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Study shows pesticide exposure can dramatically impact bees’ social behaviors



A bumblebees worker (Bombus impatiens) who seeks out, with a unique tracking tag ( BEEtag). Credit: James Crall

For bees, being social is everything.

Whether it's looking for food, taking care of young people, using their bodies to generate heat or ventilating the nest, or building and repairing nests, a colony of bees does practically everything as a single unit.

While recent studies have suggested that pesticide exposure could have an impact on foraging behavior, a new study, conducted by James Crall, showed that those effects could only be the tip of the iceberg.

A post-doctor who works in the laboratory of Benjamin de Bivort, Thomas D. Cabot Associate Professor of Organismic and Evolutionary Biology, Crall is the lead author of a study that shows exposure to neonicotinoid pesticides ̵

1; the most commonly used pesticide class in agriculture – has profound effects on a range of social behaviors.

Using an innovative robotic platform to observe bees' behavior, Crall and co-authors including de Bivort and Naomi Pierce, Sidney A. and John H. Hessel, professor of biology, have shown that, as a result of Exposure to the Icid parasite, the bees spent less time treating the larvae and were less sociable than the other bees. Further tests showed that the exposure compromised bees' ability to heat the nest and build insulating wax caps around the colony. The study is described in an article of 9 November in Science .

In addition to Crall, de Bivort and Pierce, the study was co-author of Callin Switzer, Ph.D. 18, Stacey Combes of UC Davis, former research assistant in Organism and Evolutionary Biology Robert L. Oppenheimer and Mackay Eyster and Harvard graduate Andrea Brown, 19.

"These pesticides have gone into use around the mid-1990s, and are now the most commonly used insecticide class in the world," said Crall. "Typically, they work through the treatment of seeds: high concentrations are dosed on the seeds, and one of the reasons why farmers and pesticide companies love these compounds is because they are absorbed systematically by the plants … so the idea is that provide resistance to the entire plant.But the problem is that they also show themselves in the pollen and the nectar bees are feeding themselves. "

In the last decade, Crall said, a number of studies have connected the exposure to pesticides with interruptions in foraging, "but there were reasons to suspect it was not the whole picture."

"Foraging is only a part of what bumblebees do," he said Crall. "These studies were taking into consideration the important effects these compounds were having on what happens out of the nest, but there is a whole world of really important behaviors happening inside … and this is a black box that we wanted to open a little. "




Automatic tracking of nesting in a colony of bumblebee (Bombus impatiens). Credit: James Crall

To do this, Crall and his colleagues developed an exclusive bench-top system that allowed them to follow bees in twelve colonies at a time.

"What we do is put a black and white tag with a simplified QR code on the back of each bee," he said. "And there's a camera that can move around the colonies and automatically track the behavior of each bee using artificial vision … so we can look at the inside of the nest."

Using the system, Crall and colleagues could dose specific, individual Bees with the pesticide and observe changes in their behavior – less interaction with nest partners and spend more time on the outskirts of the colony – but those experiments are limited in several important ways.

"One is physiological," said Crall. "Even if we were giving realistic doses of pesticides to bees, drinking your daily dose of coffee in five minutes will be different than spreading it throughout the day, so giving a big dose may not be entirely realistic – another important thing is that a colony of bees is a functional unit, it does not make sense to treat individuals, because what you are losing when you do is the natural social structure of the colony. "

With the robotic system however, researchers can treat a & # 39 whole colony as a single unit.

Each of the 12 units in the system, said Crall, houses a single colony where the bees have access to two chambers: one to mimic the nest and the other to act as a foraging space.

"This allows us to do more exposures at the colony level and to do continuous monitoring," said Crall. "We think this is much closer to how their natural behavior works, and it also allows us to automate behavioral tracking across multiple colonies at the same time."

Just as in previous studies, Crall said, the exposed bees showed changes in activity levels and socialization, and spent more time at the edge of the nest, but tests also showed that the results were stronger during the night.

"The bees actually have a very strong circadian rhythm," explained Crall. "So we found that during the day there was no statistically observable effect, but at night, we could see that they were crashing in. We still do not know if (pesticides) are destroying the regulation of the circadian gene or if this is just a little bit, maybe a physiological feedback … but it suggests that, from a practical point of view, if we want to understand or study these compounds, considering the effects during the night counts a lot. "




a hornet (Bombus impatiens) worker during acute exposure tests. Credit: James Crall

Further experiments, in which the temperature probes have been placed inside external hives, the suggested pesticides have profound effects on bees' ability to regulate the temperatures inside the nest.

"When temperatures drop, the bees close their wings and shiver their muscles to generate heat," said Crall. "But what we have discovered is that, in the control colonies, even if the temperature fluctuated widely, they were able to maintain the temperature in the colony permanently within a few degrees, but the exposed bees dramatically lose their ability to regulate temperature." [19659005] In addition to disturbing the ability of the bees to heat or cool the nest directly, the experiment also revealed that exposure to pesticides influenced the ability of bees to construct an insulating wax cap on the colony.

"Almost all of our control colonies have built that hood," said Crall. "And it seems to be completely wiped out by colonies exposed to pesticides, so they lose this ability to make this functional renovation of the nest."

Going forward, Crall said, there are some additional questions raised by the study that

"This work – particularly on thermoregulation – opens up a new set of questions, not just on what the direct effects of pesticides are, but on how these pesticides compromise the colonies' ability to cope with other stressors, "he said. "This work suggests that, in particularly extreme environments, we might expect the effects of pesticides to be worse, so it changes both the way of practically testing agro-chemicals in general, but indicates specific questions about whether we could see stronger declines in certain environments. "

Taken together, Crall believes the findings indicate the need for closer regulation of neonicotinoids and other pesticides that could impact bees.

"I think we are at a point where we should be very, very worried about how the ways we are changing the environment are undercutting and decimating insect populations that are important not just for the function of each ecosystem … but that are very important for food production, "he said. "Our food system is becoming more and more dependent on pollinators over time – today about a third of food crops depend on pollinators, and this is only on the rise." So far, we have had this abundant and natural gift of pollinators that they do all this work for us, and now we are starting to realize that it is not a fact, so I think we should be very worried about this. "


Explore further:
Neonicotinoid pesticides influence harvesting and social interaction in bumblebees

Further information:
J.D. Crall el al., "Exposure to the neonicotinoid interrupts the behavior of the hornet's nest, social networks and thermoregulation" Science (2018). science.sciencemag.org/cgi/doi … 1126 / science.aat1598

"The pesticide influences the social behavior of the bees" Science (2018). science.sciencemag.org/cgi/doi … 1126 / science.aav5273

Journal reference:
Science

Provided by:
Harvard University


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