Flower patterns improve foraging efficiency in bumblebees by guiding approach flight and landing: Podcast Transcript

In this podcast for Functional Ecology, Assistant Editor, Frank Harris, sits down with Anna Stöckl—a Group Leader at Konstanz University, Germany—to discuss her recently published paper ‘Flower patterns improve foraging efficiency in bumblebees by guiding approach flight and landing’.

Anna’s paper shows that flower patterns reduced flower handling time by up to 30%, without a reduction in nectar discovery time. Instead, the patterns were involved in guiding approach flight, landing and departure decisions. Anna et al. revealed these effects on flower-experienced foragers. Since these represent the majority of active pollinators, the nectary-independent impact of flower patterns must be considered fundamental to plant–pollinator interactions under natural conditions.

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Today, I’m delighted to welcome Anna Stöckl to the podcast. Anna is a Group Leader at Konstanz University, Germany. She is fascinated by how sensory systems process information, to give animals a gateway to the world around us. At the core of her work is the question of how neurons transport and transform the signals they receive from the eyes into a meaningful output that can guide an animal’s behaviour. At present, Anna has authored over 25 papers.

On top of finding out more about her, we are going to be discussing her recently published article – Flower patterns improve foraging efficiency in bumblebees by guiding approach flight and landing. 

Anna: Hello, I’m very well. Thank you for having me. 

Frank: We are delighted to have you on our podcast with your perfect Functional Ecology paper. I was fangirling over it before this podcast recording—it really is a perfect example of what a functional ecology paper should be like and asks all the right questions! But before we get into the paper, perhaps you can tell us who you are, where you are from, and how you came to study bumblebees? 

Anna: So actually it’s been quite a journey to studying bumblebees and actually quite a journey through many different animal model systems! So I’d say my background is very much in biology/neuroscience. I studied both and we have this beautiful term for it— neuroethology. I’ve always been studying questions on how neural processing leads to adaptive behavior, with a focus on senses and vision. Vision is absolutely my favorite sense and that is where I’ve also focused most of my work. As I’ve said, I’ve been hopping around and working with lots of different model organisms, from jellyfish, to frogs, to electric fish, and mice. I’ve also studied this in the context of different insects through my touring around Europe. I’ve studied in Germany, in Heidelberg and Munich, completed by PhD in Lund in Sweden, and had a short stint in Finland as a postdoc. After all of this, I came back to Germany. With regards to this Functional Ecology paper, I actually conducted this work in at the University of Würzburg. Recently, I’ve moved as a group leader to Konstanz.

Our main study organisms, and we can talk a little bit more about how this study came to be, are actually not bumblebees! We’re working on hawkmoths. These are very charming insects that hover in front of flowers to suckle nectar and they are our main model organisms. This study has essentially opened bumblebees as an interesting complementary model Organism 

Frank:  So, before we jump in to the nitty gritty of the paper, I was wondering if hawkmoths are your favourite study organism? 

Anna: Firmly, yes. Absolutely, I have to say that. I’ve been thinking about how to pinpoint it. I think that they’re just incredibly charming to observe and learn from. We’re working on diurnal hummingbird hawkmoths that you can easily observe in many places in Europe during the summer. These hawkmoths hover like little hummingbirds and they’re very curious. Even in the lab, they’re not easily freaked out by us, so you can observe their behaviour very nicely. In addition to what I’ve said, they also provide fantastic access to the neural processing in their brain—they have very fascinating eyes. So, with all of the things that I’m interested in—eyes, visual inputs, and neural processing behavior—they just make a fantastic package. On top of that, they’re incredibly cute and can actually get people over their fear of moths, so I think for that they’re also a fantastic ambassador. 

Frank: Ticks all the boxes! So I always ask people how they became an ecologist and it’s usually to do with a fascination with nature from when they were really young. How did you come to study biology and neuroscience? 

Anna: I would say that my back story is very similar to that. When I was a kid, I always had access to beautifully illustrated nature and animal books. I always saw myself becoming an explorer somewhere studying leopards or lions, and then I became mainly a lab biologist and I’m actually rarely outside. I think that the reason for this is that since my school days, I’ve always had a great fascination with vision, and for understanding how this sense is so important to us—for experiencing the world around us—how this percept is brought about by the activity of neurons in our eyes and in our brain. It was this fascination that pretty much guided me through my entire studies where I began to lean more towards first biology and then more towards neuroscience, and, subsequently, get more into the mechanisms and into the fine scale details of animals in laboratory conditions

Frank: So it came from an interest in books? Growing up, did you spend a lot of time outside? Was it somewhere where you could experience nature?  

Anna: So I grew up in the southwest of Germany, in the beautiful Palatinate region. Actually, I should plug this while I can. I must let the listeners know that Palatinate is fantastic wine country. For all of those who doubt the quality of German wine, come to Palatinate and check it out, or drop me an e-mail for recommendations. I must say that I have no stakes in Palatinate wine, I just love to tell people. 

Frank: Is it something like 10% commission on the wines you sell us?

Anna: Haha, something like that! As I was saying, I grew up in the the southwest of Germany. Funnily enough, I wasn’t a great bird or insect watcher. I was really into books for sure and very fascinated by exotic animals. I think it took me a while to appreciate the things around me, especially insects. In fact, I was really scared of insects as a kid and even am, in some cases, as an adult. it’s been a process and it really came from a more theoretical fascination with the topic and very much the sensory ecology and processing, as well as neuroscience, questions that this fascination has asked of me. Insects make very good models for answering these questions, so that is how I became hooked—first as a neuroscientific biological model and then from there I started to really appreciate them more holistically. I am fascinated by their amazing behavioural repertoire and beauty. I’m getting better at being around them. Moths are perfectly fine, bumblebees are fine, but, with some other insects, I still have a healthy respect with regards to proximity.

Frank: Well, surely you have to now reveal which ones evoke the most fear in you!  

Anna: Ah, that’s true. Well, I wouldn’t say fear is the right word, but I haven’t made great friendships with locusts or cockroaches. They always shared rooms with our animals, so I had a lot of exposure, but we’re not friends yet. However, I would say that we respect each other. 

Frank: Wonderful. Let’s dive into the paper. For audiences who might not be aware of the meaning of different terms and the specifics of your study, can you please explain what the novelty of your paper is, and what it contributes to our understanding of foraging efficiency? 

Anna: Really, the context of our work is the importance or the role that flower patterns play in insect foraging. In a sense, this is something we all experience because we can easily watch insects foraging on the flowers that we have in our balconies and gardens. Like these insects, we are very much attracted to the colours and the beautiful patterns of flowers. So the context of our study is what role flower patterns have for insect foraging. There’s a very long history to the ideas behind this study that started in the late 18th century. It was during that time that people started to recognise that flower patterns can actually help insects to find the nectar in a flower. This in turn has led to the term ‘nectar guides’ being used to describe flower patterns. The idea is generally that these patterns might help insects to find the nectary—akin to something like a guide which directs them toward the nectar.

So, in our study, we were looking into the impact of flower patterns on bumblebee foraging. I should also say we were by no means the first ones to look at how flower patterns might improve foraging efficiency. There are lots of studies on the time that bumblebees take—either from landing on the flower to getting to the nectaries, or generally to go from flower to flower emptying nectaries—and many studies looking at this in different species of bees, in butterflies, and in other insects. As has been hypothesized for a very long time, these studies have shown that flower patterns very much improve foraging efficiency.

However, we were specifically interested in what role flower patterns play in this entire process—from an insect approaching and landing on a flower, to an insect making its way to the nectar, and then departing the flower to go to the next one. This was the focus of our study and we looked at that by carefully tracking individual bees during the entire process of approach, landing, finding the nectary, and then also departure. What we observed, which is really central to our study, is that these flower patterns played a role at a much earlier stage than we thought. They actually guided the bumblebees to approach the flower and guided them to landing positions very much like markings on a runway for a pilot. The patterns helped the bees to approach and also land on the flowers in positions that were ‘highlighted’ by the flowers. Though, what was really surprising to us was that we didn’t see any effect from the patterns on the actual time taken for the bees to make their way to the nectary. They would land and walk straight to the nectary, regardless of whether they walked along the pattern or not. They also didn’t use the patterns as a guide for where the nectary was. They always landed at the edge of the flower, guided by the patterns, and then they made their way straight to the nectary. This was really surprising and questioned the very concept that flower patterns are used as nectar guides. For us, what we observed is that the flower patterns acted as flight and landing guides. I should say, in the context of the literature, a 2011 paper by Leonard and Papaj (another study from Functional Ecology) also showed that naive bees were much faster at finding nectaries when they had patterns to work with. However, for bees that had experience in foraging, and had experience with the artificial patterns that we use in the lab, they went to the nectary at the same speed, with or without patterns.

In our study, it’s important to note that we tested all of this on bees that had been foraging on pattern-less grey flowers, but they knew where the nectaries were. So in this context, it seems that flower patterns are not needed to guide the bees to the nectary and the patterns don’t improve their efficiency to find the nectar. In our view, patterns are not used as nectar guides for bees that are experienced. However, even though they are experienced and know how to handle these flowers, the patterns do help by improving their efficiency of approaching and landing on flowers, and quite substantially so. The improvement in foraging efficiency we saw was about 30%, which means that for every third approach to a flower you’re essentially getting a free take if you have patterns compared to if you have none. 

Frank: That’s remarkable. So, we’ve covered some of the key takeaways and highlights. Perhaps it would be useful to talk about the similarities and differences between hawkmoths and bees, and maybe other animals with regards to vision and the processing of information? 

Anna: Let me give some background to this story on how we came to look at bumblebees interacting with artificial flowers. We had been studying hawkmoths interacting with flowers and flower patterns for quite a while before this paper on bumblebees. It’s really quite impressive because hawkmoths don’t land on flowers—they hover in front of them and they have this incredibly long proboscis (about as long as the animal itself). To envision this, I ask you to imagine that you’re trying to get a straw that is as long as your entire body into the opening of a can. It’s an incredibly complicated motor control task so the hawkmoths similarly use patterns on flowers to guide their approach and to guide their proboscis toward the nectary. This is what we had actually been studying, and to my shame, I should say that the hawkmoth paper is still not out and the bumblebee study overtook these studies in the lab.

Furthermore, and I think this is probably also a topic that comes up more regularly on this podcast, this was during the COVID-19 pandemic. Because of the pandemic, we could not access our hawkmoths for a while so we decided to pivot. I will give a big shout out to the students who did most of the experimental work in this study. We had students who wanted to work in the lab who had thesis projects planned, and that’s when the idea for the paper materialised: why not try a comparative study between hawkmoths and bumblebees which were available in Würzburg universityat the time. I must add that they were only available because my colleague Johannes Spaethe, who is an expert on bumblebees and bumblebee behavior, was happy enough to collaborate on this project and lend his expertise for the experimental set up.

Now, with that context, what are the similarities between the bees and hawkmoths? Well, first of all, in terms of their visual ecology, they’re very comparable in that they have similar colour preferences. So both the bees and hawkmoths, for example, prefer blue and yellow colored flowers and also blue and yellow contrasted patterns. So we were able to test with exactly the same stimuli. They also have very similar color vision systems—trichromatic color vision, UV blue and cream, and spectral and sensitivity peaks—so in that sense they’re very nice for comparative studies. The big difference, when it comes to flower interactions, is that one of them lands on the flower and one of them hovers in front of the flower, which means that there are great similarities and contrasts with regards to how they forage. The hawkmoths essentially land their proboscis and the bees land their whole body. For the bees, this of course gives them a different sensory experience of flowers compared to hawkmoths. Because we were interested in visual cues, we decided to take any other consideration of senses out of the study. Flowers have a 3D shape, they have mechanosensory cues on their surface, and they have an odor. Of course, insects that land on the flowers experience cues that are completely different to the experiences of hawkmoths that are at a distance from the flower.

However, ultimately, the approach experience I think is very comparable. It is very comparable in terms of what we see in the data—both insects, hawkmoths and bumblebees, use patterns to align their body as they’re approaching. These cues function as ‘runway guidance’ for landing at the edge of the flower. So bumblebees land at the edges of the flowers and the hawkmoths also land their proboscis at the edges of flowers. The big difference is that, at least in our study, bumblebees didn’t follow patterns to the nectary, but hawkmoths do with their proboscis—they essentially tap along the patterns as if they were little walking paths.  

Frank: That’s amazing. So I’d like to ask you to get your crystal ball out and think about the future. Where would you like your research to be directed next? Are there any upcoming themes you’re looking forward to explore, and what changes do you hope your work will precipitate? 

Anna: I really like this question. So we’re very much anchored in quantitative behavioral studies. We have also done quite a lot of work on the kinematics of flight—neural and behavioral flight control. I think what’s really interesting about this study is the findings. The patterns of flowers are really important for bumblebees to control their approach flight and to control their landing. This is a persistent effect for all bumblebees, whether they are experienced in foraging or not.

There is a big community of neuroscientists, behavioral researchers who work on flight and landing control in insects, and sensory ecologists that work on the importance of insect-flower interactions and on flower pattern cues for insects. There currently isn’t that much desire, in my personal opinion, to actually bring these fields together—of having very quantitative investigations of how, for example, flower patterns could actually guide a bees flight and landing. We have all these tools and theories (or many theories, not all of them) so there’s lots of opportunity for collaboration. There’s a lot of questions around insect flight and landing control, and I think this is a really cool place where we could bring these things together. This study has involved lots of quantitative work, and I should say it has been conducted on very abstract and controlled stimuli—fake flowers. This work can actually be put into a more natural context which is something that I would really love to test in the future. It would be fascinating to test what we found on more natural flower patterns, and ideally also on real flowers and a variety of flowers in order to see how much of this is actually applicable to a real-world scenario. So, from all of our beautiful checkerboard and stripe patterns that we use to test bees and hawkmoths, what is actually relevant for an insect that lands on a natural target? I think that this is a beautiful example of where the neuroethological studies that we’ve been doing can provide a crossover in different study disciplines in the future. Both neuroscience and ecology can learn so much and gain many insights from each other. 

Frank: So I was wondering if you could speak on foraging and bees and the impact of your paper then under an ecological umbrella. What’s the relevance there? 

Anna: Essentially, what our results show is that humans must have a bit more of a differential perspective on flower patterns and their ecological importance. Our results, together with previous studies, suggest that flower patterns are really important, especially for inexperienced bumblebees that haven’t foraged flowers and haven’t learned where to find nectar on a flower. However, for bumblebees that have foraged on flowers, flower patterns might not play such a big role as nectar guides, but they are still very important for guiding their approach and landing. We have to keep in mind that a bumblebee, and a honeybee as well, can visit hundreds of flowers a day. Experience in our experiments means that they have visited a few dozen flowers. This means that a honeybee or bumblebee will count as having foraging experience after a day, in our experiments. So flower patterns might not be so relevant as nectar guides after a day, but they’re still important for guiding flight approach and landing on flowers.

Flight control and landing control are behaviors where animals can really only rely on their sense of vision, whereas once they have landed, they have many other cues to actually find the nectary. Therefore, in an ecological sense, I think it’s really important to stress how flower patterns are vital for this first stage of interaction. The interesting part for the future is whether this finding translates to real flowers and are we able to observe the aforementioned 30% improvement in foraging efficiency using real flowers? If you think about it, that would be a very impressive improvement—if a bee visiting hundreds of flowers a day is able to visit a third more flowers due to the ‘guidance’ provided by flower patterns. 

Frank: That’s fantastic. I was wondering if you could talk a little bit about policy, perhaps? Policy people always ask questions about synthesis and the applicability of scientific research for things like conservation, so I was wondering if you have anything to say on that?  

Anna: So I’ve been thinking about this and again it’s made me think deeply about the features and patterns on real flowers. We use very artificial ones in our study so we must wait to see if we get the same improvement in foraging efficiency with patterns on real flowers. If we do, then this becomes very interesting from a conservation perspective.

Humans are very good at changing the types of colors and flower patterns that insect-pollinators experience because we love ornamental and ‘beautiful’ flowers. If you look around our gardens and balconies, we have a lot of flowering plants; however, many of them are not types that a bumblebee would probably have experienced in a given natural context, especially in cities. So a question that I find myself asking is how do ornamental flowers—altered to fit our own standards of beauty—affect insects visiting these flowers? In our specific question, how would altered colours and patterns on flowers impact the approach, landing and foraging of the nectaries? I have to keep this question open. I think it’s a really interesting question to look at in the immediate upcoming future, but I can’t say that I am close to finding an answer. 

Frank: Fantastic. Thank you for that. Perhaps we can do some shout outs just before we wrap up?  

Anna: Sure. As I said before, a massive part of this study was actually performed by undergrads. So I’d like to shout out Alexander Dietz and Robin Richter, who did their bachelor’s and master’s thesis on this, respectively. They really carried the big bulk of the experimental work, and were also instrumental in really improving our experimental designs. Many thanks to both of them. It was a really fantastic experience to work with them and I think it was a great example of what you can do if you’re really excited about working with animals and are interested in these kinds of big questions.

I’ve already mentioned Johannes Spaethe, without whom none of this would have been possible because I had no experience working with bumblebees before we started this study. He was sweet enough to share his expertise and really invest in this collaboration. He has convinced me that bumblebees are an amazing model organism and that we really want to keep on studying them in the future! So thank you Johannes for essentially starting this whole complementary line of research. I should also thank James Foster who’s been involved in the work of this study (not just the number crunching). He was very helpful in setting up statistics that allowed us to process a lot of this quantitative data. 

Frank: Perfect. Thank you. I always like to ask what would your one single piece of advice be for other early career researchers? Or, better yet, what would you say to a younger version of yourself?  

Anna: It’s a very good question. Personally, I would have told young me to not be scared of insects because they’re amazing creatures. And I think, as the study shows, you can actually go a long way with relatively simple methods. There are so many incredibly fascinating questions around insects, especially insect pollinators. I can only encourage everyone to watch these insects in the outside world. By observing them, you can come up with these simple experiments—this study was not complicated in terms of equipment—to answer complex questions. The last thing I’d say is to go for it and do your best to get into this field if you’re interested. Don’t feel afraid, especially don’t be afraid of creatures with 6 or 8 legs because they’re amazing! 

Frank: That’s wonderful. Thank you so much. I’ve really enjoyed the chat today and I’m sure our listeners have too. I’d just like to wrap up by saying thank you to Anna.  

Anna: Thank you for having me. 

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