The common eastern bumble bee (Bombus impatiens) was a top pollinator of pumpkin flowers in a recent Journal of Economic Entomology study. Photo by Carley McGrady.

By Leslie Mertz

Leslie Mertz, Ph.D.

Commercial pumpkin growers routinely rent honey bees so they have enough insects to pollinate their crops, but a new study published in the Journal of Economic Entomology suggests that wild bees can do the job for free. The three-year study found that wild bumble bees and squash bees could easily handle the pollination required to produce a full yield of pumpkins in all of the tested commercial fields, according to Carley McGrady, the lead author of the study.

The pumpkin study was part of a broader initiative, called the Integrated Crop Pollination Project, or Project ICP (http://icpbees.org/), which was headquartered at Michigan State University and funded by the U.S. Department of Agriculture’s Specialty Crop Research Initiative. “The goal of Project ICP was to assess the pollinators in specialty cropping systems throughout the United States, so there were researchers collaborating across institutions looking at the different systems, such as watermelon, almonds, cherries, blueberries, and pumpkins, and I was part of the team looking at the pumpkins in Pennsylvania,” McGrady said. She did the research while she was a master’s student at Pennsylvania State University and is now a doctoral student at North Carolina State University.

Carley McGrady and her Penn State master’s advisor Shelby Fleischer are shown in a large commercial pumpkin field, where they showed that wild bees could easily handle all of the pollination necessary, and without the need for the additional honey-bee pollination that is typically provided via rented hives. Photo by Nick Krause.

“We Were Blown Away”

For the study, the Penn State research group spent three summers counting the number and type of bees visiting flowers in 30 pumpkin fields ranging from 5–140 acres in size. They verified previous research showing that the three top pollinators were honey bees (Apis mellifera); bumble bees, mainly common eastern bumble bees (Bombus impatiens); and squash bees in the genus Eucera. In fact, she said, those three did more than 95 percent of the pumpkin pollination.

Taking it a step further, McGrady pored through the scientific literature to determine how much pollen a female pumpkin flower needs to initiate fruit production and the amount of pollen carried per visit by each type of bee. Honey bees and squash bees need to visit a female flower 8–15 times to accomplish pollination, but bumble bees can do it with half the visits. That’s because a bumble bee’s extra-fuzzy body carries at least twice as much pollen as that of a honey or squash bee, and its rather clumsy movement in the flower leaves behind more pollen.

“When we multiplied the number of visits times how much pollen they were depositing, we were blown away to find that bumble bees and squash bees combined were doing more than 10 times the pollination that was necessary,” McGrady reported.

The bar on the left shows the diversity of bee species the researchers identified on the flowers in 30 pumpkin fields. The bar on the right shows the proportion of visits, with three types of bees doing more than 95 percent of the pollination: honey bees (Apis mellifera); bumble bees, mainly common eastern bumble bees (Bombus impatiens); and squash bees in the genus Eucera. Bee illustrations created by: Emily May/The Xerces Society for Invertebrate Conservation.

The bumblebees were less successful in the largest fields in the study, but still got the job done. McGrady speculated that the lower rate in large fields is a function of population size. She remarked, “It suggests that there’s a set number of bumble bees in the landscape in these areas, and as your fields get really big, it kind of dilutes how many bumble bees [are available in] a larger field. But even at the lowest visitation rates in the largest fields, they were still doing 1.5 times more pollination than was necessary.” (Bumble bees only fell short in one field, which was facing horticultural issues that made the plants unenticing to bumble bees.)

Supporting a Resilient Pollinator Community

Even before they finished collecting the data and performed the number-crunching, McGrady said it was pretty clear that wild bees were doing a stellar job of pollination, and honey bees rentals weren’t as vital as growers had thought. In fact, she said, one of the commercial growers went ahead and cut his honey bee rentals in half, going from the extension-recommended one hive per acre to one hive per two acres, “and it didn’t change anything for his pumpkin yield—he got all the pumpkins he wanted.”

McGrady was quick to note that she is not advising growers to drop honey bees completely in favor of wild bees, but rather consider how best to bring a community of pollinators to their fields. That way, one type of bee can have a bad year due to unfavorable weather conditions or some other detriment, but other bees are there to pick up the slack, she remarked: “What’s important is supporting a resilient pollinator community.”

Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.