Published literature has long called for growers to place four hives to the acre when pollinating blueberries. While in practice that may be good advice, it doesn’t take into account several features that growers should take into account when considering stocking density. And with the advent of new cultivars and higher planting densities, the formula’s value comes into question.

“We have new cultivars with higher bloom density, and we have tighter plant spacing relative to when (stocking density) recommendations were produced,” said Lisa Wasko DeVetter, Washington State University associate professor in horticulture. “It’s possible that we might just need to increase our honeybee hive density to provide the pollination services for that number of extra blossoms that we have relative to say ten, twenty years ago.”

In a presentation at the Oregon Blueberry Conference February 4 in Salem, DeVetter noted that published literature also generally fails to account for colony strength (also interchanged with colony size) in its recommendations of stocking density.

“Colony size is really important to consider because it relates to the worker population in that colony or the number of foragers that can engage in pollination services,” she said. “So, say for example you have a field, and you stocked it with four hives to the acre. Well, do those hives have an average of about 15,000 workers, 30,000 workers or 60,000 workers? That matters because at the higher population density, that means you’re going to have more honeybees that can engage in foraging services.

“And again, the traditional stocking density recommendations of four hives to the acre, doesn’t really account for the population size or strength of the colonies,” she said.

In her presentation, DeVetter highlighted a research project from Laura Goldstein, a former master’s student at Michigan State University, who probed what is an appropriate stocking density when taking into account colony size, colony strength and other factors. The project involved research in Washington, Oregon, Michigan and Florida.

The key variable that related to honeybee activity and overall pollination turned out to be stocking strength, DeVetter said, which is a metric that takes into account both the density and strength of colonies. Researchers measured stocking strength by analyzing a hive’s returning foragers, bee visitations to flowers, pollination and yield.

“What we saw is that increasing bee visitation improves pollination to a point,” DeVetter said. “It certainly plateaus. It is not a linear effect that the more honeybee visits to a flower, the greater the berry size. There is a limit there.

“And we found that honeybee visitation is best predicted by stocking strength, not stocking densities,” she said. “So, I think this really underscores how important the strength of those colonies are when it comes to pollination services, and growers should monitor the strength of those colonies and have a conversation with their beekeepers if this is a point of concern for them.”

Growers can learn more about how to non-invasively monitor for strength of their colonies by using return forager counts by visiting the project website and watching the training video: https://blueberrypollination.org/index/php/videos/