A honey bee (Apis mellifera) forages on blueberry flowers. While the bee forages on one flower in a cluster, the rear leg of the bee is contacting the stigma of a different flower in the cluster, illustrating a behavior that permits honey bees to successfully serve as pollinators of highbush blueberry crops. (Photo credit: George Hoffman, Ph.D.)

By John P. Roche, Ph.D.

Blueberry plants depend on insects for their pollination. The U.S. produces more than  700 million pounds of highbush blueberries per year, so blueberry pollination is important economically as well as ecologically. Because blueberry flowers are bell shaped, with their interior recessed in a tube of petals, effective release of pollen from the flowers depends on an insect behavior of rapidly vibrating wing muscles, called buzz pollination. Honey bees (Apis mellifera) do not perform buzz pollination, and thus are thought to be inefficient as blueberry pollinators. Despite this, all large blueberry farms of five or more acres rent honey bee hives, and the honey bees successfully aid in crop pollination.

To determine the mechanisms with which honey bees achieve successful blueberry pollination, George Hoffman, Ph.D., of Oregon State University and colleagues studied how pollen gets picked up by honey bees on blueberry flowers and how pollen is transferred back to blueberry flowers to achieve pollination. Their findings were published in a new report in November in Environmental Entomology.

The pollination of flowers by insects relies on a complex interplay between the anatomy of insects and the anatomy of flowers. Each leg of an insect is divided into a femur at the top, a tibia in the middle, and a structure called a tarsus (plural: tarsi) at the lower end. And a typical flower is divided into a cone of petals that make up a corolla, structures called anthers that hold pollen, and a central stigma that accepts pollen and transfers it down a tube to the ovary for fertilization.

In their study, Hoffman and colleagues collected honey bees from 11 Oregon blueberry farms and quantified the amount of pollen found on four main body parts: head, thorax and abdomen, upper legs, and tarsi. They also made 327 observations of honey bees foraging on blueberry flower clusters, noting how many times each bee probed the corolla of a flower, grabbed a stigma with the tip of a tarsus (a tarsal claw), brushed a stigma with a tarsus or leg, or probed deep enough into a flower with a tarsus to touch an anther of a flower.

When quantifying pollen loads, the investigators found the largest amount of pollen on the honey bee tarsi, with less on the head and legs, and the smallest amount on the thorax and abdomen. In their behavioral observations, they discovered that the pollen transfer was accomplished via the following mechanisms, ranked in order of most to least:

• head entering corolla
• grabbing stigma of adjacent flower with end of tarsus (tarsal claw)
• leg stretching across corolla opening and contacting stigma of flower
• leg entering the corolla opening and contacting the stigma.

Therefore, a lot of transfer of pollen happened when body parts holding pollen touched the stigmas of flowers during non-foraging behaviors, such as grooming and walking across clusters of flowers. Interestingly, the last three behaviors listed above often happened when a bee stabilized its body by grabbing the stigma with a leg, and they often resulted in touching the stigma in a flower adjacent to the flower from which nectar was being gathered.

A honey bee (Apis mellifera) forages on blueberry flowers. While the bee forages on one flower in a cluster, other parts of the bee come into contact with the stigma of a different flower in the cluster, illustrating a behavior that permits honey bees to successfully serve as pollinators of highbush blueberry crops. (Photo credit: George Hoffman, Ph.D.)

Regarding the key takeaway from this study, Hoffman says, “When investigating the role of pollinators of small flowers in densely packed inflorescences such as blueberry, pay attention not only to the flowers from which the insect is feeding but also to the adjoining flowers when their stigmas come in contact with the legs and tarsi of the pollinator.”

Pollen grains in blueberry plants often occur as four grains bound together in a pollen “tetrad.” Each of the four grains on a tetrad can potentially pollinate an ovule of a flower, heightening the effectiveness of pollen transfer by honey bees. Work by Margriet Dogterom and Mark Winston in 2000 observed that in a variety called Bluecrop, adequate pollination of a blueberry flower requires 125 pollen tetrads. In the present study, Hoffman found that the first segment of the tarsus of honeybees, called the basitarsus, held 200 pollen tetrads on average, which is more than sufficient to achieve pollination.

The results of Hoffman’s study have important practical applications to highbush blueberry cultivation. “There is already evidence from Project Integrated Crop Pollinations and other studies,” Hoffman says, “that, as the number of honey bee hives per acre on large commercial blueberry farms increases, so do the number of honey bee flower visits. There is a corresponding decline in lost yield due to inadequate pollination. Our findings suggest a mechanism for how this occurs even when little blueberry pollen is returned to the hives.”

Regarding future research steps his group is pursuing, Hoffman says, “Our study documented that 66 percent of the time a honey bee lands on a blueberry cluster, a leg or tarsus (usually covered with pollen) comes in contact with the stigma of one or more flowers in that cluster. We need to determine how much pollen is transferred to the stigmas of those flowers. We are part way through this investigation, and first year results are leading us to further delineate the behavior of the bees when visiting blueberry clusters.”

Hoffman and colleagues discovered that although the access honey bees have to blueberry flowers are constrained by the bell shape of the corollas, when widening the frame of reference to look at the full range of honey bee behaviors, it is clear how adequate pollination can be achieved. This is a powerful finding for our understanding of highbush blueberry pollination dynamics and also for understanding the process of designing effective scientific tests.

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and an Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 190 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com.