Nutritional Ecology of Honey Bees in a Changing Landscape
with Pierre Lau, Pierre Lesne, and Spencer Behmer
Honey bees (Apis mellifera) are responsible for pollinating approximately 35% of the world’s overall food production, providing an estimated US$235 to US$577 million to the global economy annually. In the U.S., honey bees contribute over $15 billion annually to the economy, providing pollination services for the production of major agricultural crops. While the demand for bee-pollinated crops continues to rise with an ever-increasing human population, the number of managed honey bee colonies available for pollination has been declining steadily in the last decade. A recent survey reported that beekeepers across the U.S. lost up to 40% of their colonies between 2018 and 2019, the highest of such losses reported in the past decade (Bee Informed Partnership, beeinformed.org). One of the major drivers behind this decline in honey bee health is poor nutrition, considered as a seasonal lack of resources in terms of abundance, diversity, or both.
As eusocial, generalist pollinators, honey bees utilize a diverse range of floral resources to meet the nutritional requirements necessary for proper development and growth at the individual and colony levels. Flowering plants provide honey bees with nectar, their primary source of carbohydrates, and pollen, their primary source of proteins, lipids, and micronutrients. Nectar and pollen are collected and brought back to the hive by older, forager worker bees, and are then processed and stored within the hive in the form of honey and beebread, respectively. Older bees consume primarily honey, as the carbohydrates within nectar are responsible for fueling an individual’s everyday metabolic activities. Young nurse bees feed primarily on beebread, which they convert into the jelly that they feed to developing brood. Moreover, the proteins and lipids within pollen have been shown to be critical for physiological processes related to glandular development, immunocompetence, and cell membrane support in nurse bees. Not surprisingly, honey bees are better at tolerating the stress caused by pathogens, pesticides, and antibiotics if they have access to sufficient pollen stores within their hive.
Honey bees require a constant supply of pollen from a diversity of plant species throughout the year to maximize colony health and productivity. Every plant species produces pollen grains that are unique in size, color, odor and nutritional composition. The protein content in pollen grains can range widely from 2.5% to 61%, while the lipid content ranges from 1% to 20%. Pollen also contains critical micronutrients including vitamins, minerals, and sterols. While several studies on honey bee nutrition have demonstrated that colonies need to collect pollen from a wide variety of plant taxa, less is known about the factors that drive bees to select which plants to forage from in the environment.
Like many other social insects, including bumble bees, ants, and termites, honey bees exhibit dietary preferences based on the nutritional content of their diets. Forager bees may set their foraging preferences based on the nutritional content of pollen, other non-nutritional cues such as odor, shape or color, the relative abundance of a given pollen source, or a combination of these factors. Previous studies have suggested that honey bee foraging preferences are based on prior knowledge of the type of food that had already been collected, with colonies being able to perceive, discriminate, and complement nutritional deficiencies in their diet by making subsequent foraging decisions. Laboratory studies conducted with artificial protein diets have also suggest that honey bees are inclined to consume diets with a particular nutritional content. Results from those studies would imply that foragers are able to seek out pollen sources that fulfill nutrient deficiencies within the colony to maximize fitness, an idea that has not been fully tested yet.
While proper colony nutrition is pivotal for honey bee health, we still lack critical information about honey bee nutritional requirements, particularly with respect to the macronutrients present in pollen. The goal of this presentation is to go over the nutritional preferences of honey bees in our everchanging landscape, and to present a recent study, which identifies the patterns of macronutrient consumption that drive honey bee pollen foraging in a nutritional geometric framework. We did so by exploring whether and how honey bees regulate their protein and lipid intake through a combination of field and laboratory choice and no-choice experiments using monofloral Brassica and Rosa pollen, as well as artificial pollen substitutes. In the field experiments, bees regulated their protein-to-lipid (P:L) consumption by foraging non-randomly between Brassica and Rosa pollen to reach an intake ratio of 1.4P:1L. This ratio was similar to the 1.5P:1L diet preferred by nurse bees in the no-choice laboratory tests. Nurse bees that consumed the 1.5P:1L diet also had significantly larger hypopharyngeal glands compared to bees in all other treatment groups. Our results suggest that honey bees have a preference for consuming macronutrients at a significantly lower P:L ratio (1.4P:1L) than previously thought, suggesting that lipid consumption is of undervalued importance in our current knowledge of honey bee nutrition.
Factors that Affect the Reproductive Quality of Queens and Drones
with Elizabeth Walsh
Reproductive division of labor is one of the defining traits of honey bees (Apis mellifera), with non-reproductive tasks being performed by workers while a single queen normally monopolizes reproduction. A queen’s developmental fate is highly plastic, and her reproductive physiology is greatly affected by the queen-rearing environment. There are several environmental and biotic factors that affect the reproductive quality of queens and their drone mates. For example, previous studies have shown that in queenless colonies, the age of larvae when chosen by workers to be raised as queens can range from the first to the third larval instars. This plasticity in queen phenotype leads to inherent variation in the reproductive potential of mated queens, with queens that develop from third instar larvae (compared to those that develop from first instar larvae) being more “worker-like.” These queens usually exhibit lower individual fitness and head colonies with lower growth and productivity. A number of studies have also shown variation in the reproductive fitness of honey bee queens raised from different worker larval instars, as measured by body size, ovariole number, and the diameter, number, and viability of drone spermatozoa stored in the queen’s sperm-storing organ (spermatheca) and mandibular gland pheromone composition. Furthermore, there is significant positive effect of queen grafting age on a colony’s production of worker comb, drone comb, and stored food (honey and pollen), although we did not find a statistically significant difference in the production of worker and drone brood, worker population, and colony weight.
Unfortunately, honey bee queens and drones multiple health risks including nutritional stress, exposure to pests and pathogens, and pesticide contamination, which cause problems at the individual and colony levels. One of the gravest problems faced by honey bees is parasitization by the mite Varroa destructor, which is typically controlled through the application of miticides such as tau-fluvalinate, coumaphos, and amitraz. In addition to miticides, colonies are also exposed to pesticides brought by foragers from agricultural settings, including the fungicide chlorothalonil and the insecticide chlorpyrifos. Here, we explored whether exposure of wax to combinations of these pesticides during development affects honey bee queen physiology and worker behavior. To do this, we reared queens in plastic cups coated with molten beeswax that was either pesticide-free or containing field-relevant concentrations of tau-fluvalinate and coumaphos, amitraz, or chlorothalonil and chlorpyrifos. Once queens mated naturally, we placed them in observation hives to measure egg-laying rate and worker retinue size. We then dissected the queens and used the contents of their mandibular glands to measure worker attractiveness in caged bioassays and to analyze their chemical components using GC-MS. Exposure of wax to field-relevant concentrations of the tested pesticides during queen development significantly lowered the adult queens’ egg-laying rate and worker retinue size. Miticide exposure during development also lowered the attractiveness of queen mandibular gland contents to workers and affected the relative amounts of the glands’ chemical components. Our results support the ideas that mandibular gland pheromones act as honest indicators of queen reproductive fitness and that pesticide exposure of wax during bee development is an important and concerning factor impairing honey bee health.
Our results provide evidence that in honey bees, queen developmental plasticity influences several important measures of colony fitness. Thus, the present study supports the idea that a honey bee colony can be viewed (at least in part) as the expanded phenotype of its queen, and thus selection acting predominantly at the colony level can be congruent with that at the individual level.
Born in Colombia, South America, Juliana Rangel obtained a BS in Ecology, Behavior, and Evolution in 20104 from the University of California, San Diego. In 2010 she obtained a PhD in Neurobiology and Behavior from Cornell University in Ithaca, NY, working with Dr. Tom Seeley. She was a National Science Foundation Postdoctoral Research Fellow from 2010 to 2013 working with Dr. David Tarpy at North Carolina State University. In January 2013, Juliana became Assistant Professor of Apiculture in the Department of Entomology at Texas A&M University (TAMU) in College Station, TX. She was promoted to Associate Professor with tenure in 2018. Her research program focuses on the biological and environmental factors that affect the reproductive quality of honey bee queens and drones, the behavioral ecology and population genetics of unmanaged honey bees, and the quality and diversity of honey bee nutrition. She is an active member of the Texas Beekeepers Association and has spoken to dozens of beekeeping associations across the USA and internationally. She teaches the courses Honey Bee Biology, Introduction to Beekeeping, and Professional Grant and Contract Writing. Since 2014 she has been the coach of TAMU’s undergraduate and graduate teams of the Entomology Games at the branch and national games of the Entomological Society of America (ESA), earning first and second place nationally three years in a row. She is the 2021 Secretary for the Southwestern Branch of the ESA and is the past elected chair of the National ESA’s Diversity and Inclusion Committee. She currently serves as the elected chair of her department’s Faculty Advisory Committee and has been part of several committees at the departmental, college, and university level. She received the 2021 James I. Hambleton Memorial Award, which was established by the Eastern Apicultural Society of North America to recognize research excellence in apiculture. She also received the 2020 John G. Thomas Award for Meritorious Service from the Texas Beekeepers Association for her contributions to the apiculture industry in the state. She received the 2019 Dean’s award for Excellence in Diversity and the 2016 Dean’s award for Excellence in Early Career Research from TAMU’s College of Agricultural and Life Sciences. She also received the 2018 Outstanding Achievement in Mentoring award from the Entomology Graduate Student Association. She was 2014 President and 2013 Vice-President of the American Association of Professional Apiculturists.