Two Prolonged Bee Visits Suffice to Maximize Drupelet Set for Red Raspberry by Corey J. Andrikopoulos and James H. Cane, Utah State University, Logan, HORTSCIENCE 53(10):1404–1406. 2018. https://doi.org/10.21273/HORTSCI13124-18
Everyone knows that insect pollination is critical for fruit set in many commercial berry species. The red raspberry is no exception. It produces an aggregate fruit composed of individual drupelet fruits. If the weather is bad, the insects are not flying, or something else interrupts pollination, you can easily get misshapen fruit and lower yields. Researchers at Utah State found that it only takes two prolonged visits of 1 minute or more to get great fruit set. You don’t need hundreds of bees visiting the flowers. Here are a few tidbits about red raspberries from this article:
- Red raspberries are mostly self fertile but because the pollen bearing anthers are not located directly adjacent to the female stigmas, self pollination can be spotty.
- A single flower bears 60 – 90 stamens with ample pollen, but bees and other insects are needed to move the pollen to the central region of female stigmas. Each individual stigma needs to be pollinated to get a good fruit.
- Previously scientists estimated that up to 68 visits per flower were necessary to get adequate fruit set, but these authors showed that 2 visits were great.
- For two visits to be adequate, the bee must really work the flower for a prolonged period of time (up to a minute or more). Bees often spend long times on a newly opened flower first thing in the morning because that’s when nectar flow is at its peak after a night time of storing up nectar in the flower’s nectar pool.
- Individual flowers remain receptive for about 2 days, and every morning the nectar pool is recharged. Nectar release is temperature dependent.
- By individually covering flowers as they bloomed and comparing them to uncovered controls, the researchers showed that bee visits increased drupelet formation by 2 to 4 times. If a flower was visited first thing in the day for a prolonged period, two such visits would be sufficient to have great fruit formation.
- There is also a condition of over visitation where bees actually damage the female stigmas if there are too many of them working the flowers.
Adding honey bee hives to a raspberry garden is a good idea for most commercial businesses. However, Alaska temperatures, especially in the a.m. can be cold – too cold for honey bees to be very active. Since nectar flow is greatest first thing in the morning, wild bumble bees become even more critical for great fruit set in northern gardens.