Strawberry flowers need to be pollinated. Uneven pollination usually results in misshaped fruit. Strawberry flowers are most effectively pollinated by honeybees. A recommended honeybee hive size for greenhouse strawberry is one with 6,000 to 8,000 bees per 1,000 m2 greenhouse with a typical planting density of 8-10 plants per m2 (Mitsubachi Kyogikai, Japan), which is translated as approximately 11,000 sq feet per small hive. Such small hives with a honeybee population of several thousand per hive are available to rent in countries like Japan where average size of greenhouse strawberry production is small (~20,000 square feet). However, in US, as far as we are aware, such a rental business offering small honeybee hives (less than 10,000 bees per hive) has not been developed yet due to the limited market. Typical honeybee hives used for open field crop production seem to be at least several times bigger than what future strawberry growers would need for small scale local production in urban or suburban settings.
Use of bumblebees is more common for greenhouse and high tunnel growers in the US, because honeybees do not function well in most American high tunnels (the typical glazing limits UV light transmission) and honeybees are more challenging to manage when there is competition from other flowers available in nature or in other neighboring crop fields. Winter strawberry production in greenhouse, however, is a unique situation where not much competition exists from flowers outside the greenhouse (i.e., off season), and therefore the flowers in greenhouse are the main attraction for the bees. Bumblebees work well in greenhouse tomato and their use is an industry standard. And, introduction of bumblebees to strawberry greenhouse is possible; however some careful consideration is needed before doing so.
Bumblebees are more active than honeybees, being able to visit more flowers per flight. This is why considerably fewer bees (~20 bees per 1,000 sq feet, per Koppert Biological Systems) are introduced for bumblebees than for honeybees for pollinating the same number of flowers. Therefore when the bumblebee population is high relative to the number of flowers, bumblebees tend to damage the flower (and also the receptacle that develops into fruit) by making too many visitations, and too vigorously harvesting pollen during each visit, resulting in abnormally shaped fruit. When growers must use bumblebees for strawberry, care should be taken to manage bee visitations to the flowers by limiting the bee’s access to the crop by temporarily closing flight holes (exit for bees) on the beehive for prolonged periods during the day. Providing supplemental pollen to the hive is probably necessary with restricting pollen gathering by the bees.
Use of bumblebees for greenhouse strawberry (photos shared by Koppert Biological Systems)
Per several available sources for information of honeybee management in greenhouse (e.g., Mitsubachi Kyogikai, Japan), there are several key issues.
1) Avoid using UV protected greenhouse glazing. Honeybee visible light ranges from 300 nm to 650 nm (for humans it is 380 to 780 nm) and complete exclusion of UV range (300-400 nm) substantially affects honeybee activities.
2) Using honeybees in winter is contrary to their natural life cycle. Understanding their life cycle and seasonal environmental effects on the bees, especially temperature and light, is crucial in bee management in greenhouse.
3) When you need to apply pesticides, potential impact on bees must be known. If necessary, moving bee hives to a temporary location needs to be considered.
Strawberry flowers are also pollinated by wind that vibrates the flowers to shed pollen from anthers onto pistils. An alternative way to pollinate strawberry flowers is using a tool to vibrate the flower at a high frequency. An electric pollinator is an effective tool for small scale greenhouse operation. A few seconds of vibration can be applied for individual flower trusses. This needs to be done as frequent as once a day. But visiting all trusses manually becomes labor intensive management and applicable in small R&D type of production (~100 m2 or 1000 sqft). Another option is to use a small leaf blower to provide wind over your plants to vibrate trusses for a few seconds. When you apply these methods, flowers (anthers) need to be dry so that pollen will disperse more evenly. Pollination needs to be done everyday or every other day, considering a typical life of viable flowers.