A study by researchers at the University of California, Riverside, examined the effects of reduced water availability on both bumble bees and honey bees (Apis mellifera), shown here. The study found that reduced water availability affects floral resources that plants produce and that these changes in floral resources can negatively impact bees consuming nectar and pollen from affected plants. (Photo by David Rankin)

By John P. Roche, Ph.D. 

By serving as pollinators to natural and agricultural ecosystems, bees are vital to society. The total economic value of pollination to crops worldwide is estimated at more than $147 billion. Among the potential threats to bees as pollinators is climate change, which in many parts of the world may cause ongoing drought conditions. Does reduced water availability have effects on bees? A new study by researchers at the University of California, Riverside, tested the effect of reduced water availability on a plant used by bees and examined whether changes in plant nutrition content affect honey bees and bumble bees. The study, authored by Erin Wilson-Rankin, Ph.D., associate professor in the UCR Department of Entomology, former student Sarah Barney (now at the University of Michigan), and research assistant Giselle Lozano, was published in October in the open-access Journal of Insect Science.

A study by researchers at the University of California, Riverside, examined the effects of reduced water availability on bees. Honey bees and bumble bees were offered diets from tomcat clover, Trifolium willdenovii, the test plant species used in this study, grown under optimal or reduced water conditions. (Photo by Sarah Barney.)

Restricted water conditions are predicted to increase in many regions of the globe as a result of climate change, as increased air temperatures speed evaporation of water from soil and plants. In many arid and semi-arid regions, water use must be strictly regulated as populations cope with a limited water supply. Therefore, understanding any effects of reduced water availability on the production of nectar and pollen by plants, and any subsequent effects on bees, is crucial because it can help inform management that will optimize pollination under water limitation.

Rankin and her colleagues addressed two questions: One, does reduced water affect the resources that plants produce? And, two, does reduced nutritive value in food resources affect bees? Their study used tomcat clover, Trifolium willdenovii, as a plant test species. For bee test species, they used the honey bee, Apis mellifera, and a bumble bee, Bombus impatiens.

Experimental plants were raised in greenhouses. In the reduced water condition, plants were watered at the same frequency as in the optimal condition, but received a 30 percent lower quantity of water. In tomcat clover, the inflorescences consist of tiny florets that are grouped into a larger cluster. One day after the first florets opened on tomcat clover plants, the investigators removed the inflorescences for measurements.

The investigators examined both the quantity and quality of floral rewards. In the low-water treatment, they found that plants had significantly fewer inflorescences, significantly fewer florets per inflorescence, and significantly fewer total florets compared to the optimal treatment. In addition, in the low-water treatment, the amount of nectar per floret and the concentration of sugar in the nectar were significantly lower than in the optimum treatment.

There was no difference in the mass of pollen produced per floret between optimum-water and low-water conditions, but the total protein was significantly lower in the low-water treatment. Also, the total amount of pollen produced per plant was lower in the low-water treatment, correlating with the lower number of florets produced in that treatment.

For honey bees, the researchers created two types of artificial diets by manipulating sugar and protein content based on the observed nutrition in tomcat clover in the low-water condition. One type of diet was optimum and the other diet mimicked the effect reduced water has on plants by providing 10 percent less protein and 17 percent less carbohydrates. They grafted honey bee larvae onto artificial diets in cups; 48 larvae received the optimum diet, and 48 larvae received the reduced-water diet.

For bumble bees, the investigators created two artificial bumble bee diets. One approximated the higher-quality nutrition provided by clover experiencing optimal water (“optimum” diet), and the other approximated the lower-quality nutrition provided by clover experiencing a reduced watering regime (“low-water” diet). They studied 74 microcolonies. Bombus impatiens larvae from half of the microcolonies were raised on the optimum diet, and B. impatiens larvae from the other half of the microcolonies were raised on the low-water diet. They also raised four whole B. impatiens colonies, assigning two of these colonies to an optimum diet and two colonies to the low-water diet.

A study by researchers at the University of California, Riverside, examined the effects of reduced water availability on bees such as common eastern bumble bees, (Bombus impatiens), shown here in a nest in one of the experimental treatments in the study. The study found that reduced water availability affects floral resources that plants produce and that these changes in floral resources can negatively impact bees consuming nectar and pollen from affected plants. (Photo by Sarah Barney)

Their results were striking. Honey bees, bumble bees in microcolonies, and bumble bees in whole colonies all had lower survival with the diet based on the reduced nutritive value of pollen and nectar of clover experiencing reduced water conditions than with the diet based on the nutritive value of clover experiencing optimal water conditions. In the two full colonies receiving this low-water diet, bumble bees produced fewer eggs and fewer bees than in the two full colonies receiving an optimum diet. Also, bumble bees in whole colonies receiving the low-water diet produced fewer worker bees and fewer reproductive females.

Summarizing the significance of the study, Rankin says, “We found that water limitation directly impacts the floral resources produced by plants and this can have cascading effects on pollinators consuming the nectar and pollen of affected plants.”

It may seem obvious that reduced water levels would affect bees, but until specific experimental tests such as those in this study are done, it is unknown exactly how the bees are being affected, and such results are essential to helping inform effective management of pollinators.

Looking ahead, Rankin says, “In this study, we manipulated both nectar and pollen concentrations simultaneously, as this is what we observed occurred when we reduced water to clover plants. In the future, we would like to manipulate nectar and pollen resources independently.”

Many questions remain about the interplay among water availability, floral resources, and bee development and success. Pollination management efforts must include finding ways to increase floral rewards, thereby increasing the number and health of bees. One way the quality of floral rewards could be increased is by earmarking part of a restricted water budget in a given region specifically for plants or habitats used by bees. A potential avenue for increasing the number of bees is through changes in resource management, incorporating the promotion of bee-friendly habitat and bee health into management strategies. Another possibility is genetically engineering bees to make them better able to thrive in low-water conditions. But all potential management initiatives will benefit from continued studies on plant–bee mutualisms and the role of water on bee health.

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com/.