Bumblebees hate pumpkin pollen, which may help pumpkins

In the study, Brochu and colleagues created diets that represented different defenses to test which cucurbit pollen characteristics deterred bumblebees. One diet of wildflower pollen collected by honeybees served as a control. A second consisted of unadulterated cucurbit pollen, which is nutritionally poor food for bumblebees, has large and spiny grains, and contains natural chemicals. In a third treatment, the team extracted the chemicals from the cucurbit pollen and added them to the control diet of nutritionally rich wildflower pollen.

Microcolonies of five bees were each fed a separate treatment. The bees fed the wildflower pollen thrived, as expected. Under a natural cucurbit diet, the cumulative effect of the pollen’s physical defenses, poor nutritional content and chemicals led to bees ejecting their offspring from their brood cells and killing them. Bumblebees do this when stressed, possibly because they can’t take care of the larvae, Brochu said.

With the crushed pollen diet, where the pollen’s physical defenses were removed, eggs and larvae failed to mature. “Over the course of the 50 days of the experiment, in both the crushed and natural cucurbit treatment, no offspring made it to adulthood,” Brochu said.

In the crushed treatment, the adults also died at a higher rate, possibly due to a release of additional toxic chemicals. And with the chemical wildflower treatment, larvae made it to adulthood most of the time and the bees ate more, possibly to compensate for something in the chemicals, Brochu said. Their abdomens became hard and dark, a process called melanization, which indicated trauma to the guts.

“We tend to think that all pollen resources are great for all bees, but I don’t think that’s true,” Brochu said.

For the sake of bumblebees, she said, pumpkin and squash growers may think twice about bringing commercial bumblebees into their fields and may provide wildflower strips as alternative food sources.

Study co-authors include Brian Nault, professor of entomology at Cornell AgriTech, and Jessica Peterson, a former postdoctoral researcher in Nault’s lab who is an invertebrate ecologist at the Minnesota Department of Natural Resources. Nault and Peterson conducted initial research identifying how generalist bumblebees avoided gathering cucurbit pollen.

The study was funded by the Canadian Natural Sciences and Engineering Research Council; a U.S. Department of Agriculture National Institute of Food and Agriculture Hatch Grant; and the Griswold Fellowship, which supports graduate-student research in Cornell’s Department of Entomology.

Header image: A top view of a micro-colony. The blue cup contains a pollen diet. In the center, a bee has her head in a honey pot and directly below her in the image are a series of little yellow bumps (resembling popcorn), which are the early brood cells. They contain eggs or very small larvae. Photo by Kristen Brochu/Cornell University

This article also appeared in the Cornell Chronicle.