“We’re Hungry and We’re Sick!”: Synergistic Effects of Diet and Stress on Honey Bees

Honey bees face a myriad of interacting stress factors including pesticide exposure and poor nutrition in intensive agricultural landscapes. Andrew Garavito spent his masters looking at how these factors interact in real-world landscapes to affect honey bee health. Comparing factors such as pollen diet, pesticide contamination of pollen, and drought stressing of pollen, he gained some interesting insights into what bees face on any given day in the field. 

Honey bees (Apis mellifera) are vital pollinators to many fruit, nut, and specialty crops1. Today they face a myriad of interacting threats to their health and success. From changing agricultural landscapes affecting bee nutrition to the risk of pesticide exposure2, it’s no wonder these little insects have seen increasing mortality rates over the past ten years3. Scientists all over the world, including Andrew Garavito (vanEngelsdorp lab, Bee Informed Partnership (BIP), have taken up the cause of bettering bee health. Much is known about how nutrition and pesticides affect bees separately, but Andrew is interested in their effects when interacting with each other and with other stressors such as drought. His research focuses on real-world situations that bees face, working directly with beekeepers to find tools to help protect them.

Andrew set out to test bees’ response to various diets and levels of pesticide exposure. Knowing that higher parasite loads are an indicator of poor health2, he used the honey bee gut endo-parasite Nosema ceranae (Figure 1) as a proxy for bee health. Using pollen trapped from four crops (black cap raspberry, meadow foam, crimson clover, almond; Figure 2), Andrew set out to test differences in pesticide exposure in different crops, as well as stress caused by pesticide-laden pollen. He also wondered whether a more diverse diet could better combat that stress. Feeding bees various combinations of different types of pollen with differing levels of pesticides, Andrew found some unexpected things.

Figure 1. Nosema ceranae spores under a microscope taken from a sample of bees.

Figure 2. A pollen sample taken from a pollen trap on a colony of honey bees.

One would expect pesticides to increase stress on the bee and thus increase Nosema infection2, but no significant difference was found between bees fed pollen with or without pesticides. The same was true for bees fed a more diverse diet vs. a diet of only one type of pollen. Bees with a diverse diet of varying amino acids and nutrients would also be expected to better combat Nosema infection4, but diet variety did not appear to have an effect on Nosema infection. Andrew attributes these results to his hypothesis that Nosema replicates better in a strong, healthy host so that sick bees don’t support as high of Nosema levels.

In the summer of 2012, BIP heard from a North Dakota beekeeper who mentioned that bees used to pollinate sunflower crops (Figure 3) had a particularly bad year. North Dakota was in the middle of a severe drought that year, and the beekeeper thought this may have had an effect on the sunflowers and thus maybe the bees. Andrew sprang into action. He thought there may be some connection to pollen nutrition and pesticide exposure.

Figure 3. Rented honey bee colonies in a North Dakota sunflower field. http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=7977

To put his hypothesis to the test, Andrew planted two types of sunflower seeds: those untreated with pesticides and those treated with a seed coating of one neonicotinoid pesticide and three fungicides. Pesticides used in seed treatments are systemic and thus affect all parts of the plant, including the pollen the bees forage on. The sunflowers were grown under three watering regimens: well-watered, moderately drought stressed, and severely drought stressed. Resulting pollen was fed to bees who were either infected with Nosema or were not (Figure 4). In this way, Andrew could differentiate whether honey bees were more stressed by pesticide exposed pollen, and by drought stressed pollen.

Figure 4. Live honey bees in a feeder cage being fed a specific pollen diet (bottom) and sugar water (top).

He found that sunflowers grown under drought conditions were smaller in size, affirming that they were in fact stressed by the lack of water! Furthermore, the pesticide load in pollen collected from treated plants was higher in drought stressed plants than in well-watered plants. This showed that drought does affect the way the plants take up systemic pesticides. Does this have an effect on the bees foraging on them? The answer is probably. Bees infected with Nosema died faster than uninfected bees who were fed the same diets of severely drought stressed pollen. This was true regardless of whether the pollen came from flowers grown using treated or untreated seeds! With regards to the moderately drought stressed pollen, infected bees died faster than their uninfected counterparts only when the moderately drought stressed pollen came from seed-treated plants. This finding points to both drought and pesticide exposure having

an effect. The consequences of pesticide exposure increase in drought and are eventually overridden by the effect of drought on the pollen.

Andrew’s work shows us that while we know much about honey bees and factors they face, there is still much to learn about how these factors interact. Honey bees are very complex social organisms and they continue to surprise us with how they handle their multitude of stressors.
 
References:
1. Morse, R. A., & Calderone, N. W. (2000). The value of honey bees as pollinators of US crops in 2000. Bee Culture, 128(3), 1-15.
 
2. Pettis, J. S., Lichtenberg, E. M., Andree, M., Stitzinger, J., & Rose, R. (2013). Crop pollination exposes honey bees to pesticides which alters their susceptibility to the gut pathogen Nosema ceranae. PloS one, 8(7), e70182.
 
3. vanEngelsdorp, D., Underwood, R., Caron, D., & Hayes Jr, J. (2007). Estimate of managed colony losses in the winter of 2006-2007: A report commissioned by the Apiary Inspectors of America. American Bee Journal, 147(7), 599-603.
 
4. Di Pasquale, G., Salignon, M., Le Conte, Y., Belzunces, L. P., Decourtye, A., Kretzschmar, A., ... & Alaux, C. (2013). Influence of pollen nutrition on honey bee health: do pollen quality and diversity matter? PloS one, 8(8), e72016.
 
About the Author:
Kelly Kulhanek is a PhD student in the vanEngelsdorp bee lab. She is studying the effect of beekeeper management practices on honey bee colony health and survival.