Researchers from Cornell University in Ithaca, New York recently found that the chemical, nutritional and physical properties for defense in pumpkin and squash pollen are not beneficial to bumblebees.

These plants are in the cucurbit family of plants. A recent study, published in the Nature Scientific Reports journal, suggests that they may have evolved an evolutionary way to repel bumblebees from obtaining and ingesting their pollen.

The study reports on the negative impact of pumpkin and squash pollen defenses on generalist bees and their fitness and foraging. The study’s senior author is entomology professor Bryan Danforth. According to him, bumblebees experience many physical problems when they consume cucurbit pollen, some quite severe. Their digestive tracts are damaged and distorted, and bumblebee colonies that feed on this pollen fail to produce offspring. Even the bumblebee lifespan decreases.

Bumble bees have complex interactions with their environment. They visit pumpkin and squash flowers in pursuit of nectar, but do not collect the pollen. When they visit these flowers, pollen sticks to their legs, which makes them good, if unwitting, pollinators of pumpkin and squash plants since stray pollen on the insects’ bodies potentially rubs off on other flowers of the same sort, resulting in pollination.

Here's a sweet 4:38-minute video by Organic Slant of cute bumble bees in squash plant flowers... note the pollen that clings to them even though they are not looking for pollen. Gotta love the uniquely bumblebee buzz:



Kristen Brochu, former doctoral student in Danforth’s laboratory, is currently a Pennsylvania State University post-doctoral researcher. She is the first author of this study.  According to her, bumblebees hate the pollen, and use their legs to wipe it off on leaves and remove it from their bodies.

She and her colleagues set the study up by creating cucurbit pollen diets with various defense mechanisms, to determine what bees eat and what they avoid, as well as which of the pollen’s characteristics serve as the bumblebee deterrent.

To this end, the authors set up bumblebee microcolonies with 5 bees each, and these were given separate treatments. Bumblebees that received wildflower pollen thrived, as expected. For bumblebees that received a natural diet of cucurbit pollen, the chemicals, pollen’s defenses and low nutritional content of the pollen cumulatively caused the bees to kill their offspring by ejecting them from the brood cells. According to Brochu, bumblebees do this when they are stressed. They probably realized that they could not take care of their larvae in this case.

In another treatment, the authors extracted cucurbit pollen chemicals and crushed the pollen, removing its exterior, and added it to the control diet of the very nutritious wildflower pollen. They observed failure of the eggs and larvae to mature.

During the full 50-day duration of the study, the bumblebee colonies that consumed diets of both natural and crushed cucurbit pollen failed to produce adult offspring. Those that received crushed pollen had a higher adult mortality rate, most likely caused by the release of toxic chemicals to the bumblebees.

As Brochu remarked, we tend to think all pollen is good for all bees, but clearly that is not true.  

IMAGE ON LEFT: 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 are a series of little yellow bumps (like popcorn), which are the early brood cells. They contain eggs or very small larvae. Image Credit: Kristen Brochu, Cornell University

Squashes and pumpkins must produce sweet and delicious nectar for bumblebees to chance being covered with their toxic defense pollen. There are so many flowers to forage, generalist bumblebees should leave the pumpkins and squashes to the specialist squash bees, since they solely subsist on cucurbits and don’t have any toxicity problems with the pollen.

For those who enjoy reading scientific papers, here is the original source.