Fertiliser affects how bees see flowers

Fertilisers are critical to modern agriculture: without the higher crop yields they provide, we’d all starve. But, fertilisers are typically made using ammonia, meaning they have a significant environmental footprint in the form of greenhouse gas emissions. They also produce algal blooms in rivers, reservoirs and even the sea. Now a new study suggests that - by affecting the electrical fields around flowers - fertilisers can also alter the way that pollinating insects, like bees, see them. Bristol University’s Sam England told Will Tingle how plants generate their electrical fields, and what fertilisers might be doing to put bees off…

Sam - The electric fields around flowers kind of have two main sources that we're aware of. One comes from their interaction with the atmosphere. So these flowers are conductors to some extent in the same way that the metal in your wires and your electrical appliances at home are conductors. Actually, plant tissue is quite conductive too. Now, the interesting thing is that these plants are obviously stuck in the ground, but they're pointing up above the ground into the atmosphere. And the atmosphere actually has an electric field in it too. And when this gets really strong, that's when you see things like thunder and lightning. But even in fair weather, there is still some amount of vertical electric field in the atmosphere. What this means is that the grounded plant relative to the air around it, actually has an electric field coming off of it because it's not at the same electrical charge as the surrounding air. But also in addition to these kinds of electric fields that are generated by their interaction with the atmosphere, the plant itself also has all kinds of physiological processes going on inside it, where it's transporting ions and signaling within the plant between cells also generate an electric field external to the flower. So those are the two main sources of electric field around flowers that we know of at the moment.

Will - And how would insects such as bees use these electrical fields to detect the presence of flowers?

Sam - Something that's really cool and that we've only really discovered over the last decade or so is that a couple of different species of insects are able to detect electric fields in air. And the way that they do this is that basically hairs on their body get pushed around by the electric field. If you think about the same way as when you charge a balloon up with your hair, the balloon can kind of push your hair around. Essentially the same thing is happening with the tiny little hairs on these bees as they're entering into the electric field around flowers. And the cool thing is that it might actually provide some useful information to them. In theory they could be using it to just detect the location of flowers, but of course they have excellent senses of smell and also vision that help them do that. But something that we have discovered previously is that when a bee visits a flower, it modifies that electric field around the flower for some amount of time on the order of a few minutes. And so then a bee that comes along after that first bee might be able to detect that difference in the electric field and essentially use it as a cue as to whether a bee has visited that flower recently and that maybe the nectar might be diminished because it's been visited recently. So it might basically give these bees a bit of a clue as to whether there's a good amount of nectar in that flower or not.

Will - What is it about synthetic fertilisers that alters the detectability of these flowers?

Sam - We try to unravel this a little bit in our study, but for sure this is something where the exact mechanisms need to be delved into in a little bit more detail. We think there's probably, again, two main ways that this is happening. One is that it just modifies the electrical properties around the surface of the flower, but also in the air around it by changing things like the humidity, but also the conductivity like the electrical conductivity on the surface of the flower. And this essentially means that the electric fields travel in a different way through the material which can make them stronger or weaker. And in the case of the fertilisers that we looked at, it seemed as if they actually make the electric fields stronger, which might be quite confusing to the bees. The other way that it might be having an impact is that a lot of these harsh chemicals that are found in synthetic fertilisers can stress the plant out. And when they do this, that will basically cause the plant to start moving ions around, kind of signaling within the plant between different cells to essentially tell other parts of the plant that maybe we're in a bit of trouble here, which again, that will change the electrical profile of the flower.

Will - Finally, what would be the ramifications on insect populations and their relationship with flowers because of this discovery?

Sam - This is one of those things where there's really a huge arsenal of evidence that's beginning to accumulate that there are just so many different ways that a lot of our activity and a lot of the chemicals that we're introducing into the environment are having quite adverse effects on quite a few different species of insects. We already know that a lot of fertilisers and pesticides are quite harmful towards pollinating insects like bees. And our study really just adds another dimension to how that's happening. But of course that does somewhat optimistically open potentially new avenues for how we can try and mitigate these negative impacts in the future as we continue to develop these technologies.