Plight of the Bumblebee

What comes to mind when you hear the word bumblebee? Fear? Reflection on a previous unfortunate encounter with a hidden bumblebee nest? Or perhaps, you just imagine all the species of these fuzzy, majestic flying beasts you have seen methodically inspecting and foraging on the flowers of your garden. Whatever your memory invokes, it is likely striking and not easily forgotten. Bumblebees are a diverse genus (Bombus), encompassing more than 250 species, of which the majority live in the northern hemisphere in small, mostly underground colonies. Like their well-known cousin, the honeybee, bumblebee colonies consist of one reproductive female (the queen), sterile females (the workers), and reproductive males (drones)

B. impatiens queens. Photos by L. L. Richardson

Being an insect, bumblebees engage in a unique state, a physiological throttler of sorts, called diapause. Diapause originates from the Greek word diapausis, which means “pause”, and was first officially described in the world of entomology by William Wheeler in 18931.  Diapause is a pause button for the progression of insect life. Why does it exist? Well, it allows organisms to remain in a dormant state during unfavorable conditions, like drought, temperature variability, or lack of food. Diapause can exist at any stage of insect development, from eggs to pupae to adults.  As you can imagine, sometimes it is just easier to survive in an area without food if you live off your body’s nutrient stores and decrease your metabolism so much that you don’t really have to eat.

Bumblebees put all their eggs in one basket. Literally, newly fertilized bumblebee queens are the only members of the colony to survive cold winters. The future of the colony’s survival is entirely dependent on her making it through the long winter months. Therefore, like in Aesop’s fable, The grasshopper & the ants2, preparation is everything. To survive, queens must accumulate sufficient nutrient stores in their bodies prior to entering diapause. Lipids are generally sourced from pollen and carbohydrates from nectar. Both lipids and carbohydrate (stored as glycogen) are stored in a multi-purpose organ in bees called the fat body. Unlike adipose tissue in humans which serves primarily as a fat storage organ, the fat body of insects is also important in immune function and detoxification processes (like the spleen and liver in humans). This fat body also produces important hormones that control social structure in bees (e.g vitellogenin3), and is also the target of feeding of some large bee mites, like the varroa mite on honey bees4.

So, the fat body is really important in bees, and is especially important in overwintering bumblebee queens that carry the weight of their colony in their ovaries. But, how much do we know about the all-important preparation that a queen must go through to survive the winter? What if she doesn’t have enough nutrient stores in her body? Will she make it through? These are questions addressed in a recent paper in Integrative Organismal Biology by Kristal Watrous et al3 from the Woodard lab at The University of California, Riverside.

From Watrous et al, 2021: Weight change through time as a function of diet treatment. Nectar starvation treatment groups (NS3d and NS6d) did not differ significantly from controls by 12 days.

Watrous and colleagues studied the impacts of pollen or artificial nectar deprivation on bumblebee queen survival, weight, and nutrient composition. They found that while queens did lose weight during their deprivation periods, they were able to regain their weight and were able to restore nutrient levels to that comparable to control queens. This compensatory recovery has been reported in vertebrates and invertebrates alike, and points to conserved abilities of organisms to withstand periods of nutrient hardships. 

B. impatiens queen in the laboratory (left, photo by S. H. Woodard), and bumble bee research in the Woodard lab (right, photo by L. L. Richardson)

These findings are important because they provide us with the knowledge of what a bumblebee queen can withstand and may allow us to improve our understanding of how nutrient-poor habitats could impact bumblebee queen health.  Because if bumblebee queens don’t survive, there will be no bumblebee colonies in the spring! So now when I see bumblebees, and especially if I see a bumblebee queen, it invokes in my mind the saying, All hail the (bumblebee) queen!

Carol Fassbinder-Orth is a professor at Creighton University.

1Wheeler, M. W., 1893. A contribution to insect embryology. Journal of Morphology 8:1–160.

2Pinkney, J., & Aesop. 2015. The grasshopper & the ants.

3Corona, M., R.A. Velarde, S. Remolina, A. Moran-Lauter, Y.Wang, K.A. Hughes, and G.E. Robinson. 2007. Vitellogenin, juvenile hormone, insulin signaling, and queen honey bee longevity. Proceedings of the National Academy of Sciences 104: 7128-7133.

4Ramsey, S. D., R. Ochoa, G. Bauchan, C. Gulbronson, J.D. Mowery, A. Cohen, D. Lim, J. Joklik, J. Cicero, J.D. Ellis, D. Hawthorne, and D. vanEngelsdorp. 2019. Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. Proceedings of the National Academy of Sciences, 116: 1792– 1801.

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