Stellenbosch University: Honey bees can be handy pollinators of blueberries

Honey bees seem to be quite handy pollinators of blueberries as they help to increase fruit yield and quality, a new study at Stellenbosch University (SU) found.

Titled “Assessing the effectiveness of honey bee pollinators for cultivated blueberries in South Africa,” the study was conducted by Keanu Martin, Bruce Anderson, Corneile Minnaar and Marinus de Jager from SU’s Department of Botany and Zoology. It was published recently in the online version of the South African Journal of Botany.

“We found that honey bees are usually extremely beneficial and can act as reliable blueberry pollinators, even in areas without access to native blueberry pollinators such bumble bees. Honey bees increase fruit production and quality, and decrease ripening times compared to when pollinators were absent,” say the researchers.

They conducted the study to determine the effect of honey bee pollination on the fruit yield of five commercially grown blueberry varieties in South Africa, namely Emerald, Eureka, Snowchaser, Suziblue and Twilight. For each variety, they calculated 1) the benefit of the bees — a comparison of fruit yields after exposure to honey bees and fruit yields after their exclusion — and 2) the pollination deficit — the difference in yield between hand pollination (manual transfer of pollen from male blueberry anthers to the female stigma for maximum yield potential) and yields after exposure to the bees.

On a plot at a Western Cape blueberry farm, the researchers stocked honey bee hives at 15 hives per hectare, which falls within the range of commercial blueberry farm hive densities. Here they focused on fruit produced when flowers were bagged and had no access to honey bee pollination, fruit produced when flowers were open to honey bee pollination, and fruit produced through hand pollination. The researchers point out that for the approximately 280 hours they spent at the plot, honey bees were the only pollinator seen visiting blueberry flowers.

Following these pollination treatments, the researchers examined the development of the blueberries from pollination to harvesting to determine whether they were ready to be picked. Fruit were considered ready for picking when the entire berry had transformed to a uniform dark blue. They then harvested the blueberries and determined their weight, diameter and the period of development.

They also calculated the adjusted fruit mass by combining the fruit mass and percentage fruit per treatment. According to the researchers, the adjusted fruit mass is a good proxy for realised yield because blueberries are typically not thinned by farmers and so adjusted fruit mass is likely to correlate with measurements of traditional yield (e.g., total mass per 100 flowers).

Improved yields

They say their study showed that honey bees increased the set, size and mass of blueberry fruit across multiple varieties, when compared to fruits produced in the absence of pollinators. Exposure to honey bees also appeared to reduce fruit ripening time across varieties.

“Pollination by honey bees can result in very impressive yield increases for Emerald, Snowchaser, Twilight, and Ventura varieties, but for Eureka and Suziblue, honey bee exposure appeared to have relatively small effects on fruit production.”

According to the researchers, this may occur if a variety can reproduce with itself and doesn’t need any kind of stimulation to move pollen from the anthers to the stigma, or if honey bees are depositing high loads of pollen from the same variety.

They point out that hand pollination results in the best fruit quality as it enhances pollen deposition and crosses can be made with highly compatible varieties.

“The presence of the honey bees significantly increased the mass and diameter of the fruit by 30% and 13% respectively. Honey bees also significantly increased adjusted fruit mass by 62% compared to fruit produced when blueberry flowers were bagged. Hand-pollination, on the other hand, increased the adjusted fruit mass by 26%, compared to fruit produced from honey bee pollination.

“Honey bee pollination decreased the ripening time of blueberry fruit by a week compared to fruit produced in the absence of pollinators (bagged flowers). Additionally, hand-pollination decreased the ripening time by a further two weeks compared to fruit produced through honey bee pollination.”

The researchers point out that although honey bee pollination consistently resulted in improved yields, the magnitude of this improvement (i.e., the benefit of bees) was dependent on the different varieties. Similarly, the pollination deficit also varied considerably across varieties and while some varieties appeared to perform close to maximum potential, others yielded well below their maximum potential when pollinated by honey bees.

They add that while honey bees are useful pollinators of blueberries in areas where native blueberry pollinators are absent, it is important to select blueberry varieties that perform well with honey bees as their primary pollinator.

“More research is needed to identify which blueberry varieties do best under commercial honey bee pollination conditions, what makes varieties successful or unsuccessful, which varieties can be effectively pollinated by honey bees, and which varieties need other effective means to increase fruit quality.

“It is important to quantify the benefits of honey bees for different blueberry varieties so that objective decisions can be made about the perceived needs of importing new pollinators. This may also enable us to farm more intelligently with varieties that do better under honey bee pollination.”