How to make plants more attractive to pollinators

The engine of any garden is its pollinators – organisms lured into a flower by the promise of a tasty treat, and who leave, often unknowingly, with a dusting of pollen destined for another bloom. Pollinators come in many forms: birds, bats, and occasionally even the odd lemur. But by far the largest group is insects. Insects rely on a mix of signals to decide whether a flower is worth visiting. Some are to do with smell, some even with electricity and one of the most important is visual. So important are these factors, in fact, that certain plants are simply less attractive to pollinators and get passed over. So, what factors in a plant’s advertisement system are the key to getting them back in the pollinators’ good books? Beverley Glover is the Director of the Cambridge University Botanic Garden...

Beverley - Well you're looking for food and you're looking for advertising that tells you that that food is there. So the first thing as you said is the reward, something tasty to eat, usually nectar, sometimes pollen because it's quite protein rich, occasionally other weird things, oils, resin, scents, but usually nectar. And you need something you can see that you can learn to associate with the presence of that reward. Of course what you can see is going to depend on what sort of animal you are.

Will - Is this something that we can see with our human eyes or is this something that due to our evolution of the visible light spectrum is something that escapes us?

Beverley - So yeah it's very different for all sorts of different animals. So we're standing here in the systematic beds at the Botanic Garden at the Boragineaceae family and we're looking at this cyanoglossum which is covered in bees and they are liking it because that purpley blue colour you can see is well cued to their visual system. So bees have like us a trichromatic visual system. We see red green blue, they see green blue ultraviolet. So it's different from its blue shift and this particular hue of flower is obviously very attractive and stands out to them. But behind me on the next bed there's a bright red penstemon here. It's got no bees on it at all. If we have a look that will have plenty of nectar in it but the bees aren't seeing it very well because that red colour doesn't pick up in their visual system. It essentially looks muddy green to them. That particular flower has evolved to signal to a hummingbird and birds have tetrachromatic visual systems so they see the red green blue that we see and the UV that the insects see. And that is perfectly visible to them and they know that it will have lots of nectar in it because big fat annoying bumblebees can't see it and won't have already got in there and drunk all the nectar up.

Will - So if we wanted more bees to visit a plant, is it in the realms of science fiction if we understand the genes that code for these molecules and these enzymes. Could we pop a couple in to therefore make a plant a bit bluer a bit more attractive to a bee?

Beverley - So that's a good question people ask me this a lot because I work on the production of those diffraction gratings that make the blue colour and I don't really imagine that I'm going to make you know iridescently blue oilseed rape and fava bean all around the countryside. Because actually the pollinators are particularly bees if you're talking about bees. They are quite smart so the key thing for them is the food. If the nectar reward isn't there then you can make it look as pretty as you want but they won't stick with it they won't keep coming back to it and you won't get pollination. So in the work I do with crop pollination the focus is first on what's the reward we've been for instance exploring different fava bean varieties and cultivation in the UK looking at which ones make the most nectar and the most sugar-rich nectar and that's got to be the starting point because this is a smart animal. It'll learn what's good for it. Once we've got that right yeah we can make it a little bit more interesting we can think about the size of the flower and the length of the the tube that the nectar is in and the colour of it and you know blue wouldn't hurt but it's it's sort of the icing on the cake it's a clever animal. In the end I hope your listeners are smart enough to buy the product they want however good or bad the advert is. The advert might catch your eye in the first place but it's the product that matters.

Will - No comment. So therefore is it as you said if we're looking to use genetics to improve a plant and improve its relationship with pollinators are we therefore looking for the pathways to create a better meal for the bee instead?

Beverley - Yeah and it's not just about the quality of the meal it's about the energy you expend getting that meal out as well. So it's about energy gained for energy spent so part of that's about the reward how sugar rich how much of it but it's also about how hard is it to get into the flower so if you think about something like a snapdragon flower there the dark pink one over there are the yellow ones behind, only a heavy bumblebee can actually open that flower and so if what you wanted was to attract honeybees to those flowers you'd have to do something to make them less difficult to get into. A honeybee will spend an awful lot of time watching the hoverflies indefinitely and they'll never figure out they're not smart enough so they're wasting energy unable to access the flower whereas a bumblebee lands on it the lip opens and it gets into the nectar. That's actually an evolutionary advantage for the flower and for the bumblebee because again as with the red flower we talked about earlier the bumblebee knows that that snapdragon is likely to be nectar rich because he knows that hoverflies and everything else can't get in. But if we wanted to think about how to improve the attraction to flowers not just the reward itself but also how much energy you spend getting to it so you can think about how you open the flower does the flower provide grip so if it's slippery and difficult to access that makes it more energetically inefficient If you provide conical type cells on the surface that the insect can get a grip on like you might on a climbing wall that reduces energy expenditure. If you make the tube length such that you don't have to you can literally just land and drink rather than land and have to crawl and wiggle your way around thinking back to visual cues things like nectar guides so lines or spots or a bullseye pointing to where the nectar is can make the difference between taking three seconds to land in the right place and two seconds to land in the right place. And if you're visiting 100 flowers then that's a lot of seconds of energy spent flying so all of these things help maximise the energy reward in exchange for the energy spent getting it.

Will - If we're looking to create this ultimate energy efficient garden then for our pollinators is this something that is as simple as selective breeding do you think or is this something that requires a bit more of a modern genetic toolkit?

Beverley - Well it can be done with selective breeding i mean gardeners and plant breeders have been breeding the horticultural industry for centuries and have produced all sorts of different colours and combinations and patterns some of which you'll see in your own garden are really attractive to pollinators and some of which aren't. Of course knowing a little bit about genetics and how these things are built the developmental biology of these traits makes it much easier for us to identify where we should be focused. But i think the thing is i'll bring you back and say i don't think there's ever going to be a perfect flower because there are hundreds of thousands of different pollinator species so actually what suits a bumblebee isn't what suits a hummingbird is not what suits a hoverfly and if we want to keep that range of pollinator species alive and well then we're going to need a range of different flower types.

Will - I would be remiss as a final question then given that you are the director of this wonderful garden not to ask if the listeners at home as you said wanted a good vibrant garden keeping pollinators well-fed is it about that diversity do you think there's no there's no one shoe size fits all type thing

Beverley - Yes absolutely and the one key thing i'd ask people to think about which people tend to forget is the time at which the plant flowers so diversity of flower types but you've got pollinators foraging from march when bumblebee queens hatch all the way through to october november and so you want plants in your garden that produce food for that entire duration We're all very good in the uk at may and june looking gorgeous in the garden and the rest of the year it being a bit dry and sad try to think about things that will give you flowers for that whole duration.