Research Shows Bee Colony Strength a Significant Factor in Yields

A multi-pronged research project has shown that variations in population size of honey bee colonies pollinating blueberry fields can significantly affect blueberry yields.

According to a research paper on the two-year project, by using stronger colonies, estimated yield increases of 25,000 kilograms per hectare, or 22,300 pounds per acre, could be achieved without increasing hive density or stocking rates.

OSU Pollinator Health Extension Specialist, Andony Melathopoulos at the Oregon Blueberry Conference in Salem February 14, discussed the importance of having strong hive colonies.

The research further showed that colony strength in working hives varied considerably, ranging from 10,300 to 30,700 adult worker bees per colony. And it showed that by using returning forager counts, researchers could accurately and rapidly assess adult honey bee colony populations.

The project, launched in 2019 by OSU Pollinator Health Extension Specialist Andony Melathopoulos, Kennedy Judith Grant, a researcher in his lab, and Lisa DeVetter, an assistant professor in small fruits for Washington State University, also confirmed the presence of European foulbrood disease in colonies pollinating blueberries, a concern of beekeepers.

The researchers came to their findings by analyzing results from two seasons at 13 commercial blueberry fields in Oregon. The fields were stocked with identical densities of ten colonies per hectare, or four colonies per acre.

Growers currently use stocking rates of between 6.2 and 12.3 colonies per hectare, according to the report.

The findings are significant in that they could provide both blueberry growers and beekeepers with higher economic returns in cases where growers would pay more for stronger colonies, according to the research paper.

If the average price per pound of blueberries is $2.20 per kilogram, the difference in gross returns between fields with populous versus weak colonies would be $49,000 per hectare, according to the paper. “Given that colonies for blueberry pollination rent, on average, for $543/ha, it is possible that blueberry growers who incentivize the delivery of stronger colonies, could realize more than 100 percent returns on investment for beekeepers delivering premium strength colonies, even when factoring in a modest rental fee increase for those colonies,” the paper stated.

The findings essentially validated the researchers’ stated hypothesis that growers could increase yields by using stronger hives and that it would benefit both beekeepers and growers to incentivize beekeepers to deliver stronger hives.

The current standard for minimum colony strength for blueberry pollination is defined as having six Langstroth-sized frames covered with adult bees. One concern, however, is that blueberry growers often lack the tools necessary to assess internal frame counts and, therefore, adult colony populations, according to the paper.

Researchers found that by using returning forager counts they were able to arrive at similar results to the more invasive means of determining colony strength. In the research, the researchers also further refined current standards of minimum colony strength under the returning forager method.

“We were able to refine the estimates of adult colony population previously developed by Sagili & Burgett (2011), who estimated that a colony meeting the minimum colony strength pollination standard of six frames of adult bees should have 100 returning foragers per minute,” the paper states. “These findings suggest the returning forager count for six frames of adult bees should be 2.6 times lower than previously reported, or 38 returning foragers per minute, and that air temperature does not influence this relationship.”

The researchers were unable to determine if higher stocking rates contributed to an increase in brood diseases or if stronger hives and lower stocking rates led to a decrease in the diseases.

“There remains a need for research that investigates the relationship between colony density and the severity of EFB (Europoean foulbrood) during blueberry pollination,” according to the paper.

The research did find, however, that colonies experience elevated numbers of brood diseases when pollinating northern highbush blueberries, in this case both the Duke and Liberty varieties, with 63 percent being free of symptoms before pollination and only 19 percent free of diseases by the end of pollination.