Best Management Practices for Using Commercial Bumble Bees on Horticultural Crops in Delaware

Jacquelyn Marchese, Graduate Student, Department of Entomology & Wildlife Ecology; Deborah Delaney, Assistant Professor Entomology & Wildlife Ecology,; Gordon Johnson, Extension Vegetable & Fruit Specialist,

A common native pollinator in Delaware is the Common Eastern Bumble Bee (Bombus impatiens). The behavior, physiology and morphology of bumble bees make them ideal pollinators because of the speed at which they transfer pollen, the efficiency with which they gather pollen within various crops, and the increased endurance to fly in adverse weather for longer periods of time. The bumble bee also has the ability to buzz pollinate the flower for pollen, a pollination technique not seen in honey bees. Buzz pollination occurs by bumble bees vibrating the flower by pumping their wings at a certain frequency, to dislodge pollen. Bumble bee foraging activity starts earlier and ends later in the day than managed honey bees and they forage in lower temperatures.

In the early 1980s, commercial rearing of bumble bees for pollination services was developed in The Netherlands. By the 1990s commercial bumble bee production made its way to the United States. In the United States, there are two companies that distribute commercial nests of B. impatiens, Koppert Biological Supply Company and Biobest®. Bumble bee colonies are reared and placed within plastic boxes, equipped with enough pollen and a sugar substance to sustain bumble bee during shipping. Two holes are engineered into each plastic box that allows the grower to control the activity of their bumble bee nest. These entrance holes are controlled by a plastic flap that either opens or closes all or one entrance hole (Figure 1).

Bumblebee1 Figure 1. The colony entrance reducing tab, closed, one open hole, two open holes. The one open hole allows for bees to enter the colony but prevents bees from escaping, a useful tool to keep bees protected from pesticide sprays.

Bumblebee2Figure 2. Pictures of the different components of a commercial bumble bee quad. Top left, inside the individual colony unit. Top right, the whole individual colony unit. Bottom left, inside the quad with three out of the four colony units. Bottom right, the full quad.

Bumble bees differ greatly from their more popular, honey bee, counterpart and thus have different biological requirements in order to optimize their use to growers in the field.

Bumble bees were studied in 3 crops in Delaware: Strawberries, watermelons, and pickling cucumbers. We found that bumblebees are a viable alternative pollinator for strawberry and watermelon and can be considered for those crops. During our two year study that looked at bumble bees in strawberry and watermelon field crops, bumble bees were constantly and consistently detected in the fields throughout both seasons. Although we determined that strawberry and watermelon pollen were not the main pollen sources being brought into the colonies, foragers were seen with enough frequency and abundance that we are confident in their ability to pollinate these crops. The bumble bees observed on strawberry and watermelon blooms were most likely nectar foragers, but if foragers are strictly on the crops for the intention of gathering nectar, transfer of pollen should still be occurring.

Bumblebees did not perform well in pickling cucumbers in our studies. In two years of sampling pickling cucumber fields with commercial bumble bees, we found that they made up at most 8% of all pollinators collected and they were frequently seen on weedy forage such as morning glory, ragweed, horse nettle and other common flowers. Honey bees were the most abundant pollinator found in these fields, followed by native sweat bees and pollinating hover flies. Therefore, at this time, without additional research, we cannot recommend bumblebees in pickling cucumber crops. Growers should continue to rely upon and use honey bees in pickling cucumber plantings.

Place bumble bees in the field after crops have begun to bloom. Like honey bees, bumble bees need access to forage to sustain themselves. Bees that have found unintended forage in the beginning of the season are likely to continue to forage on this unintended source, especially if it is more favorable than the intended crop. Place bees in the middle of the field to encourage in field foraging.

Allow time for bees to settle before opening units. Always follow instructions provided by the bumble bee supplier when placing bees within the field. Give the allotted time before opening up the colonies for the first time. Although bumble bees will need to chew out of the hole in order to begin foraging, colonies should be given at least 30 minutes to settle after being handled during shipment and placement. Also, be sure to check on each colony 2-3 hours later to make sure that the bees have successfully chewed out of the hole and exited the nest. On occasion, bees will not successfully chew out of the hole and will need to be cut out of the colony. Although this has been known to occur, it is not common and most colonies will successfully find their way out of their colony and into the crop, on their own.

Close bumble bee units before each pesticide application. During the season, change each bumble bee colony entrance to one open hole at least two hours before all pesticide applications. This will allow time for bumble bee foragers to return and be kept in the colony in order to limit forager exposure to pesticides, see the pesticide section below for more information.

Place bumble bees under shade, to increase their productivity and longevity. Bumble bee units placed in natural shade (along forest/field edges) or fitted with a shade structure last longer and are significantly more productive than units in the sun. Placing bees under shade is especially important during the warm summer months of Delaware’s watermelon bloom. Bumble bees constantly and actively strive to keep their colony temperature at around 86°F. Colonies exposed to direct sunlight have to work harder and use more energy to thermoregulate. Colonies placed in full sun without shade cannot maintain normal worker activity (pollen and nectar foraging and duties within the colony) for as long as the colonies with shade.

Keep bumble bees away from honey bees. Bumble bees should be placed as far from honey bee hives as possible. This is especially true when crops are not in bloom. When forage is low for the commercial pollinators they should be greater than 1 mile from each other. Honey bees are very resourceful and a bumble bee colony is a great source of pollen and nectar which honey bees are constantly seeking. If surrounding forage is low or not agreeable to honey bees, bumble bees will be susceptible to honey bee robbing causing a weakened colony and overall loss in productivity from both pollinator species.

Bumble bee units should be weighed or strapped down, especially when placed within a shade structure. These units may be susceptible to being flipped or carried by strong winds. Not only does this disrupt the normal orientation of the colony, causing helpless larvae, nectar and pollen to fall out of their individual waxen cells, but can cause blockage to the unit openings, trapping bees within the unit.

Bumble bees may be transferred to another field for additional pollination services throughout a season. Before moving, close the plastic opening tab to the one-hole open position. Allow forager bees at least two hours to return to the colony. The bumble bee colony may then be transferred to another site.

Close up colonies before each pesticide spray. Bumble bees very easily accumulate pesticides within the wax of their brood clump and their bodies by foraging in crops that have been treated with various chemistries. Although bumble bees will inevitably have some exposure to sprayed pesticides within the field, growers can limit exposure by using the plastic opening tab within each colony box. Growers are urged to close up the commercial nests at least two hours before spraying to decrease the exposure of bees to the pesticides.

Dispose of bumble bee colonies in a timely and humane fashion. There is a risk of commercial bees breeding with native populations. Commercial bumble bees are mass reared, and therefore the genetic diversity of the commercial bees does not mirror what is naturally found and occurring in the wild bees. The integrity of this wild genetic stock is important because it allows for the bees to be adapted to a wide variety of environmental conditions and exposure to various pathogens that they may encounter. If commercial bees mate with wild bees, the commercial bees will be diluting the genetic stock of the wild bee population.