Pollinator Power

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Published on: Jul. 2, 2001

Last revision: Nov. 9, 2010

"Without bees, no trees-and none of these," the old forester said. With a flourish, he directed our eyes to a sunlit patch of blooming trout lilies where a black and yellow bumblebee scrambled from flower to flower, buzzing threateningly. I backed up a step. I was nine years old and liked the forester's rhyme, but wasn't quite convinced of the connection.

I've since come around.

Missouri's native bees are an abundant and diverse group of insects that live in virtually every kind of terrestrial habitat in the state, including prairies, forests, wetlands, backyards and hedgerows. At least 450 species in six families live here. Many are small and easily overlooked. Others, like bumblebees, are universally recognized. Most species of native bees do not have a common name, although groups of species do. The name "leafcutter" bee, for example, applies to a number of similar species, all of which use cut leaves to build nests.

Bees are important pollinators of native plants. Without native bees, many native plants could not reproduce effectively.

The honeybee (our state insect) is an Old World species that was brought to North America by European settlers long ago. It has been living wild in Missouri for at least 200 years. There are no honeybees native to Missouri or anywhere else in the New World. The strain of honeybee known as the "Africanized" or "killer" bee, is not established in Missouri.

Honeybees are extremely important pollinators in managed agricultural systems such as orchards. Much of the world's agricultural production and economy depends on this little creature. Its value in North American natural communities, however, is debatable. Like many exotic species, honeybees may have a negative impact on native species that have similar ecological roles.

Like butterflies, moths, beetles and flies, bees go through egg, larval, and pupal stages before becoming adults. Larval bees in the nest are entirely helpless and cannot obtain their own food, so the adult female provides it for them. Female bees collect pollen, nectar and, sometimes, oils from flowers, transport the pollen to the nest and place the pollen load into a cell. It may take several to many trips to provide enough pollen to provision a cell. Some species add glandular secretions to the pollen mass. A female usually lays a single egg on the pollen mass.

Larval bees feed and grow quickly, consuming the pollen, or most of it, in about a week. After feeding, most Missouri bees stay in their cells for nearly a year before emerging as adults.

Some bees, however, like certain leafcutters, yellow-faced bees, many sweat bees and small carpenter bees, produce several generations a year. Social species like bumblebees and some sweat bees produce multiple broods of workers. Male bees have no role in building nests, collecting pollen or caring for offspring. Their primary role is mating.

Ants and termites live cooperatively and are considered social, unlike butterflies which are solitary. Bees display a variety of social living arrangements, and some of them are solitary. Amazingly, certain sweat bees have built-in flexibility to live a solitary or social existence. Most bees, however, are one or the other.

Solitary species live alone and are self-dependent. A single female mates, builds her own nest, collects her own pollen, provisions her own cells and lays her own eggs; all without the assistance of another female. She does not interact with her offspring and usually dies before they become adults. Solitary bees often nest in dense aggregations, with many individual nests closely packed together. This gives the appearance of an organized colony, but aggregations are only a collection of individual nests. Most Missouri bees are solitary.

Females of social species work collectively to provision and sometimes defend a nest, and to produce offspring. A group can be as few as two or three (for some sweat bees species) to as many as several hundred (bumblebees). With some non-native social species, such as the honeybee, nests can include tens of thousands of individuals.

Bumblebees are the most well known group of native social bees. After mating in the summer or fall and then overwintering, a single fertilized queen begins building her nest in the spring, just as a solitary species would. She collects pollen, lays a small clutch of eggs and feeds her offspring, which soon turn into adult females. However, instead of leaving to start their own nests, these females stay with the queen and become the first of her worker force. They help her and the colony by collecting pollen, defending and maintaining the nest, feeding the offspring and caring for the queen.

The queen stays in the nest and is the sole or primary egg-layer. At some point in the summer, the queen begins laying some eggs that become males, as well as some that become next year's queens. These individuals mate, and the potential queens enter dormancy underground or elsewhere. By late summer or fall, the resident queen wears out and dies, and the nest ceases to function as a colony. The workers and males die when the weather turns cold.

Sweat bees (Halictinae) are Missouri's other group of social bees. There are at least 70 species of sweat bees in Missouri. Most are small and inconspicuous and have not been studied. Some are solitary, but a number are known to be social. Like the bumblebees, they live throughout the state in all habitats. True to their name, they have the habit of landing on human skin to lick perspiration.

Bees are among the few insect groups that build nests, and the females do all the work. Nests contain one to many cells. A cell is where a single bee usually develops to adulthood. The architecture of these nests varies tremendously, depending on the kind of bee. Many ground-nesting bees build simple, shallow holes in the ground, sometimes with side branches, while some social sweat bees create elaborate underground labyrinths with many branches. Certain leafcutter bees string together a linear series of leafy cells inside a hollow twig. Other species related to the leafcutters attach fragile, resinous globes to twigs or under rocks.

Some bumblebees may convert a mouse or bird's nests into a wax-based dwelling. Nests are valuable in the bee world and are sometimes aggressively stolen by other bees.

Certain bees take this behavior a step further. While most bees make their own nests, there are some kinds of bees that mimic the habits of the cowbird. Referred to as cleptoparasites (or parasitic bees), these bees do not collect pollen or make nests, but use the nests of non-parasitic species as their own. The "host" bee usually doesn't even realize it.

Unlike the cowbird, which parasitizes the nests of many different kinds of birds, parasitic bees are usually much more particular about their hosts. Bees that attack leafcutter bee nests don't attack sweat bee nests, and vice versa. Because of the absence of pollen-collecting hairs on their bodies and their often slender physiques, parasitic bees often look more like wasps than bees. Nearly 100 species of bees in Missouri are parasitic.

Bees are important pollinators of plants in Missouri, and nearly everywhere else. Without pollination, plants could not reproduce sexually, and there would be far fewer seeds or fruits. To produce their full complement of seeds, most flowers require multiple visits by pollinators, not just one. Bees are superior pollinators because of their reproductive requirements for pollen and nectar, their adaptations for collecting pollen and their habit of visiting flower after flower. They transfer many pollen grains to plant stigmas (female reproductive organs in plants) and effect cross-pollination more dependably than any other insects.

In addition, bees display flower constancy, which is the habit of visiting flowers of the same species consecutively during a foraging trip. In this way, plants are cross-pollinated with "the right kind" of pollen.

Over time, many plants have developed pollination systems that depend on bees. Missouri examples include most of the flowers in the bean family (Fabaceae), the daisy family (Asteraceae), the rose family (Rosaceae), gentians and many others. Many native fruits eaten by birds and other wildlife develop from flowers that are bee-pollinated, such as blueberries, plums, serviceberries, buckthorns and ground cherries.

Some flowers require certain kinds or sizes of bees for pollination. Wild indigos, which are common on prairies and glades, have large flowers that can be pollinated only by large, strong bees, primarily queen bumblebees.

Far from being threatening, our native bees are the core component of a pollinator force that powers much of natural Missouri. They carry it-and us-across the void of winter into the growing seasons, from one year to the next, from generation to generation.

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