Thrifty flowers

Pollen is a valuable commodity. The less pollen needed, the more energy is left for other functions. Flowers resort to several strategies to economize on pollen. Some use a method called explosive pollination, others resort to buzz pollination. Both methods are well illustrated by official state flowers.

Mountain laurel in bloom. © Beatriz Moisset

Explosive Pollination of Mountain Laurels

Mountain laurel (Kalmia latifolia), Pennsylvania’s and Connecticut's state flower, has a singular way to ensure that its insect visitors carry pollen to other flowers of its species. The unopened blossoms present little knobs which give them a funny look. Their function becomes apparent when the flower opens. They are pockets that hold the anthers trapped. Anthers are the part of the flower that produces male cells, pollen.

Mountain laurel flower before an insect's visit.  © Beatriz Moisset

Flower tripped by an insect. © Beatriz Moisset

In most flowers the anthers are free and exposed to spread their pollen at the slightest touch of a flower visitor. The mountain laurel has a different strategy. The pollen is well protected against rain and wind; but, when a pollinator lands on the flower searching for nectar, the weight acts as a trigger, causing the taut stem of the anther to spring. The anther hits the pollinator gently on the back and gives it a dusting of pollen.

This method allows mountain laurel flowers to produce only a moderate amount of pollen because most of it ends up where it is intended, on the body of a pollinator, rather than being wasted in other ways.

Connecticut: Mountain laurel

Pennsylvania: Mountain laurel

Explosive Pollination of Pea-like Flowers

Texas Bluebonnet. Wikicommons © Jacopo Werther

Other flowers encase both the pollen-producing parts and the female parts, or stigma, into a sheath. Some members of the pea family, Fabaceae, have flowers that resemble a butterfly, this is why this subfamily is called Papilionoideae, papilio meaning butterfly. The top petal is large and shaped as the sail of a ship and is called the banner. Two side petals are called wings, and the bottom two petals are merged into one forming a sheath that encloses both the anthers and the stigma. This structure looks like the keel of a boat and it is so called.

Structure of a papilionoid flower. Wikicommons. © David Richfield

When a pollinator lands on a blossom, its weight triggers a mechanism causing the keel to spread open and the anthers and stigma jump up out of the enclosure. This is called tripping. The insect has to be heavy enough to cause such reaction. Some bumble bees are pros at this task; honey bees, on the other hand, dislike being pounced upon. After a few times, they learn to sneak around and steal the nectar through a small opening at the base of the keel. Only novices perform pollination. Once they learn this trick no more pollen is transferred to the stigmas. Nevertheless managed honey bees are widely used to pollinate alfalfa and clover. Bee hives provide a large labor force that makes up for this deficiency. Some bumble bees, especially the short-tongued species, are also inclined to a little larceny. However, they are generally considered highly competent pollinators of alfalfa and clover.

Clover, another papilionoid flower. © Beatriz Moisset

Texas’ bluebonnet (Lupinus) and Vermont’s red clover (Trifolium pratense) need to be tripped. Six species of lupines, all of them called bluebonnets because of their appearance, live in Texas. All six of them are considered the state flower. The red clover, Vermont’s state flower, on the other hand, is not a native plant. It is an important crop used to feed livestock.

Texas: Bluebonnet

Vermont: Red Clover

Buzz Pollination or the Salt Shaker Technique

Other flowers make the pollinators work for pollen in a different way. Interestingly enough, honey bees never learned how to do the job I will describe, but native bumble bees and numerous species of bees are real pros.

Azalea Andrena bee pollinating azalea. Observe the pores at the tips of the anthers. © Beatriz Moisset

In most flowers the pollen is made available to pollinators as soon as it is ripe. The anther splits open. We all have seen the golden dust many flowers have. If we touch it, it sticks to our fingers. Rhododendrons and azaleas do something else. They keep the pollen enclosed inside the anther. It can only come out through a small opening at the tip. Several other native flowers, like tomato and blueberry have a similar characteristic.

In order to extract it, the bee has to cling to the anther and give it a good shake. It accomplishes this by vibrating its flight muscles while keeping the wings still; it is like running the car engine in neutral. Pollen comes in clouds and clings to the body of the pollinator. Later, the bee proceeds to package its loot in the little baskets of the hind legs.

Buzz pollination of azalea. Video. © Beatriz Moisset

Two states have official flowers that exemplify this method of pollination, Washington has chosen the Coast Rhododendron (Rhododendron macrophyllum) and West Virginia has another species of Rhododendron (Rhododendron maximum). Georgia’s official wildflower, azalea (Rhododendron prunifolium), follows the same system of pollination; its official flower is the rose; but, fortunately, they decided to honor a native plant in addition to a non-native flower.

Georgia (official wildflower): Azalea

Washington: Coast Rhododendron

West Virginia: Rhododendron

Other plants select their pollinator clientele through other methods. Some hide the nectar rather than the pollen. A different set of skills are needed to deal with these flowers as we'll see in the next post.

References

Pollination of mountain laurel

Image illustrating the parts of a Papilionoid flower

Red clover pollination (also alfalfa)

Legume pollination

Bluebonnet pollination

List of articles
Beginners Guide to Pollinators and Other Flower Visitors

© Beatriz Moisset. 2015

Alfalfa Pollination

Large commercial alfalfa field

requiring managed beehives because of its size

© Vapampidis. Wikicommons

What is the most important insect-pollinated crop? Not a fruit or a vegetable but alfalfa. Yes, alfalfa, not because we consume large amounts of their sprouts, but because cattle needs this food. Thus: no alfalfa, no beef or milk.

This crop was first introduced in California during the gold rush. Alfalfa farming grew from there at a steady pace to what it is today, the third largest crop, after corn and soy bean. A few species of bumble bees and solitary bees took a look at this exotic flower, found it to their liking and proceeded to pollinate it. It wasn't a big leap; this plant's blossoms resemble those of other members of the pea or bean family. The native pollinators were familiar with bean flowers and adapted easily to the new arrival.

Alfalfa blossom. H. © Zel. Wikicommons

Peas, beans, alfalfa, clover and several other plants have butterfly-like (papilionaceous) flowers. The lower petals form an enclosure, shaped like the keel of a boat. This structure holds the sexual parts, the anthers and pistil. When an insect lands on the flower, a trigger mechanism makes it snap open or trip the flower. This is how pollination is accomplished.

Honey bees were not present in California when the earliest fields of alfalfa were cultivated. The first beehive arrived in 1853, just one solitary hive; a few others had died in transit. It took many years for the honey bee populations to build up to significant levels. This didn't matter because the wild bees did an excellent job.

New Zealand wasn't so lucky when it started growing alfalfa to feed the recently introduced cattle. The few native pollinators were stumped by the new flower. They lacked the necessary equipment and dismissed the strange blossoms. Year after year, hay grew luxuriantly, but no seeds. Alfalfa growers resorted to importing a few species of bumble bees from Europe to cut down the expenses of having to buy seed every year. Thus, industrious bumble bees saved the day in that country.

Scotch broom, a papilionaceous flower, being tripped. © Beatriz Moisset

 Honey bees detest the rough treatment alfalfa flowers subject them to and soon learn a trick of their own. They enter the flower from the side and help themselves to nectar without tripping its mechanism. Only naïve, youthful ones pollinate flowers. Despite the honey bees' poor performance, most alfalfa farmers resort to them because of the convenience in using a managed species.

In addition to the honey bee, two other species of alfalfa pollinators are managed to some extent and used commercially. They are the Japanese alfalfa leafcutter bee and the native alkali bee.

The populations of native bees have been severely reduced in the last one hundred years largely because of intensive agriculture. Restoring their numbers and putting them to work in alfalfa fields would be an arduous task but a worthy one. Some farmers find that, when they include flowering and nesting site places in their fields or orchards, the health of the crop improves and the need for pesticides diminishes. Thus, alfalfa pollination could be done entirely and more efficiently by native bees. This form of agriculture would be more sustainable than the present methods.

List of articles
Bring Back the Native Pollinators
Will We all Die if Honey Bees Disappear?
Growing Insects: Farmers Can Help to Bring Back Pollinators
Organic Farming for Bees

Beginners Guide to Pollinators and Other Flower Visitors

© Beatriz Moisset. 2014