Pesticides affect bees inside their nests

How do pesticides impact bees when they get home?
13 November 2018

Interview with 

Dr James Crall, Harvard University


Worker bees


Neonicotinoids are the world's most popular pesticides; and they're often added to seeds so that, as plants grow, they're protected from insect attack. But the chemicals also get into the plant's nectar, meaning that harmless insects, like pollinating honey- and bumble bees, are inadvertently exposed too. This affects the bees' foraging behaviour; but what about their behaviour when they get home, inside their nest? No one knew, but now, speaking with Chris Smith, Harvard scientist James Crall has developed a way to tag individually an entire nest of bumble bees so he can follow what happens to their behaviour when they're exposed to neonicotinoids, and it's not good news...

James - Neonicotinoids mimic chemicals that work in the insect’s central nervous system and they were developed in the late 80s and came into the market in the 90s are now really widespread. They are the most common group of insecticides used across the globe and over the past few years we've realized more and more these compounds, even at low levels, they're still having negative impacts on bees. So what we were interested in understanding is a little bit more of why that might be happening. And in the past 10 years or so we've got a really great understanding now, of the ways that these compounds affect foraging behaviour. But what's going on inside of the nest, all of this important behaviour within colonies has really been kind of a black box, and so what we wanted to do in our work is understand what these compounds are doing to that social life of the colony within nests.

Chris - Is that easy to do?

James - Yes and no. So it's easy if you're willing to have a few angry bees in the face. It is easy enough to open up a colony and look inside of it. One of the challenges of watching individual bees is it's really hard to scale up in a few different important ways. So one is of course, it is hard to focus on 100 different things at once, so if there are 100 bees in a colony it’s really really hard to understand what every one of those is doing at the same time. It's also really hard to do it for a long time period. And it's also really hard to do across multiple colonies and so what we wanted to do in our work is get some automatic tools that let us do that work without having to look by eye. What we do is outfit every bee in the colony with a special little paper backpack, sort of like a very simple QR code that we've made really simple so we can make it really small, and just glue it to the back of a bee. That lets us look inside of the nest with a camera and identify the location and movements and interactions uniquely identified workers, automatically, without having to look by eye.

Chris - So basically all the bees are tagged in a computer via a camera, and can watch where individual bees go, and what they do and presumably then you can compare bees that are exposed to pesticides and those that aren't and you can ask that crucial question: Does this make a difference to how they behave?

James - That's exactly right. So one of the first things that we wanted to do with this technology of being able to look inside of a nest and follow what individual workers were doing, is we pull out individual workers, feed them different amounts of a really common neonicotinoid pesticide called imidacloprid, and we can feed different amounts to workers that have been brought out of the colony and made a little bit hungry. So they're more likely to eat a little artificial sugar, and then we can put a controlled amount of this compound in there. And so within the same colony we can feed different bees different amounts of the pesticide. And because we know that individual behaviour of every bee in the colony, what we can look at is the changes in behavior within individuals. So is a specific worker acting differently and behaving differently than it was after we give them a little bit of this pesticide.

Chris - And is the exposure what we would dub field relevant. In other words if these were bees in the wild and they were foraging in an area where a farmer had used some of this particular type of neonicotinoid, would the exposure that you’re giving to them be equivalent to what they would probably encounter naturally?

James - Yeah that's a great and really important question for this kind of work. The concentrations we are looking at are in the range that we expect to see in nectar and pollen in plants that have been treated months before, as seeds, with these compounds.

Chris - And when you do this, do you see a change in the way that the bees behave?

James - Yes we do. So we see a couple different things change, so one thing is that workers within the nest move away from the nest centres. So they seem to be a little further away from where a lot of the centre of action of the nest is. They're also less active and they also interact less with nest mates.

Chris - And do you think this makes a difference though? Because some of these changes they sound quite subtle. Does it make a difference to the fitness of the nest?

James - Yeah we think it is likely to have important ties to colony function in the long run, even though we didn't directly look at that in this study. There is now a lot of other really great studies and great work showing that even these same concentrations that we studied here, can negatively impact the colony growth. And so we think these changes in behavior, for example bees spend less time nursing and doing other kinds of nest care. All of those are really important for the growth of a colony. So this work suggests that we might be disrupting a lot of these other important behaviours within the nest and not just foraging.

Chris - So what should we do about this then? It sounds to me like we have pretty incontrovertible evidence that these compounds, that are being routinely used, are very disruptive to at least some species of insect. So can we justify continuing to use them?

James - I think that's a really important question, and I think at the very least we certainly need to be having a very serious conversation about the role of these compounds in our food systems. We know for sure that these compounds can have negative impacts but there are still huge open questions in terms of different species, how they vary over time and place. So I think we need to be thinking very very hard about the benefits that we get from using these compounds and what the clear risks are.


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