By John Palka — Posted August 30, 2020
The paths that fan out from the Eastman Nature Center near our home in Maple Grove, Minnesota, continue to be a source of inspiration for me. I go walking there almost every day, and each time something new catches my eye and finds its way into my camera.
I especially relish an area of restored prairie that is rich in flowers and, correspondingly, rich in pollinators, primarily insects. Here is a glimpse of the prairie in mid-July when it abounded in bee balm, also known as wild bergamot (Monarda fistulosa, Family Lamiaceae) with its globular lavender heads, and in gray-headed coneflower (Ratibida pinnata Family Asteraceae) and black-eyed Susan (Rudbeckia hirta, also Family Asteraceae), both with bright yellow petals.
It is hard to believe that scarcely two hundred years ago, prairies like this extended for literally thousands of miles down the center of North America—tallgrass prairie more easterly, where the soils were richer and the rains more abundant, shortgrass prairie further west where the soils were less rich and the climate drier. Only a handful of prairie remnants have been preserved. With great effort and not a little expense, small patches of prairie have been restored. Such is the prairie at Eastman.
The insects come to prairies for sweet and nutritious nectar and in many cases also pollen. They do the flowers a great service by spreading pollen from one individual flower to another, thereby effecting cross-fertilization. Cross-fertilization results in the mixing of genes from different individuals of the same species. Mixing genes leads to greater variability among the offspring. And greater variability offers both a greater chance of the survival of the species under changing environmental conditions and the possibility of generating characteristics that are more advantageous than any that have come before. This is the fundamental reason why sexual reproduction evolved. The evolutionary success of sexual reproduction, then, is why butterflies and bumblebees abound on Eastman’s prairies!
We met butterflies in the previous post, “The Dragons and Damsels of Elm Creek,” but there we focused on dragonflies and damselflies, both of them dramatic predators. Here we focus on two nectar feeders: the butterflies we already know something about and also the much more abundant bumblebees.
HOW BUTTERFLIES FEED
All butterflies feed in much the same way. They cruise over the prairie, meadow or flower garden looking for nectar-laden flowers. They spot flowers primarily by a combination of color and fragrance. Each species of butterfly has its preference for particular species of flowers, but the relationships are usually not exclusive.
The butterfly you see here is the Giant Swallowtail (Heraclides cresphontes, Family Papilionidae), a species of conspicuously large insects that is abundant all the way from southern Canada to Mexico. This swallowtail is feeding on the nectar of one of the abundant prairie flowers, Monarda, the wild bergamot or bee balm. Both butterflies and bumblebees seek out Monarda. The yellow flowers in the foreground are gray-headed coneflowers. We saw swaths of both of these in the first photograph.
To access the nectar made by flowers, butterflies utilize a highly maneuverable proboscis, a drinking tube that is kept coiled up under the butterfly’s face until it is needed and then is extended to a length that can equal more than half the length of the body. The butterfly inserts the proboscis into the throat of a flower and extends it until it reaches nectar—if, indeed, there is any nectar in that particular flower. Then the butterfly drinks, pulls the proboscis out, and goes on to the next flower. Having many flowers in a head makes the whole process more efficient.
In the two pictures below, you see an Eastern Tiger Swallowtail (Papilio glaucus, Family Papilionidae) drinking from a Monarda.
And below you see a different Tiger Swallowtail that has just finished drinking from a blue giant hyssop (Agastache foeniculum, Family Lamiaceae) and is in the process of curling up its proboscis for the flight to another flowerhead.
This feeding mechanism is common to all butterflies, and also to their relatives, the moths. Below, are some examples from the prairies at Eastman.
A Common (or Clouded) Sulphur (Colias philodice, Family Pieridae) on a clover (Trifolium sp., Family Fabaceae).
A Great Spangled Fritillary (Speyeria Cybele, Family Papilionidae) on Monarda.
A Common Wood Nymph (Cercyonis pegala, Family Nymphalidae) with its proboscis still curled, getting ready to feed on a gray-headed coneflower.
A Great Spangled Fritillary feeding on Echinacea (Echinacea glauca, Family Asteraceae).
Let’s pause a moment to note that coneflowers and Echinaceas are not actually single flowers. Rather, each apparent flower is an arrangement of hundreds of flowers, all of them tiny. The flowers around the edge of the disc bear showy petals and are usually sterile; the flowers in the interior have tiny petals that we never notice and are fertile. The whole arrangement looks and functions like a single flower.
And finally, in the images below we see not a butterfly but one of its relatives, a moth, again on Monarda. And a remarkable moth this is! Appropriately called the Hummingbird Clearwing Moth (Hemaris thysbe, Family Sphingidae), it beats its partially transparent wings much like a hummingbird does and often hovers by a flower while it drinks. This is truly a sight to see!
In the upper picture you see a Hummingbird Moth hovering. The wings are edge-on and virtually invisible to the camera. You can see them much better in the lower picture, including the clear areas through which the lavender of the Monarda and the green of nearby leaves are visible.
HOW BUMBLEBEES FEED
Butterflies and bumblebees look very different overall, but they both feed on nectar so some similarities would be expected. Indeed, both insects have a proboscis. However, a bumblebee’s proboscis is not extravagantly long and flexible like that of a butterfly or a moth.
When a bumblebee is flying, it folds its stiff proboscis under its head. When it starts to feed, it swings the proboscis forward and pushes its whole head into the opening of the flower so that the distance to where the nectar is located is as short as possible. As you would expect, bumblebees also feed on many varieties of flowers that do not hide their nectar inside long tubes!
The stiff proboscis of a bumblebee is a tube formed by two highly modified mouthparts whose edges overlap tightly and form an air-tight seal.
Within this tube is a long, hair-covered tongue that can be extended out of the tip and into the nectar. If the nectar is very fluid, generally because its sugar concentration is low, the bumblebee (or honeybee) sucks up the nectar through the tube. If the sugar concentration is high and the nectar is viscous, the bumblebee laps it up using its tongue.
Below you see the whole proboscis, with the tongue inside it, caught in fortunate light that makes it glow.
And in the photograph below, at the limits of what my camera can capture using a telephoto lens, is the tip of a bumblebee’s proboscis. You can just see the tongue protruding. This remarkable close-up video will show you much more detail.
Bumblebees are not shy about sharing a precious feast.
In fact, even though their feeding mechanisms are somewhat different, sometimes butterflies and bumblebees feast together. Below, you see both a Monarch butterfly and a bumblebee on a single flower head of milkweed (Asclepias sp., Family Apocynaceae).
SETTING OUT THE FEAST
The flowers belonging to different species differ from each other anatomically and are arranged in different ways on the plant. In addition, the details of what they offer to pollinators change as the season progresses.
For an example, let’s look at Monarda, whose common name itself, bee balm, suggests that it is a favorite among insects. The picture below, taken in mid-summer, shows three flower heads of Monarda, as well as a spike of blue giant hyssop behind them. Individual Monarda flowers are clustered at the top and center of each flower head. Below them and around the perimeter are rows of buds that have not yet opened.
Later in the season, as the early-emerging flowers the flowers mature and then drop off, patterns like that in the next picture become predominant. The top of the flower head loses its flowers. What remains there is developing fruits.
Even when there is a single ring of flowers remaining, some individuals within that row open sooner and others later. This way, fresh flowers are always coming in. When fresh flowers first open, they are especially rich in nectar. Pollinators are highly attracted to this, so they keep visiting a flowerhead even as it is nearing the end of its flowering period. Individual flowers whose nectar has been consumed by a pollinator do make additional nectar, but it takes about five hours to fully replenish the supply, and a single visitor, whether a bumblebee or a butterfly, can empty a lot of flowers. Having fresh flowers emerge late in the season and throughout the day keeps the whole head attractive to pollinators.
Once the flower heads have thinned, it becomes easier for us to examine a single flower, as in the image below. This image also shows in greater detail the barrel-shaped fruits, within which fertilized eggs are developing inside protective seeds.
A single Monarda flower is long, slender and tubular. The nectar is in the interior at the base. A nectar-feeder has to have a long proboscis to reach this nectar, or else, like the bumble bee, use other tricks. Bumblebees, as we have seen, insert their heads as far as possible into the opening of the flower. Sometimes, however, they take an entirely different approach. They go to the base of the flower from the outside, use their mouthparts to make a hole, and insert their stiff proboscis through the wall of the flower straight into the drop of nectar on the inside. This maneuver provides little or no pollination, so it is generally referred to as “robbing” the nectar!
What is it about nectar that draws so many pollinators? Into this secretion the plant releases a variety of sugars, notably sucrose, fructose and glucose. These sugars provide the insects with their main source of energy. In addition, there are small amounts of amino acids, the building blocks of proteins, a few other kinds of organic molecules and some minerals. The chemistry of nectar is in many ways similar to that of the tree sap out of which maple syrup is made.
It is important to remember that a plant has to use its own metabolic energy to synthesize all the sugars that it so freely gives away to visiting insects. It even takes energy to move those sugars out of the plant’s own cells and into the nectar outside of them. Flowering plants have evolved to do this because, as we said at the very beginning, the reward is so great—cross-fertilization that enhances the opportunity for adaptation and further evolution.
Let us look at one more aspect of the biology of nectar. Nectar is primarily a sugar solution that is exposed to the environment. It is not sterile. Therefore, microbes readily grow in it. In this way, as well, nectar resembles the sap of sugar maples.
The metabolism of the microbes produces a variety of compounds that are potentially detectable by nectar-feeders, effectively modifying the taste of the nectar. In addition, some of those compounds are volatile and this gives the nectar an aroma. Wines and beers have aromas that we humans find attractive, produced by the action of microbes. Research on bumblebees published just last year indicates that nectar has a similar effect on the bees—they can detect the volatiles and exhibit preferences for the blends produced by specific microbes.
Microbes are ubiquitous on Planet Earth. In this sense, the sophisticated evolutionary and functional relationship of flowers and their nectar-feeding pollinators includes vast numbers of invisible organisms, just as the microbiome that inhabits our guts is integral to our own utilization of food. It’s an interesting and unexpected parallel, is it not? Stay tuned for details!
Not only restored prairies but all areas in which plants grow and flower and which insects can visit are filled with jewels waiting to be noticed and appreciated. It takes a little patience, however. The air is only rarely filled with butterflies or dragonflies and not every flower has a bee or bumblebee hiding in it!
The features of flowers that are so striking to us—their color, their size and shape, their fragrance, the times of day and weeks of the seasons when they open—all are related to their reproduction. Because most flowering plants are pollinated by insects, their own reproduction is related to the lives of “their” insects.
So when you go strolling, pay attention. You will see life in action! And when you see life in action, ask yourself—what is there under the surface that makes life not a solo affair but a dance between partners? And what is there that links this specific instance of life to other lives and to our whole Earth? If you keep asking, you will uncover wondrous things! We’ll take up two of the most surprising ones in the next post.