By Nancy Anderson
The story of bumble bees visiting blossoms provides a beautiful example of insect and flowering plant coevolution for mutual benefit. In the fall when the leaves start to turn, bumble bees are still a familiar sight, providing a pollination service that helps fall blossoms produce fertile seeds. On a sunny fall day, a bumble bee is easy to identify because of its large size, fuzzy appearance, and loud buzz. Bumble bees have yellow and black hairs, two pairs of wings, long antennae, eyes on the side of their head, and really buzz when they move their wings. Their scientific name, Bombus, is Greek for a “buzzing sound.” The workers are female bees with a “pollen basket” on their hind legs. Most bumble bees seen in July are workers, but later in the season male and queen bumble bees are often observed. Queens are larger than workers; males lack the “pollen basket” and with one more abdominal and antennal segment than females, appear elongated.
In terms of their evolutionary history, bees evolved as a strictly vegetarian group from ancient predatory wasps, most likely in the dry western parts of the supercontinent Gondwana. Subsequent fragmentation of land to form South American and African continents and then the expansion of bees into the Northern Hemisphere happened at a time when northern latitudes had tropical plants. Floral diversification allowed bees to diversify and thrive. The earliest bumble bee fossil was found in Europe as a more temperate climate and open vegetation (deserts, tundra, and grasslands) started replacing the tropical forests. The earliest North American Bombus fossil dates back to a period of cooling temperatures. Their large size and furry body still help them succeed on cool fall days and at higher altitudes where a founder queen must get through the winter as a solitary bee.
On their branch of the insect tree of life, within the Apidae bee family, members of the genus Bombus thrive in annual colonies. Each colony is multi-generational with a division of labor where the vast majority of individuals cooperate as non-reproductive adults that specialize in caring for the brood or in aiding a single reproductive queen bee. After colonies are done for the summer, the males and queens have a mating season. A mated queen seeks out a place to hibernate in a cavity protected from wind, rain, and predators. The solitary founder queen emerges in spring. She makes a nest under tufts of long grass, in brush piles, or in any location that is protected from disturbance by predators and sheltered from rain. For example, an abandoned rodent burrow with soft, dry nesting material, provides room for the colony to grow from 50 to a few hundred bees living cooperatively together for one growing season.
Flowering plants attract bumble bees for feeding in exchange for pollination services. Like most bees, bumble bees feed entirely on pollen for protein and fat nourishment and nectar for carbohydrates. Meanwhile, bees transport pollen from the male parts to the female parts of blossoms so that sperm in the pollen grain can join with an egg to complete the fertilization process in the flower. Seeds that develop from the products of fertilization pass on a new combination of genetic information to the next generation of plants that will grow from fertile seeds. Flowers evolved particular shapes and colors, sticky pollen, sweet nectar, and scents inside the flower to entice bees to brush against the male and female flower parts, thus providing pollination services. Since adult female worker bees gather pollen and nectar to feed the brood, the young can stay protected in their nest throughout their development, nourished by food from the adult worker bees. Nectar and pollen provide the only nourishment that all bees need throughout their lives.
Bumble bees and other bees do important work for people who grow edible plants. The seeds they help plants to produce also feed songbirds and other wildlife. However, some flowers hide their pollen deep within their anthers, the male parts of the plant. Among these are the Solanaceae (plants in the potato family) including peppers, potatoes, eggplants, and tomatoes, as well as wildflowers like toxic woody nightshade and horse nettle. These so-called buzz-pollinated plants benefit from the buzzing that gives bumble bees a unique role as an important pollinator. The anthers in these plants enclose pollen that is only released from pores or slits when vibrated by a pollinator. With their “buzz,” a bumble bee helps the blossom release pollen by sprinkling it out like a salt shaker onto the female flower parts – or onto the bee, which gathers the pollen as food. Without vibration-assisted foraging, the pollen is not efficiently released from the blossom for fertilization to take place. As a bumble bee moves from one flower to another, it more efficiently cross pollinates the plants. Honey bees cannot “buzz” pollinate, so bumble bees are critical for agriculture and in nature.
Unfortunately, the current decline in bumble bees is a cause for concern due to habitat loss, competition and nutritional stress, pesticide exposure, and diseases transmitted from bumble bee and honey bee colonies managed for agriculture. Bumble bees that face an uncertain future in our area include seven that are currently listed as Species of Greatest Conservation Need according to the current North Carolina Wildlife Action Plan. However, we lack the data needed to inform effective conservation measures. To address this need, the Southeast Bumble Bee Atlas (SEBBA) is a community science effort to track bumble bees in our region. This project, led by the Xerces Society for Invertebrate Conservation, involves volunteers in gathering data to answer questions like the following:
- In bumble bee habitats, what blooming plants are available (regardless of whether or not they have bumble bees), and which blossoms do bumble bees visit?
- What features are found where threatened bumble bee species are surviving?
- What plants are foraged by bumble bees from late summer into the fall, making them an important late-season bee food source?
- How common are any of the strategies for queens preparing to overwinter, such as digging in to loosen soil, mossy ground cover, leaf litter, or burrowing into sod?
The new data will make it possible to assess species distribution, population shifts, habitat associations, and more. In fact, a SEBBA volunteer collected bees nearby that were identified as a species in decline, the Half Black Bumble Bee (Bombus vagans), on Hoary Mountain Mint and Summer Bluet blossoms. The SEBBA data is helping to identify regions with healthy bumble bee populations, as well as those in need of restoration or management.
To help bumble bees, get to know them and their vital contributions to the survival of so many plants. Steps you can take in your own garden:
- Protect and enhance plants for feeding pollinators and maintain your garden with a diversity of blossoms, including plants that are native to your area, especially those that are visited by bumble bees.
- Enhance potential nesting sites: Leave some leaves where they fall. Leave brush piles, dead trees, and fallen logs, and leave an area without mowing. Tall native bunch grasses are especially important for bumble bees. The grasses clump and create protection from predators and rain. Any area preferred by field mice will potentially provide shelter for bumble bees in abandoned rodent burrows.
- Reduce the use of pesticides.
- Finally, in the fall if you notice larger bumble bees visiting flowers or crawling on the ground, they are probably queens. Look carefully for larger bumble bees on the ground near well-drained soil or in the leaf litter under trees. If you observe a queen digging or searching for a place to hibernate, submit your observation at http://QueenQuest.org, a collection by Xerces and other collaborators to gather anecdotal observations of overwintering queens.
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