Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Watermelon and other growers using bumble bees have expressed concerns about not seeing the bumble bees active in fields. Growers used to watching honeybees will not see the same amount of activity by bumble bees.
Compared to a honeybee, bumble bees are about 10 times more efficient as a pollinator due to their size, the speed at which they transfer pollen, the efficiency with which they gather pollen within various crops, and their 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. Because of these characteristics, many less are needed to achieve the same crop pollination and commercial colonies only have about 200 bees each (800 per quad).
When assessing bumble bee activity, flag out 10 areas in your field and observe each area on three different days during bloom. These observations should last one minute under sunny, windless conditions, between 9 a.m. and noon. Approach each plot with care so as not to disturb the foraging bees. Stand about three feet from the crop to avoid blocking the flight path of the bees. Count and record the number of bumble bees at each flag, then calculate the average for your observations. You should an average one one bumble bee per ten flags (0.1 bees per flag) to have adequate pollination.
Bumble bee colonies should be shaded and can be placed along shaded field edges. However, if there are other wild flowers nearby, they will also work in those areas, reducing their field effectiveness. Therefore, when placing bumble bees in watermelons or other flowering vegetable or fruit fields needing pollination, it is recommended that bumble bee quads be placed in the field middles to have more foraging in the target field.
For more information on using bumble bees see this 2013 weekly crop update article: http://extension.udel.edu/weeklycropupdate/?p=5408