Mites alight! Sunflowers for Varroa resistance in honey bees?

Dr, Evan Palmer-Young, October 28, 2021

This post references our Biorxiv preprint on sunflowers, honey bees, and Varroa mites:

Mites alight! Sunflower crop area and pollen supplementation enhance honey bee resistance to Varroa destructor

Evan C Palmer-Young, Rosemary Malfi, Yujun Zhou, Bryanna Joyce, Hannah Whitehead, Jennifer I Van Wyk, Kathy Baylis, Kyle Grubbs, Dawn Lopez, Jay D Evans, Rebecca E Irwin, Lynn S Adler

doi: https://doi.org/10.1101/2021.08.04.454914

For a summary presentation, see our related Youtube video: Mites alight-- sunflower and honey bee resistance to mites

For the last 40 years, honey bees around the world have been plagued by the invasive mite Varroa destructor. Existing mite control strategies rely heavily on chemical treatments that can adversely affect bees; they can also be costly and labor-intensive to apply. What if honey bees had self-service access to medicinal plants that allow them to better resist mites on their own?

Over the last ten years, Professors Lynn Adler at the University of Massachusetts Amherst and Rebecca Irwin at North Carolina State University have been studying how different floral resources and the plant compounds they contain can affect resistance to gut parasites in bumble bees. In this project, they teamed up with Kathy Baylis at the University of Illinois, Jay Evans at the USDA Bee Research Laboratory, and the Bee Informed Partnership to examine the effects of a specific floral food source—cultivated sunflower, in the form of cropland and pollen supplements—on honey bee resistance to Varroa mites. 

Why sunflowers? Recent experiments on bumble bees—by former Adler lab undergraduate and newly minted Irwin lab Ph.D. Jonathan Giacomini—had shown that consumption of sunflower dramatically enhanced bumble bee resistance to the trypanosomatid gut parasite Crithidia. In addition, pollen of plants in the Asteraceae family—of which sunflowers are a member—conferred resistance to brood parasitism in mason bees, suggesting that sunflower could enhance resistance to the brood parasite Varroa in honey bees as well.  

The first hint of a sunflower-Varroa connection came from combining Varroa mite counts from colonies in the APHIS National Honey Bee Disease Survey with a landscape analysis that quantified the amount of sunflower cropland within 2 miles (3.2 km) of apiaries. This showed that each doubling of sunflower cropland was associated with a 28% reduction in Varroa loads. However, just because sunflower is associated with lower mite counts doesn’t prove that sunflower is what causes reductions in Varroa numbers.

To evaluate causal relationships between sunflower and resistance to mites, field colonies at the University of Maryland were fed supplements containing either sunflower pollen, wildflower pollen, or a protein-rich formulation commonly used by commercial beekeepers. After five weeks, sunflower-fed colonies had 2.8-fold fewer mites than did colonies fed the protein supplement. This suggests that providing honey bees with access to sunflower pollen—whether in the form of landscape resources or pollen supplements—could reduce Varroa infestation. 

Do current land use trends ensure the safe access of honey bees to sunflowers for the foreseeable future? Quite the opposite, unfortunately. Recent changes in agricultural markets and land use policy have had large effects on the Northern Great Plains, where most of the nation’s sunflowers are grown. Sunflower crop area has decreased by 2% per year since 1980 and 3.5% per year since 1995, according to data from the National Agricultural Statistics Service. Although that doesn’t look promising for realizing sunflower-mediated benefits against Varroa—or for wild bees that rely on sunflower as the main food source—it at least suggests that agricultural land use is responsive to policy changes, and that this trend could be reversed if appropriate incentives are provided.    

Before recommending new policies, however, more research is needed to confirm this result and explain the underlying mechanisms. Sunflower pollen is notoriously low in protein, and is considered a poor food source for honey and bumble bees. If reductions in mite counts simply reflect reduced brood-rearing or even brood abortion—which could preclude the ability of mites to reproduce—then sunflowers might not actually help colonies in the long run. On the other hand, if sunflowers contain compounds that stymie mites without harming bees themselves (think cardiac glycoside-sequestering monarch butterflies and other chemically defended prey), then feeding bees with sunflower pollen could provide transgenerational immunity to Varroa. In addition, providing bees with such compounds in purified form could confer sunflower pollen-like benefits to colonies in seasons and regions where sunflowers are unavailable. 

Still, these preliminary results suggest that reversing declines in sunflower cropland could provide valuable resources not just to bees that specialize on Asteraceae, but to Varroa-plagued managed honey bees as well. 

Photo credit: Photo by Jordan Cormack on Unsplash