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Pollination syndromes can predict who will visit a flower

Those new to the world of bees often believe that honey bees can pollinate anything. Some beekeepers are barely aware of other pollinators or the great diversity among them. But the single-pollinator theory couldn’t be further from the truth. Instead, millions of years of evolution have brought plants and specific pollinators together, each trading for something they need, and each vitally dependent on the other.

This article first appeared in American Bee Journal, Volume 160 No 6, June 2020, pp. 673-676.

When I first heard the term pollination syndrome I thought it was some kind of disease. It didn’t sound good. But pollinator syndromes are simply sets of characteristics that make a certain plant attractive to a specific type of pollinator. The first definition of syndrome at Merriam-Webster.com is the one we most often think of, and it sounds like a medical text: “a group of signs and symptoms that occur together and characterize a particular abnormality or condition.” But the second definition is the one we need here: “a set of concurrent things that usually form an identifiable pattern.”

The little things add up

Taken together, the characteristics of a flower make it more or less attractive to a certain kind of pollinator, which is why we see different pollinators on different flowers. As an example of a plant characteristic, think of the length of the corolla — the tube made by a whorl of petals. Some plants, like honeysuckle (Lonicera), have a long and deep flower. The only creatures that can pollinate this type of flower are those with equally long tongues or with very tiny bodies that can crawl down to the nectar despite their tongue length.

Conversely, some of the disk-shaped flowers such as those in the family Asteraceae, have very short flowers which are preferred by short-tongued animals, but can also be used by others.

As the name suggests, a syndrome is not one characteristic but “a set of concurrent things.” In addition to corolla length, these characteristics may include the presence or absence of nectar, landing platforms, and aromas. In addition, visual patterns known as nectar guides — sometimes evident only to those that can see UV light — may point to the flower’s sweet reward.

Other characteristics of a syndrome may include flower color, the amount of pollen, the size of the pollen grains, the sweetness of the nectar, and whether the flower opens during the day or in the evening. Just as an individual goes to the restaurant that carries his favorite dishes, a pollinator goes to the flower where conditions are just right and the food is perfect.

Bumble bee on foxglove: Dividing plants into pollination syndromes is tricky because many flowers attract more than one type of pollinator. These deep, bell-shaped foxglove flowers are frequently visited by bumble bees and hummingbirds. The hummingbirds hover as they drink, but the bumble bees climb right in and disappear from view. All photos by Rusty Burlew
Bumble bee on foxglove: Dividing plants into pollination syndromes is tricky because many flowers attract more than one type of pollinator. These deep, bell-shaped foxglove flowers are frequently visited by bumble bees and hummingbirds. The hummingbirds hover as they drink, but the bumble bees climb right in and disappear from view. All photos by Rusty Burlew

A flower for the birds

If we look at pairings of plants and their pollinators, we can see how they work together. For example, birds prefer flowers with deep corollas and lots of nectar. They also choose flowers that are red, white, or orange. Some things, like nectar guides and a sweet odor, are irrelevant to birds, but some prefer a landing platform to stand on.

If you look at a commercial hummingbird feeder, you can see how the designers use the hummingbird’s pollinator syndrome to attract the birds to your yard. The nectar pool is deep, but not scented. The fake flowers have long red corollas but no nectar guides, and the flowers are elevated off the ground. In their search for food, the hummingbirds recognize their preferred characteristics and flock to the pseudo-plant for a drink.

Types of pollination syndromes

It’s not surprising that names have been assigned to all the different types of pollination syndromes. The names are old and, in some cases, their descriptions have changed as we’ve learned more about pollination. Then, too, the groups are flexible, often crossing over one another, making each name somewhat fluid. Still, to understand the concept of syndromes, it helps to review the common types.

All the pollination syndromes are divided into two main categories, abiotic and biotic. The abiotic syndromes include wind pollination (anemophily) and water pollination (hydrophily). The root “phily” means “love” or “affinity,” so these plants have a love for wind or water.

Bees and flies: This early willow is a favorite of both bees (left) and flies. Willows are one of the earliest nectar plants and attract a large following.
Bees and flies: This early willow is a favorite of both bees (left) and flies. Willows are one of the earliest nectar plants and attract a large following.

Abiotic pollination syndromes

Anemophily: Wind pollinated plants have no need to attract animals, so their flowers are not colorful or large, and they don’t produce sweet nectar or piquant odors. When you calculate the energy saved by not having to produce showy flowers spiked with attractants, it seems like a plus for plants. After all, wind is everywhere and it’s free to use, so it seems like the plant is getting something for nothing.

However, wind is not very targeted. It blows randomly without a plan or sometimes it doesn’t blow at all. To make up for this randomness and uncertainty, anemophilous plants need to produce a ton of pollen to assure that a least some of it gets where it needs to go. Without an animal charged with transporting the pollen from the anther of one flower to the stigma of the target flower, a plant must expend copious energy in over-production.

In fact, egregious amounts of pollen can often be seen smoking from trees and misting windshields and windowsills with yellow dust. Unlike sticky pollen from showy flowers, wind-pollinated flowers have tiny grains especially designed to float on air currents for long distances and remain suspended for the duration of the trip. These are the pollens that cause allergic reactions such as sneezing, sniffling, and bloodshot eyes. Without a doubt, wind-borne pollen is the stuff that gives pollen a bad rap.

When you find a plant with dull or nearly non-existent petals lacking color, odor, and nectar, and if that plant has loads of powder-like pollen, you can almost be assured it is anemophilous. Good examples are beech, alder, birch, and hazel whose long catkins dangle in the spring breezes and release choking clouds of cream-colored dust. Bees, especially honey bees, often collect this pollen for early spring brood rearing, arriving back at their hives with tightly packed, tallow-colored baskets. The early pollen is a win for the bees, but any pollination caused by the bees isn’t necessary for the trees.

Hydrophily: Water pollination is another abiotic system that works very much like wind pollination. In some cases, aquatic plants release pollen into the water, which is then carried by currents to female flowers. In other cases, the pollen is released just above the water so that it floats to the next flower on the surface. Although relatively rare, it is an important mechanism to the plants that use it.

Biotic pollination syndromes

The remainder of the pollination syndromes are animal mediated. Some animal pollinators are shockingly numerous, such as the bees, beetles, flies, and butterflies, while others are far less common, including mammal and bird pollinators. The following types are listed in alphabetical order.

Cantharophily: Flowers attractive to beetles are often flattened like a dinner plate and green or murky white, not bright. The flowers are usually without nectar guides and the odor, if present, is fruity with a hint of decay. Pollen is usually abundant and nectar may or may not be present.

Beetle: Each beetle pollinates only a few flowers in a day, but the number of species and individuals is huge, all of which adds up to a lot of pollination.
Beetle: Each beetle pollinates only a few flowers in a day, but the number of species and individuals is huge, all of which adds up to a lot of pollination.

Chiropterophily: Bats, which are mammals, prefer large and showy bowl-shaped flowers that open at night. These may be green, purple, or white and contain large amounts of both pollen and nectar. Depending on location, some bat-pollinated flowers have a sulfur-like scent, while others have odors described as musty or “off.” Since the flowers open at night, they are devoid of nectar guides. In addition, the flowers are positioned well away from entangling leaves and stems to facilitate bat flight at night.

Melittophily: No surprise, melittophily is the name for bee pollination syndrome. In a way, grouping all bee pollinators under one heading is an odd choice. Bees range from fruit fly size to monstrous, reminding me of shrews with wings. Their tongues range from very short to wickedly long. Some bees live in the trees while others live in the ground, some can bore holes while the majority cannot, and some can even shake pollen out of a flower (sonication) while others just wish they could. Other bees collect oil from flowers, while most pay no attention to it.

Regardless of bee diversity, the syndrome traits generally listed for bee-pollinated flowers include white, yellow, purple, or blue flowers, with or without UV markings or nectar guides, that exude a pleasant but mild odor. Depending on species, bees are attracted to both shallow and tubular-shaped corollas with or without a landing surface, but they all search for flowers that provide nectar or pollen, or both. However, the huge variation in bees leads to diverse foraging preferences within the syndrome. A bee is a bee, but they’re not all alike.

Sweat Bee: Melittophily is pollination by bees.

Myophily: Of the two types of fly pollination, myophily is the more common. These flowers attract flies that eat both nectar and pollen in their adult stage. Since they enjoy the same food as bees, they are often seen on the same flowers as bees and sometimes butterflies, often simultaneously, especially flowers that are white, purple, blue, or violet. Bee flies, drone flies, and hover flies all belong to this group.

Ornithophily: Bird pollination is also quite diverse, depending on species. Nectar-consuming birds, including hummingbirds and honeyeaters, do the lion’s share of bird pollination, reaching deep within the flowers to collect a meal. But some short-billed birds perform random pollination as they scour flowers for insects. The nectar-eaters prefer flowers that are large and red, orange, or white with deep corollas, whereas the short-billed birds like flatter flowers with a place to perch.

Phalaenophily: Moth pollinators come in various styles. The hawk moths are excellent pollinators that behave much like hummingbirds when they drink nectar, hovering in front of the flower while they uncoil an impossibly long tongue. Many of the hawk moths are nocturnal, preferring large, white, late-opening flowers with long corollas, a sweet odor, and lots of nectar. Smaller, less spectacular moths also pollinate, landing on a variety of different flowers that are frequently white, red, purple, white, or pink.

Psychophily: Most butterflies like showy flowers that are red or purple, but some accept other colors as well. Like hummingbirds, they seem especially attracted to deep corollas with a plentiful supply of nectar and a convenient landing platform. Like bees, they can be lured into a flower by colorful or contrasting nectar guides. For the most part, butterflies are not interested in pollen, so pollination is merely incidental, occurring as the butterfly travels from flower to flower looking for nectar.

Skipper: Skippers are small butterflies that spend their time drinking from flowers and moving pollen as they go.
Skipper: Skippers are small butterflies that spend their time drinking from flowers and moving pollen as they go.

Sapromyophily: The second type of fly pollination, sapromyophily, is relatively rare. The syndrome comprises flowers that are dull dark brown or purple and often riddled with translucent patches that appear partially decayed. The odor arising from the shallow or funnel-shaped flowers may be mildy or strongly putrid.

Therophily: Although it’s uncommon in North America, pollination by marsupials and other small mammals other than bats is found in certain parts of the world, including South America. These animals specialize on pollinating flowers that are hard to get into, especially those where the flower is protected by a sheath or a set of tough bracts. The flowers — reminiscent of Fort Knox — require muscle and dexterity to penetrate — the perfect job for some of nature’s most surprising pollinators. Recent research in Brazil has demonstrated that the primary pollinator of a strange tuberous plant with tough scaly foliage (Scybalium fungiforme) is the big-eared opossum (Didelphis aurita), a marsupial. Other mammalian pollinators include various rodents and shrews.

Using pollination syndromes

Every spring, delicate yellow violets fringe the edge of the forest behind my house. Perfect little creations, they announce spring by nodding their approval in the dappled sunshine. I sometimes toss a handful of fresh violets into a salad or scatter them on the side of a dinner plate where they add sparkle, warmth, and a touch of hope for the future. But I always wonder, who pollinates the violets? Each year, they seem devoid of pollinators.

A few weeks ago, while I was mulling over pollination syndromes, I decided to discover who was working my violets. Since the flowers have bright yellow petals, bees were a definite possibility, and the purple nectar guides suggested a pollinator that flies and has a thirst for sweets. The landing platform also suggests a pollinator that flies, but the small petals are too tiny for birds. When they first open, the lightly fragrant flowers appear at the sunny edge of the forest where other flowering plants will eventually bloom. Lastly, the flowers are shallow, well-suited to a short-tongued species. If there’s pollen down in the corolla, I didn’t see it.

Because the flowers are so early, I imagined a bee pollinator that is early to emerge and perhaps stays low to the ground. Sun-warmed soil could initiate both flower bloom and bee emerge, so I thought the pollinator might live in the soil near the violet patch. That’s not much to go on and some of it is more conjecture than science, but that was my bet.

Selecting from the earliest bees in my area, I decided a bee in the genus Andrena was a good choice. Many Andrena are early to emerge and nearly all are ground dwellers with short tongues. Now I just had to prove it.

Caught in the act

I chose a warmish sunny day, the first week in April. I found a stand of violets in a clearing near a creek. The ground was wet, but the violets were lovely, specked with dew that splintered the sunlight.

Hunkering down on a wet log, I decided this was one of the most ridiculous things I had ever done. After all, in 25 years of living in one place, I never saw anything visit the violets, so what made me think this day was any different? But after I sat quietly contemplating my sanity for five minutes, I was rewarded. Not with one bee, but with a whole regiment. Once I became part of the landscape, as shiny and slick as the sodden stumps that surrounded me, the bees came from all directions — both male and female — to feast on the yellow violets. I couldn’t believe my luck.

My first four photos were ruined by my cat who insisted on caressing my lens with her tail. Once I got rid of her, I thought the bees would be gone for good, but after a few tense moments they came back and I was able to capture the proof. Although I don’t know the exact species, they were the Andrena I imagined.

Andrena on violet: I captured this Andrena bee in the act of flying from violet to violet, sipping from nearly every one.
Andrena on violet: I captured this Andrena bee in the act of flying from violet to violet, sipping from nearly every one.

More useless than not?

While pollination syndromes are fun to play with and afford unique opportunities to utter mind-bending words like chiropterophily, not all pollination biologists believe they are useful. The reason stems from the wide diversity of plants and the fact that many plants rely on multiple pollinators. For example, it’s hard to fit something like foxglove (Digitalis) into a syndrome when it attracts both bumble bees and hummingbirds.

Furthermore, as biologists spend more time studying obscure plants within the rainforests and other isolated regions, they keep finding more types of plants that attract pollinators in unique ways. The amazing array of variation is the result of where the plants live and the potential pollinators that share the landscape, meaning many plant species cannot be chunked into specific groups.

A tool like any other

Personally, I like the idea of pollination syndromes even though I see their shortcomings. While dividing plants into syndromes isn’t a perfect system, the classifications can sometimes be helpful or just plain fun to use. Just as a screwdriver isn’t the answer to every problem under the hood of your car, a syndrome doesn’t answer every pollinator question. Still, sometimes it’s just the ticket and one that helps you learn in the process.

Next time you wonder, “Who pollinates this plant?” give it try. See if you can hone in on a few potential pollinators based on the plant’s traits, then wait them out with a beer, a book, and camera-in-waiting. Sounds like beekeeper fun, right?

Rusty
Honey Bee Suite

Tree frog: I have found very little information on pollination by amphibians, but small frogs that hunt from flowers are often covered in pollen and no doubt contribute to pollination as they hop from flower to flower.
Tree frog: I have found very little information on pollination by amphibians, but small frogs that hunt from flowers are often covered in pollen and no doubt contribute to pollination as they hop from flower to flower.


Comments

Granny Roberta
Reply

Beautiful pictures. But it’s the Internet—when your cat is “helping” you photograph flowers, we want pictures of the cat as well.

Also, WHY does “psychophily“ mean butterflies. I couldn’t find any hint on the Internet. Maybe I was too easily distracted by cat pictures.

Rusty
Reply

Roberta,

Very good question. I can’t find the answer either, but I’ve got a cat photo around here someplace…

Pam Phillips
Reply

Many cultures, including the ancient Greeks, believe there is a connection between butterflies and human souls. Which leads to scientific terms using “pysche” to mean butterfly.

Fantastic write-up of pollination syndromes! Thank you!

Rusty
Reply

Pam,

Thanks for that explanation; it makes sense!

Granny Roberta
Reply

Pam,

I, too, thank you for that explanation.

Rick A - Warner
Reply

Always blessed to read any of the wonderful information you present. Thank you, so much to learn and enjoy, thank you Rusty.

Chip Clements
Reply

Thanks, Rusty, for illuminating a hidden world of flowers and pollination. As I started to read the section on wind-pollinators, I thought I’d finally have an answer for all those people who ask me whether honey from my bees will help with their allergies. And I would tell them no, because the trees that cause allergies are mostly pollinated by wind and not bees. But nothing is so simple when it comes to bees and it turns out they are attracted to wind-borne pollen even though the trees don’t need them to be. But even supposing honey did contain the pollens we’re commonly allergic to, have you come across any scientific evidence that eating honey could help stave off allergic reaction?

Rusty
Reply

So, Chip, you had to ask the dangerous question! I will get blasted for this, but I do not believe honey can cure allergic reactions to pollen, nor have I read any scientific evidence to prove it. Honey bees do indeed collect pollen from wind-pollinated plants (corn is a good example) but not much of that pollen is going to land in the honey. Some will, but very little. In addition, I’ve heard people swear by honey for curing allergies, even when the honey was sourced in California and they were “curing” Idaho allergies. It doesn’t make any sense.

The other thing I’ve noticed is that people will swear that spring honey cures their fall allergies or vice versa. How can that be when the pollen in those products is completely different?

So no, I don’t believe it, but if it makes people happy…

Lisa Robinson
Reply

I spent more time sitting among the Spring Beauties for the same reason, but even without a cat, I didn’t see any bees. Flies yes. But it was a chilly windy day. Will have to try again next spring! (I know there is a Spring Beauty bee back east, and hoped we have one out west too.)

Rusty
Reply

Lisa,

I love to see those eastern spring beauty mining bees (Andrena erigeniae) all covered in pink pollen! I too wish we had them here.

Herb Betz
Reply

Rusty

How can I send you a photo or load one into a reply?

I was out in a goldenrod patch this weekend (southeastern PA) and got an interesting shot of a large black narrow-waisted waspy looking critter with just a bit of fuzz on the thorax tending the flowers along with honeybees. I thought I would show you (and or others) but I can’t seem to paste it here or on the blog comment section.

Herb

Rusty
Reply

Thanks for the great photos. I finally got my favorite wasp specialist at iNaturalist to take a look, and some others did too. Your very cool wasp is Isodontia auripes, the brown-legged grass-carrying wasp.

According to PennState Extension, “Several species of Isodontia, the grass-carrying wasps, are found in the United States. The most frequently encountered by Pennsylvania homeowners is Isodontia mexicana, although Isodontia auripes is also found. The Isodontia spp. wasps are one of the thread-waisted wasps in the family Sphecidae and use pre-existing cavities to provision a nest for brood. Nest sites include abandoned carpenter bee holes, hollowed branches, and occasionally, holes in soil banks and between rocks. Around homes, they commonly nest in the tracking for aluminum storm windows.”

Isodontia auripes by Herb Betz
Brown-legged grass-carrying wasp by Herb Betz

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