Bumble bees are managed for their pollination services in a variety of crop systems, including tomatoes, fruit trees, berries, and soybean (Abrol & Shankar, 2012; Feltham et al., 2014; Stanley et al., 2015; Suzuki et al., 2007). Most commercial bees are used in greenhouses, including for the growth of berries, squash, eggplant, and peppers (Abak et al., 1995; Shipp et al., 1994; Strange, 2015). However, around 95% are specifically used in tomato greenhouses (Velthuis & van Doorn, 2006).
Bumble bees were first discovered to have value in greenhouses in 1985, by Belgian veterinarian Dr. de Jonghe (Velthuis & van Doorn, 2006). He discovered that bumble bees could replace the need for manual pollination by human workers, a practice that formerly required shaking the plants three times a week. After this discovery, three major commercial bumble bee productions began, and those same three companies are still the premier suppliers of bumble bees globally: Biobest (founded by Dr. de Jonghe), Koppert (a Dutch company), and Buntin Brinkman Bees (another Dutch company). Koppert is the biggest supplier in the United States.
Around the world, there are five commonly reared species of bumble bee (genus Bombus): B. terrestris, B. lucorum, B. occidentalis, B. ignitus, and B. impatiens (Winter et al., 2006). Bombus terrestris is used most frequently at the global scale, but in North America, B. impatiens is most widely used. The wide-spread use of B. occidentalis in the Western United States was halted due to decimation by disease (Winter et al., 2006). Bumble bees are better suited for indoor pollinating than the more commonly managed honey bee because of their unique pollination tactics, smaller colony size, and comparatively smaller foraging ranges (Morandin et al., 2001; Velthuis & van Doorn, 2006).
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| Figure 1a. A commercial bumble bee box in a tomato greenhouse. Photo by Adobe Stock. | Figure 1b. The interior of a commercial bumble bee box. The largest bee is the queen; all other bees shown are female workers. The bottom left of the photo shows a clump of un-emerged brood cells. Open brood cells of previously emerged workers contain nectar. Photo by Emily Runnion © 2023. |
Economic Importance
In 2010, the commercial bumble bee industry provided over 140,000 colonies of managed bees for the purpose of supplying pollination services to crop systems (Strange, 2015). The two main suppliers of managed bumble bees across North America are Koppert and Biobest.
A commercial bumble bee box is shown in Figures 1a and 1b. Commercial bee boxes delivered to growers come in a variety of sizes, but a typical box will include a mated queen and about 100 female workers. It is important to understand how to sustain and properly manage bumble bees in these systems, especially when used in tomato greenhouses, which had a retail value across North America exceeding $690 million in 2012 (Strange, 2015).
More than 10,000 varieties of tomatoes are grown across the world, and they contribute greatly to the agricultural economic market. Hybrid tomato seeds, on a per-weight basis, can be more valuable than gold (Heuvelink, 2018). One source estimates that, in 2021, the global economic value of tomato sales overall was $10.8 billion (Cooley & Vallejo-Marín, 2021). According to the 2019 U.S. Department of Agriculture (USDA) National Agricultural Statistics Service, U.S. tomatoes grown under protection had a national sales total of over $345 million. While most tomatoes are grown in open fields, an increasing number of tomato growers are choosing the option of cultivating plants in either a greenhouse or high-tunnel (hoop house) setting (Figures 2a and 2b). Growing under protection offers several benefits for crop production, including control for most climactic variables year-round, and added protection from pests and diseases (Heuvelink, 2018). Both high-tunnel and greenhouse tomato growers use bumble bees for their pollination services; however, greenhouse growers are more reliant on bumble bees than high-tunnel growers.
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| Figure 2a. Tomatoes growing directly in the soil under high-tunnel protection with the side panels rolled up to allow air flow. Photo by Adobe Stock. | Figure 2b. Tomatoes growing in a large-scale commercial greenhouse. Photo by Adobe Stock. |
Bumble Bees: General Biology
Bumble bees (family Apidae, genus Bombus,) include over 250 species across the world (Goulson, 2010). They are generally covered in thick fur and are much larger than other bee species (Figure 3). Bumble bees have a primitive social structure and an annual life cycle. In wild populations, a queen will emerge from hibernation in spring. This can be March–April, depending on the species. She will forage for nectar and pollen and begin to lay eggs. Approximately three to four weeks later, the first workers will emerge and begin to take on roles as foragers or caregivers of the new brood. Toward the end of the colony cycle, new queens and drones will emerge and eventually leave to find mates. The colony dies before winter.
Bumble bees live in colonies of several hundred individuals and will nest in hollow spaces or underground. A colony is founded by a single, mated queen at the beginning of spring, who first lays eggs that hatch and become nonreproductive daughters, followed eventually by new daughter queens and males called “drones” (Schmid-Hempel et al., 2019). The drones and daughter queens leave the colony and mate with members from other colonies in the fall, and the nonreproductive workers and founding queen die (Fowler et al., 2020). The newly mated daughter queens store sperm from drones over the winter to begin their own colonies the following the season (Gradish et al., 2019).
In commercially managed hives, colonies generally last 10–14 weeks in a greenhouse or field setting. Colonies arrive to growers with an already mated queen, at least one round of brood already emerged as worker daughters, and another round of brood ready to emerge shortly thereafter. In tomato greenhouses, colonies of commercial bumble bees come equipped with a container offering constant access to sugar water and a small quantity of pollen. Because tomato plants do not provide nectar, commercial sellers must ensure hives have enough sugar water supplies to last for the duration of their pollinating time frame.
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| Figure 3. General bumble bee anatomy. Notable features include a furry or hairy body that is larger than other bees. Photo by Adobe Stock. |
Buzz Pollination
For tomatoes, pollination by bees results in higher fruit set, yield, and weight than all other methods of pollination (Morandin et al., 2001). Several species of bees use a special strategy called “buzz pollination” to pollinate tomato flowers—an especially effective means of dislodging pollen from plant anthers via wing vibration (Figure 4) (Dingley et al., 2022). Bumble bees engage in this pollination behavior, which requires them to grasp the anthers of a flower and vibrate rigorously to dislodge the pollen (Figures 4a and 4b). Buzz pollination occurs when the bee vibrates its thoracic muscles at a very high frequency (Cooley & Vallejo-Marín, 2021). This causes the pollen to bounce off the anther of the flower and land on the fur of the bumble bee. When the bee travels to new flowers, this pollen is transferred between flowers, completing pollination. Because tomatoes do not produce nectar, their only floral reward to draw in pollinators is pollen. This greatly influences the type of bee best suited for use in tomato greenhouses. Honey bees do not perform buzz pollination, and when given access to other floral resources they will not readily choose to pollinate tomatoes (Greenleaf & Kremen, 2006). There are, however, several groups of native bees that will use buzz pollination when visiting tomato flowers. For example, the Urbane digger bee (Anthophora urbana Cresson) (Figure 4b) was found to consistently visit tomato flowers in Northern California (Greenleaf & Kremen, 2006).
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| Figure 4a. Bombus impatiens (common eastern bumble bee) engaged in “buzz pollination.” The anther’s darker coloration is “bruising” that shows previous pollination activity. Photo by Adobe Stock. | Figure 4b. Anthophora urbana Cresson is a native bee that utilizes buzz pollination when visiting flowers and field-grown tomatoes in California. Photo by Adobe Stock. |
It’s important not to understock or overstock a greenhouse with bumble bees purchased for pollination services. Bumble bee colonies placed in higher densities may result in over pollination. When completing buzz pollination, bumble bees cause bruising on the anthers of the tomato flowers. Anther bruising (Figure 5) is a reliable measure of pollination activity, with increased necrotic tissue on the anther representing an increased amount of bumble bee pollination activity (Morandin et al. 2001). The amount of anther bruising can be quantified at different levels. Higher levels of bumble bee pollination activity are correlated with increased seed set, size, and weight, and increased fruit diameter (Morandin et al., 2001). Level 0 is understandably undesirable because it means that no pollination has occurred. However, level 3 or higher may also be detrimental to tomato yield. Growers should be aware of the point where they see a relationship of diminishing returns between buzz pollination activity and seed count, size, and fruit diameter (Morandin et al., 2001). The ideal number of bumble bee colonies varies depending on internal greenhouse environmental conditions, such as temperature and humidity, but generally, bumble bee colonies should be placed in densities of 7–15 colonies per hectare (Morandin et al., 2001).
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| Figure 5. Simulated example of tomato anther bruising as a result of bumble bee buzz pollination. A darker anther is indicative of increased bruising, and thus more pollination activity. Photo by Adobe Stock and adapted by Emily Runnion. |
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