposure to glyphosate. Crucially, Motta et al also demonstrate that pesticide exposure also appears to affect bee health by increasing susceptibility to a common insect pathogen. Interestingly, bees that were not exposed to the pathogen showed comparable survival to bees that were never given glyphosate.

This study highlights how commonly used pesticides, even those marketed as being targeted at specific plants or animals, can unintentionally affect non-target organisms. It also highlights the importance of considering exposure over an extended period of time (>1 day) and the importance of multiple stressors, such as the effect of pesticide and disease. Both are typically overlooked when assessing pesticide safety and both are likely to be important in the wild.

“This study is part of a growing trend towards looking at more complex interactions between animals, their microbiome, and interacting stressors. Understanding these interactions is essential to quantify the hazards associated with pesticide use and is essential if we are to develop strategies that allow us to continue using pesticides, which are vital to modern agriculture, whilst minimising their effects on the natural world.”

Prof Dave Goulson, Professor of Biology at the University of Sussex, said:

“This is a well conducted study which finds that ingestion of low concentrations of glyphosate alters the natural bacterial gut community of honeybees and makes them more susceptible to harmful pathogens. In recent years it has become increasingly apparent that gut bacteria play a vital role in maintaining good health, in organisms as diverse as bees and humans. The finding that these bacteria are sensitive to the most widely used pesticide in the world is thus concerning.

“One might wonder how bees could ever become exposed to a herbicide in the real world. Glyphosate kills plants, so contaminated flowers will soon be dead and of no interest to bees. Nonetheless, glyphosate IS sometimes found in bee food stores, at concentrations similar to those used in this study.

“Those of us that study bees have long ago come to the conclusion that colony health is adversely affected by a number if interacting stressors, including exposure to cocktails of insecticides and fungicides, impacts of pathogens, and effects of poor nutrition. It now seems that we have to add glyphosate to the list of problems that they face. This study is also further evidence that the landscape-scale application of large quantities of pesticides has negative consequences that are often hard to predict.

“It is worth noting that, although this study was of bees, these findings are highly relevant to other beneficial organisms for almost all animals harbour beneficial gut microbes.”

Dr Oliver Jones, Associate Professor of Chemistry at RMIT University in Melbourne, Australia, said:

“This is quite a complex piece of work which investigates the effect of the pesticide Glyphosate on some of the bacteria that live in the guts of bees (the bee microbiome), rather than its effects on the bees themselves. The authors state that Glyphosate exposure can change the type of bacteria in the bee’s gut and this change may have negative effects on the bees’ overall health by, potentially, making them more susceptible to pathogens.

“The work is an interesting and novel approach. However, to my mind the doses of glyphosate used were rather high. The exposures were also relatively short so it is not known if any of the changes that were seen were short or long term. In addition, only 20% of the adult bees that were exposed to the pesticide were recovered and tested further so we don’t know what the effects on the other 80% were.

“A confusing result is that the bees exposed to the highest dose of Glyphosate seems to show far fewer effects than those exposed to a lower dose after three days. This effect was shown to be reproducible, but was not explained.

“It should perhaps be kept in mind that the paper shows only that Glyphosate can potentially interfere with the bacteria in the bee gut, not that it actually does so in the environment. There are also countries, such as Australia, where Glyphosate is used but where bees are generally doing well.

“So in short, I think the work is a potentially interesting piece of the overall puzzle of bee health, but not the whole picture”.

Declared interests

Dr Jones: I have no conflict of interest to declare.

Dr Arce: No interests to declare.

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