Do bees taste pesticides?

Modern pesticides, like the neonicotinoids, are extremely effective for protecting crop yields against hungry insects. The problem is that not all hungry insects are unwelcome: some, like bees, are providing a hugely important service through pollinating the plants, which drives up yields. And there’s therefore a risk that they will be indiscriminately poisoned. But the risk might be reduced if bees can taste - and therefore learn to avoid - plants laced with pesticides. So can they? Speaking with Chris Smith, Rachel Parkinson…

Rachel - Lots of different pesticide compounds are used on crops in order to deter pest insects from feeding on them. But other insects, most notably sort of beneficial insects like pollinators, so like bees, will also be feeding on these plants and could be exposed to pesticides. And a question that was sort of unanswered was can bees even taste pesticides in nectar? Would they be able to avoid drinking these pesticides by knowing that they're present based on their taste?

Chris - Does that seem like a reasonable hypothesis to you or would you think It seems a bit spurious because if the other insects are not able to taste and avoid these pesticides, then why should bees be any different?

Rachel - That's a good question. We don't know necessarily that other insects don't taste the pesticides, so that actually could be one potential deterrent effect of the pesticide, for example. We would expect that they should taste bitter to a bee. So if you put something bitter in the nectar, it might make the nectar taste bad. And so if the bee has a mechanism for detecting it, then they might avoid drinking that nectar altogether.

Chris - And obviously if they were able to learn to avoid it in that way, that would be good because it would argue that we can worry a bit less about using pesticides and not taking down our friendly pollinators in the process?

Rachel - Absolutely. So one of the crops that these pesticides are used on is oil seed rape, and this crop actually does not require insect pollination. It fares a little bit better when there are insect pollinators present, but it's not required. So if bees could taste the pesticide and the nectar and then just choose to forage on other food sources, other flowers that were not laced with these pesticides, then this would really be very protective for the bees.

Chris - How did you test it then? How did you try to find out whether they can detect these pesticides?

Rachel - It's not so easy to ask a bee whether or not it can taste something. So one of the ways that we do this is actually by recording from the taste buds. So on insects they also have a tongue and the tongue has various components to it rather than there just being one tongue. They have various mouth parts and these mouth parts are covered in what are called sensilla. So these are like the bees taste buds and we can record from individual sensilla and sort of test whether or not the responses are different when you just have a sugar solution or if you have a sugar solution that contains a pesticide.

Chris - Do they have different taste buds detecting different tastes? So how do you know you are recording from the right one or does one taste bud do everything?

Rachel - That's a really great question as well. So the taste bud itself or the sensillum itself will contain several different neurons. And then each neuron is thought to respond to one type of taste sense. But they use all potentially all of the neurons inside of these sensilla to taste sugars. So they're tasting sugar in a really, really specialised and intricate way. And we tested a whole bunch of different sensilla over different mouth parts so we're not just zooming in on one single taste bud but rather taking a sample from across the the mouth parts.

Chris - What do you do then dunk the sensilla in a sugar solution which has or hasn't got pesticide in it and see if the nerves fire off?

Rachel - Yeah, pretty much. So we make these little electrodes out of these very fine glass tubes and the glass tube can then be filled with whatever we want them to taste. So whether it's the sugar solution or the sugar solution containing the pesticide and then we put an electrode inside one end of that tube and the other end, the sharp end of the tube, we put it just over top of the sensillum so then we can both stimulate the sensillum using the sugar solution that's inside of the capillary and record the electrical activity of the neurons at the same time.

Chris - And did you make sure that the concentration of pesticide that you were putting in there was kind of relevant to what a bee would encounter for real in the environment?

Rachel - Yes, absolutely. So we used quite a broad range of pesticides across all of the sort of measured environmental field levels. We also tried a really high concentration just to kind of have, you know, another, another point of comparison because we thought well maybe they can't taste the really low ones but surely they can taste this really high concentration.

Chris - So you covered all those bases. Did they respond though? Did you get activity changes from the sensillum when you did this proving the bees could taste these pesticides?

Rachel - Sadly, no. It seems as though across the whole range of potential field-relevant concentrations and as well this very high concentration of pesticides, the responses were essentially the same whether we stimulated with just the sugar solution or the sugar solution plus the insecticides.

Chris - So bees cannot taste these pesticides. That would be your conclusion?

Rachel - That's the conclusion, yes. And we also tested this using behaviour to see if well maybe they have a different kind of mechanism or maybe we're not going after the right sensilla, you know, just to see is there any concentration that they will avoid drinking. So we, we also came out at it from the, the aspect of behaviour

Chris - And what are the implications then? Does this mean that bees are basically gonna go and forage from fields that have been sprayed and they're gonna potentially poison themselves?

Rachel - Yes, yes, exactly. So this is the implication if, if they won't avoid drinking the pesticide-laced nectar and they don't seem to really get the kind of what we call post ingestive feedback. So this is, you know, if you eat something and a couple hours later you feel really sick, you'll probably associate that with whatever the last thing was that you ate. This kind of post ingestive feedback is, is another way that bees can associate something negative with what they're feeding on. And, unfortunately, we find that when we give them this window of time in which to feed on the solution and then potentially react also to these post ingestive feedback mechanisms, they still don't avoid drinking the solution. So it's really quite concerning because a bee foraging in the field during its foraging bout will not get sufficient information from what it's feeding on in order to stop feeding on it and you know, avoid coming back there again.