Bumblebees

While these furry, clumsy, easy-going pollinators can sting you, repeatedly even, they generally choose to ignore us. [Unless they get caught in a certain young girl’s unruly long hair. Believe me. I speak from personal experience.]

The name bumblebee, or bumble bee, comes from the characteristic buzz (bumble) of this gentle pollinator. Before it was called bumblebee, it was known as a ‘humblebee’ and before that a version of a ‘clumsily flying buzzing beetle’ or ‘dumbledor’ was used. [I’ll bet you weren’t expecting that one!]

Yellow-faced bumblebee (Kevin Cole) CC BY 2.0

Charles Darwin had this to say about bumblebees in his book, On the Origin of Species (1859):

"I have [...] reason to believe that humble-bees are indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do not visit this flower. From experiments which I have tried, I have found that the visits of bees, if not indispensable, are at least highly beneficial to the fertilisation of our clovers; but humble-bees alone visit the common red clover (Trifolium pratense), as other bees cannot reach the nectar."

​Bumbling bees

Bumblebees really are clumsy fliers. They bump into stems, leaves, and branches quite frequently. Research conducted at the California Academy of Sciences and UC Davis explored how bumblebees are able to withstand the frequent collisions they experience each day. When I say frequent, I mean once a second, on average.

​Bumblebee evolution

Cousin to honeybees (Apini), orchid bees (Euglossini), and stingless bees (Meliponini), bumblebees (Bombini Bombus) have pollen baskets on their legs, making them all corbiculate. Fossil evidence of the common ancestor to these beneficial insects dates back 100 million years. Bumblebees have been around for 25 to 40 million years, depending on who you ask.

Bumblebees are more commonly found in higher latitudes and higher elevations than other bees. There are even two species found in the Arctic! This may be due to their fur coats, but scientists explain that bumblebees are able to regulate body temperature, using the sun’s radiation and ‘shivering’ to generate heat. Unlike other bees, bumblebee queens are known to incubate their eggs.

Sitka bumblebee (James Strange, U.S. Forest Service) Public Domain

Bumblebee description

Like other insects, bumblebees have a three part body made up of the head, thorax, and abdomen. The head features two compound eyes and three primitive eyes, mouthparts, and antennae. Female bumblebee antennae have 12 joints, while males have 13. The thorax is where the wings, legs, and wing muscles are found. The abdomen contains the digestive and reproductive organs and the stinger.

Bumblebees have two pairs of wings, a fore wing and a rear wing. The wings connect to muscles that are attached to the inside of the plates that make up a bumblebee’s exoskeleton. A waxy substance is secreted from glands and discharged from between the plates. It looks a lot like dandruff. Bees groom the wax into clumps and use it to build honey pots, egg coverings, and as nesting material. Bumblebees do not have ears. We do not know if they can sense sound waves through the air. They do seem to be able to sense vibration through other materials, such as the ground.

​Unlike the oblong honey bee, or the shiny black female carpenter bee, your average bumblebee looks round and furry. This is because they are covered with very soft, finely branched bristles, called setae. Bumblebees can usually be differentiated from golden male carpenter bees by the presence of contrasting bands and other markings. This aposematic coloration serves as a warning to would-be predators. Some other insects, such as hoverflies, mimic and are protected by this coloration. This is called Batesian mimicry. Bumblebees come in a variety of sizes, ranging from 1/2 an inch, up to one-and-a-half inches long. [Don’t worry, that big one lives in Chile and is affectionately referred to as a ‘flying mouse’ - okay, maybe not affectionately.]

Bumblebees store fat in their body. This fat is used up during hibernation. Bumblebees do not use the bee dances common to honey bees, but they do exhibit social learning. Youngster bumblebees are frequently bumped off of favored flowers by more mature bees. In a lab experiment, bumblebees were taught to move an object in exchange for a reward. Untrained bees who observed the task-reward process learned it more quickly than those bees who observed the same exchange performed by an inanimate object. In fact, observer bees consistently improve on the methods used initially, implying at least some level of cognition.

Bumble bee-havior

Most bumblebee species are social insects. They live in relatively small colonies, led by a queen. Honey bee colonies hold 10 to 60 thousand workers, during peak honey production, while a bumblebee colony may only contain 50 to 1,500 individuals.

Bumblebee nests are first constructed by an over-wintered queen. After collecting pollen and nectar from flowers and finding a suitable nesting site, the young queen will build wax pots to store food, and wax cells to receive her eggs. As the eggs hatch into larvae, the cells are expanded into a lumpy mass of brood cells. Nesting sites can be in or on the ground, or in tussock grasses, depending on the species. Bumblebee nests are a food source for badgers and other insectivores. Bumblebees normally range only a mile or so from the nest.

A queen bumblebee performs a mating flight before retiring to her nest. There, she will ‘decide’ whether or not to use the collected sperm to fertilize each egg she lays. Unfertilized eggs develop into males. Fertilized eggs grow up to be hormonally suppressed worker females or fertile queens. Mature males are forced out of the nest by the females. Males and new queens live independently from the colony, sleeping in flowers or holes in the ground.

Bumblebee feeding

Bumblebees feed on nectar, and pollen is collected for the young. Bumblebees lap up nectar with a long, hairy tongue, called a proboscis. The tip of the tongue may also be used as a straw. Because the tongue is so long, bumblebees are able to gather nectar from (and pollinate) deeper flowers than honey bees, which have shorter tongues. When bumblebees fly, the proboscis is often folded under the head. Some bumblebees feed on flowers from above, the way other bees and hummingbirds do, while other ‘rob’ the nectar by cutting a hole in the base of the flower. It is called ‘robbing’ because this behavior avoids pollen transfer. [The metabolic rate of bumblebees is 75% higher than a hummingbird’s.]

Some species of bumblebee leave a scent marker behind on a flower, after it has fed. This marker deters other bumblebees from feeding on that flower, until the marker fades. Once a bumblebee has eaten its fill and collected all of the nectar it can carry, it returns to the nest and deposits its riches in brood cells, for the young, or in wax cells, for storage. Unlike honey bees, which process nectar into honey, bumblebees store it as-is.

Bumblebees as pollinators

Bumblebees are powerful pollinators. As they feed, moving from flower to flower, they collect and deposit pollen. Bumblebees are commonly used in greenhouse tomato production. They are fed sugar water so that they do not need to harvest nectar, and can focus exclusively on pollen.

[Did you know that pollen is left behind at each flower because the act of flying causes the bee to build up a static charge? The plants, being rooted in the ground, are, well, grounded. The electrically charged pollen grains on the bee are attracted to the stigma, which happens to be the best grounded part of a flower, while the flower’s grounded pollen is attracted to the bee’s statically charged body. How bizarre and amazing is that?!!?]

Bumblebees also pollinate plants by a method called buzz pollination, or sonication. They do this by disconnecting their wings from their flight muscles and vibrating their flight muscles at a frequency very close to middle C. [The frequency varies depending on plant species.] The force generated during sonication can reach 30 Gs, which is almost more than a human can tolerate! This vibration causes a flower’s pollen to burst forth. This method is particularly effective on blueberries and members of the nightshade family, such as tomatoes, tomatillos, and eggplant. Sweat bees, carpenter bees, and stingless bees sonicate, but honey bees do not. PBS has an amazing video of buzz pollination.

Bumblebee species

There are over 250 species of bumblebee worldwide and several of them are in trouble. This is especially unfortunate when you realize that native bumblebees are responsible for a lot of pollinating. Some of the more common bumblebees include:

• Bombus melanopygus edwardsii
• California bumblebee (B. californicus)
• Fog-belt bumblebee (B. caliginosus)
• Sitka bumblebee (B. sitkensis)
• Yellow-faced bumblebee (B. vosnesenskii) - most common

These bumblebee species have not been seen in recent years:

• Black-notched bumblebee (Bombus bifarius)
• Brown-belted bumblebee (B. rufocinctus)
• Indiscriminate Cuckoo bumblebee (B. insularis)
• Western bumblebee (B. occidentalis)

Cuckoo bumblebees, or brood parasitic bees, do not live in colonies. Instead, cuckoo queens will invade a nearby bumblebee nest, kill the queen, and then start laying their own eggs, which end up being cared for by the murdered queen’s workers. [You can tell the difference between a cuckoo bumblebee and a social bumblebee by looking closely at the back leg: nesting female bumblebees have a pollen basket, which is bare and shiny, whereas a cuckoo bee hind leg is covered with hairs. They transport pollen by wedging it between the hairs.]

Attracting bumblebees

Like the European honey bee, bumblebee numbers are declining. This is largely due to habitat loss, pesticide use, and the mechanization of agriculture. You can counteract some of these effects by planting flowers that attract and provide for bumblebees.

Bumblebees are attracted to flowers by both sight and smell. They seem to prefer flowers that grow on spikes, such as salvia and lavender. This may also be because these flowers tend to be blue, purple, or white, favorite colors of bumblebees. Bumblebees are also attracted to members of the sunflower family. The most important consideration when planting to attract bumblebees is that the stems are sturdy enough to support these heavy pollinators. Bumblebees can also see a flower’s temperature, as well as its electrical field.

You may very well already have plants that attract bumblebees. If you allow your kale and other cabbage family members to bolt in early spring, bumblebees will take advantage of the blooms, and you will add these plants to your foodscape as seeds are dispersed later in the season. Other popular flowers that attract bumblebees include anise hyssop (Agastache foesniculum), guaras (Guara lindheimeri and other cultivars), bog sage (Salvia uliginosa), Mexican sage (S. mexicana) and many members of the mint family, including basil, thyme, sage, and rosemary. Native plants, such as manzanita, ceanothus, CA buckwheats, penstemons, currants, and gooseberries will also attract and provide for these hard working beneficial insects.

Bumblebees are housing opportunists. They build nests in upturned planter pots, abandoned mouse burrows, and under boards. If you have a square foot of bare, undisturbed soil and the right flowers, you are pretty much guaranteed a visit.

The U.S. Forest Service offers a nice poster of western bumblebees with dietary notes. You can also learn more about bumblebee conservation from the Xerces Society.