French prune flowers have pistils that are elevated above the anthers that bear the pollen. Although self-fertile, they require honeybee pollinators to move the pollen around on the flowers for effective pollination and subsequent fertilization and fruit set. Prune flowers are not wind pollinated. Dr. Robin Thorpe, emeritus entomologist at UC Davis reported on prune pollination studies in the 1970s. In several experiments, exclusion of pollinators by caging French prune trees caused a lower percentage of fruit set, less than 1.3 percent, compared with 3.6 percent to 21.8 percent for open pollination (uncaged trees), and 15.5 percent to 19.4 percent fruit set for trees caged with honey bees. These experiments demonstrated how essential honey bee pollination was for setting a prune crop.
• Honey bee populations
Supplemental pollination with honey bee colonies may be even more important today since the wild bee population has been reduced by Varroa mites. Eric Mussen, Extension apiculturist, UC Davis, indicated that the varroa mite, Varroa destructor, was pretty well distributed throughout the country by 1992 and by 1995-1996 there were very few feral colonies across the nation. Varroa mites continue to be challenges in commercial beekeeping operations and they have significantly increased costs of maintaining colonies. Wild honey bee populations have increased somewhat since the mid-1990s but they may not be able to provide the pollination that prunes require to set a good crop.
• Are honey bees present?
In some areas there just aren’t many bees left. If you relied only on “natural” pollination to set a crop in your prunes are you disappointed? In some neighborhoods, competing blooms (mustard) or lack of bees may reduce prune pollination and result in poor fruit set. Fewer available bees may not be able to visit enough flowers to set a good crop during a short bloom. On the other hand, prunes surrounded by almonds may have lots of bees in the neighborhood and will likely do better in both good and bad pollination years. If you don’t have honey bees in the area you may want to plan on having colonies brought into your prunes to be more certain of having a crop.
• Foraging behavior
Bees can forage at least 3 miles from their colony but most tend to stay within a few hundred yards of the colony if there are adequate food rewards nearby. Prune blossoms are rewarding and will receive adequate visitation from nearby colonies depending on competition from other plants. Flight activity and foraging are proportionate to colony strength. At low temperatures bees from strong colonies are more likely to fly than are bees from weak colonies. Honey bees fly when temperatures are 55 F and higher. They do not fly in rain or in wind stronger than 15 mph. Cloudiness reduces flight activity, especially when temperatures are near the 55 F threshold for flight. Honey bees often visit plants other than prunes if pollen and nectar rewards are sufficient. Thus, the density of bees and blooms within a mile or two of an orchard can greatly influence the number of bees available for prune pollination.
• What’s a good colony?
For pollination purposes a good colony is one that has an active brood nest with uncapped worker brood at the start of prune bloom. Bees feed pollen to developing larvae so open brood cells indicate the hive has a demand for pollen. When pollen is in demand in the hive, more pollen foragers are sent into the field to collect pollen thus pollinating and setting your prune crop.
• How much colony strength is needed?
Use strong colonies with eight or more frames of bees. This is not usually a problem for prune growers because colonies have just come from almonds where their populations have expanded rapidly. Always protect colony strength by being careful with spray applications around bees.
• How many colonies per acre?
The usual recommendation for prunes is one colony per acre depending somewhat on local conditions. A large number of variables affect local needs including colony strength, the number of colonies in the neighborhood, plant competition within a 3-mile radius, and weather conditions during bloom.