When farms were smaller and set within a more diverse landscape, wild native bees provided pollination services for many crops. With strong pollinator populations and smaller field sizes, some farmers relied on the pollination services provided by native bees. As agriculture has moved towards larger fields with less surrounding wild area, farmers have become more dependent on bringing honey bees in to pollinate their crops. Their large colonies and ease of transportation make them an effective and relatively easy-to-use pollinator. However, it is never wise to rely on only one method for a service as important as pollination. Bumble bee colonies are not as easy to raise as honey bees. Bumble bee colonies do not usually contain more than 300 bees compared to 30,000 bees in a honey bee colony. However, bumble bees make up for these shortcomings in their extreme efficiency as pollinators. Characteristics that make bumble bees great pollinators include long tongues in some species, ability to fly in cold weather and low light, ability to buzz pollinate, and ease of use in greenhouse conditions.
One of the most variable features between different species of bumble bees is tongue length. Bumble bee tongue lengths range from 3/16 to 19/32 inch (~5 to 15 millimeters). Tongue length is an important consideration as it directly affects a bee’s ability to access nectar from flowers. Some flowers have a narrow tube with nectar at the bottom. To achieve pollination, the bee’s tongue must be long enough to reach the nectar at the bottom of the tube. When the bee enters the tube to access the nectar, she comes into contact with the pollen from that flower and will transport it to other flowers she visits, thus pollinating the flowers. Honey bees and bumble bees with shorter tongues, which are unable to reach the nectar, will sometimes bite a hole at the bottom of the flower to access the nectar. These bees are cheating the flowers. The bees receive nectar, but the bees do not come into contact with the reproductive parts of the flower and so are not providing effective pollination services in return for this nectar. This is why bumble bees with long tongues are often preferred for pollinating flowers with long tubes.
An example of a crop that does well with longtongued bees is red clover. Red clover flowers have tubes that are 19/64 inch (~7.5 millimeters) or deeper. Honey bees have a tongue length of only 15/64 to 1⁄4 inch (~5.9 to 6.25 millimeters). Honey bees can reach the red clover nectar if they push their heads into the tube and if the nectar level is high. Although honey bees will visit red clover for pollen, which is more easily accessible, they cannot be relied on to consistently visit red clover. If there are other flowers with accessible nectar, the honey bees will focus their foraging efforts on those plants. Long-tongued bumble bees have no trouble reaching the nectar at the bottom of red clover flowers. However, shorttongued bumble bees will have the same difficulties as honey bees.
Aside from working in a greenhouse, most pollinators are exposed to extremely variable working conditions. Cold, heat, rain, light levels, and wind can all affect the ability of bees to fly from flower to flower and perform their duties. Bumble bees tolerate extreme conditions better than many other pollinators.
Honey bees are not very active at temperatures below 50°F (10°C), whereas bumble bees will continue to forage at temperatures as low as 45°F (7.2°C). While this difference may seem small, it can be significant for pollination. For example, some flowers, such as cucumber, watermelon, and raspberry, release their pollen early in the morning, making this the time when bee visitation is most effective. Bumble bees are more likely than honey bees to visit flowers in the cooler early morning. Since this coincides with pollen release from these plants, bumble bee visits are more effective. Bumble bees also forage in lower light conditions than honey bees, adding to their work hours.
Bumble bees are also tolerant of stronger winds than honey bees and will even forage in light rain. Because of these traits, bumble bees can be a more reliable pollinator for crops where inclement weather is likely. In this situation, bumble bees can be used as supplemental pollinators, filling in when other bees cannot tolerate the conditions.
For many flowers, pollen is readily accessible to anything that goes near the anthers. This is easily seen when you put your nose into a flower to smell it and your nose emerges covered with pollen. Some flowers are more conservative with their pollen, making it accessible only to certain pollinators. One strategy is to have very small pores on the anthers, so that pollen is only released when the anther is shaken. There are 64 families of plants with at least one member using this pollen dispersal method, including species in such economically important plant families as Ericaceae, Fabaceae, Rubiaceae, and Solanaceae. Pollen is released when bees grab the anthers in their mandibles and vibrate the muscles in their thorax. The vibrations travel through to the anther, and the pollen is shaken out of the pores. The distinctly audible toot that accompanies this behavior is the cause of the name “buzz pollination.”
Bumble bees are not alone in their ability to buzz pollinate. Most major bee families (Colletidae, Andrenidae, Halictidae, and Apidae) have members that “buzz.” However, most of the bees currently managed for pollination do not buzz pollinate. This has led to the use of bumble bees for many crops that have improved fruit set with buzz pollination, such as tomatoes, peppers, and blueberries.
Both pollinators and the flowers they pollinate vary greatly in size. Bees range in size from mere specks to creatures as large as a human thumb. In order for pollination to occur, bees must reach the anthers and pistils of the flowers. Bumble bees are larger than many other managed pollinators, 13/32 to 13/16 inch (~10 to 30 millimeters). This suits them well for pollination of crops with larger flowers such as cotton, where bumble bees have been preferred due to the efficiency of their pollination.
When crops require cross pollination, the movement of pollinators within the field must be considered. Often crops are planted with one variety in one row and another variety in the next row. Bumble bees tend to have a more erratic flight pattern than honey bees. Honey bees often fly straight down rows. For hybrid crops where rows are planted with different varieties that require out-crossing, bumble bees facilitate the movement of pollen across these rows. For sunflowers, there is some indication that better pollination can be achieved by having both honey bees and other bees present than when either are used alone. It appears that the presence of other bees increases the chances of honey bees crossing rows, and so increasing cross pollination.
Bumble bees are ideal for use in greenhouses. Their nests are easily transported and housed within greenhouses. Honey bees are also used in greenhouses, but there are several aspects of honey bee behavior that make this difficult. Honey bees communicate with each other about the quality, quantity, and location of floral resources. Through this communication, a honey bee colony is able to focus most of its foraging effort on the most highly productive flower patches. If the most rewarding resources are located outside the greenhouse, and there are vents open through which the bees can escape, honey bees will forage on those resources, ignoring the flowers in the greenhouse. Although bumble bees can also escape and forage elsewhere if greenhouse vents are open, bumble bees are more likely to focus their foraging efforts primarily on the resources that are most readily available.
Bumble bees are the primary pollinators used for most greenhouse tomatoes. Tomatoes form larger, more even fruits with buzz pollination. Before bumble bees became widely available through commercial producers, tomato flowers were pollinated either by honey bees or by vibrating machines operated manually. Bumble bees are much more efficient and much less expensive than the other options. The greenhouse tomato industry grew significantly after bumble bees became available commercially in the early 1990s. From the early 1990s to 2003, North American greenhouse tomato growing area is estimated to have increased by almost 600 percent to 4,260 acres (1,725 hectares).