by Mary Holland ·
Friday, June 28, 2024
The honeybee’s plight gets a great deal of attention, as well it should, for it plays a big part in the U.S. economy and food consumption, and its population is declining. However, the honeybee is not the only pollinating insect humans depend upon and not necessarily the most efficient. Bumblebees and solitary bees are responsible for much of the pollination that takes place—bumblebees pollinate about 15 percent of all crops grown in the nation, worth about $3 billion.
A queen tricolored bumblebee collects nectar from trailing arbutus. (Photo by Mary Holland)
A look at the humble bumblebee reveals several traits benefitting pollination that are lacking in honeybees. Most bumblebees are native and have evolved with our native flowering plants. They pollinate different varieties of plants than honeybees, which are European natives. Bumblebees are generally more active in cooler weather and in lower light levels than honeybees, which extends their working hours, and some species pollinate flowers in ways honeybees have never dreamed of.
Pollination, the process of transferring pollen from plants’ male to female reproductive organs, starts the process of seed and fruit formation. It is estimated that worldwide, one-third of the human diet can be traced directly or indirectly to insect pollination (Status of Pollinators in North America, National Research Council). Wind disperses a significant amount of pollen, but bees, wasps, ants, moths, flies, beetles, and butterflies are even more crucial to cross-pollination (the transfer of pollen from one flower to another, of the same species). Each insect has specialized adaptations that allows it to accomplish this task, some more than others.
What makes bumblebees such effective pollinators? To begin with, they consume only nectar and pollen, so visiting flowers is a necessity if they are to remain alive. Bumblebees are generalist foragers, not limited to one or two species of plants. That said, bumblebees tend to specialize in one or two species of flower at a time in the interest of saving time and energy. They learn a given flower’s structure, and where and how to reach the nectaries and pollen-bearing anthers, and they return repeatedly to the same species of plant. This not only saves the bee time, it also minimizes pollen transfer to the flowers of other, unreceptive plants. Bumblebees mark the flowers they have visited with scent from a gland in their leg, indicating that there is no nectar left. Bees arriving at a flower after the nectar has been taken know not to waste their energy trying to collect nectar from that particular flower. Bumblebees are capable of learning more complex foraging tasks (such as determining how to enter and extract nectar or pollen from a bottle gentian or jewelweed flower) than many other insects.
A trick bumblebees have up their sleeve that most honeybees don’t is the ability to “buzz pollinate.” To release the pollen of certain flowers, bumblebees (and some species of solitary bees) grab onto the flower and move their thoracic muscles rapidly, causing the flower and anthers to vibrate, dislodging pollen. Some plants are adapted to disperse pollen when vibrated, and some even require it. Only about 8 percent of the world’s flowers are primarily pollinated using buzz pollination (also known as sonication), but among them are vegetables humans are particularly fond of, including greenhouse-grown tomatoes, peppers, squash, cucumbers, eggplants, and blueberries. Commercialized bumblebees are used primarily in crops grown in greenhouses.
Bumblebee bodies are extremely well adapted for collecting pollen and nectar. The hair that covers much of their body is branched, and a lot of pollen inadvertently gets stuck in it. Bumblebees have a distinct advantage over many other insects in that they can remain active in cool, wet spring weather. Although, like other insects, bumblebees are cold-blooded, they can maintain a consistent body temperature regardless of the ambient temperature by generating heat. They do this by vibrating their thoracic muscles, and this allows them extended foraging time. Their tongue is long and feathery, enabling them to reach nectar that other insects cannot reach. Bumblebees’ legs have hairs and brushes that are used to gather the pollen collected in their body hairs and pack it into pollen baskets (one on each hind leg) for transport. When both baskets are full, they can make up about 50 percent of a bumblebee’s weight. Like honeybees, bumblebees possess a “honeystomach” in which they store the nectar they collect. They are capable of carrying half their body weight in nectar back to the nest, and if they are not also gathering pollen, this load can be increased to as much as 90 percent of their body weight.
There are figures documenting that bumblebees carry four times the amount of pollen that honeybees do. They visit on average twice as many flowers per minute. They will forage for up to two times as long as a honeybee (due, in part, to the ability to regulate their body temperature by vibrating flight muscles). And 60 to 90 percent of a bumblebee’s hive is pollen-foraging bees, compared to 5 to 15 percent of a honeybee hive. Even though there are only about 250 bumblebees in a colony, compared to 50,000 to 60,000 honeybees, some people feel there is good reason to further explore the potential of bumblebees as commercial pollinators, especially in greenhouses (for buzz-pollinated crops). At the very least, their vital role as pollinators should be recognized.
Unfortunately, the bumblebee population has been declining at an alarming rate over the past two or three decades. The cause of this decline is not fully understood, but it appears likely that a number of factors play a role, particularly neonicotinoid pesticides. These pesticides are having a detrimental impact on a wide range of nontarget organisms, including bumblebees. Fragmentation of habitat, the importation of parasites and diseases, and the potential effects of climate change also are thought to contribute to the decline of bumblebees. Neonicotinoid exposure interacts with the other threats and exacerbates them, as bees poisoned by neonicotinoids are less able to combat disease and forage for food, and therefore less likely to produce a successful colony.
While humans don’t reap the benefit of absconding with pounds of stored honey from bumblebees—even in the largest bumblebee colony, there are usually no more than a few teaspoons of honey, stored for consumption during rainy, cold days—their pollinating capabilities link our fate with theirs.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England,” “Milkweed Visitors,” and “Ferdinand Fox’s First Summer.” She has a natural history blog that can be found at www.naturallycuriouswithmaryholland.wordpress.com.
Editor’s note: This column by longtime former Harvard Press contributor Mary Holland first ran in the June 25, 2015, edition of the Press. With pollinator activity at its peak in the summer months, we thought its subject timely enough to publish again.