By Michał Filipiak
The larvae of the majority of solitary bees feed directly on pollen delivered by their mothers. Apis mellifera honey bees are a different case altogether: nursing workers feed their youngs with jelly, which they produce from the nutrient sources of nectar, pollen, and water. Nectar provides energy, and pollen provides building materials for the bee body. Therefore, in both solitary wild bees and domesticated bees, the quality of pollen determines the overall quality of larval food. Herein lies a problem.
Not all plants produce pollen that fully satisfies the nutritional requirements of bees. Yet, in choosing bee-friendly plants for our gardens, we tend to ignore their quality as food for bees. Bee-friendly plants are often defined based on the quantities of nectar and pollen produced and on the visitation rates of adult pollinating insects. However, bees forage on different plant species for nectar and for pollen. Furthermore, adult food, which is rich in energy, has different characteristics from those of larval food, which is rich in body-building matter. Therefore, to balance the bee diet and ensure that bee populations can thrive, we should pay special attention to pollen quality. Fortunately, to identify good-quality pollen, we do not have to study the impacts of hundreds of organic substances on the growth and development of bees. Bee bodies and their foods are built from the same 25+ atoms, although in different proportions. Based on this knowledge, we can use a shortcut to identify good-quality pollen: multielement ecological stoichiometry.
To better understand the nutritional constraints of growing and developing organisms, the ecological stoichiometry framework was developed using atomic ratios with reference to the elements that, if scarce in the environment, prevent the building of organic molecules. Adult bodies are already built; therefore, their functionality is limited mainly by energy. However, the ability to build a fully functional adult body, according to the homeostatic “elemental recipe”, is influenced by the availability of “body-building” atoms during an organism’s juvenile stage.
We compared the body stoichiometries of A. mellifera domesticated bees and Osmia bicornis wild bees with the stoichiometries of various species of pollen (here and here along with two manuscripts in preparation). Both species experience limitations to their growth and development when a set of specific elements is scarce in pollen. Furthermore, the elemental composition of pollen is taxonomically highly variable. So, what does this mean? Let’s imagine there are Plant 1 and Plant 2. Pollen 1 is rich in S but lacking in K. The converse is true for Pollen 2. Collecting pollen from only one plant would negatively impact larval development; however, collecting pollen from both plants would provide a balanced and healthy diet. Therefore, we should stop thinking about the bee-friendliness of plants in terms of how much nectar and pollen they produce and start thinking about it in terms of their quality.
Monoculture can hamper bee development by restricting dietary options. For example, sunflower pollen contains little phosphorus, an element crucial for building ribosomes and RNA. Considering O. bicornis, rapeseed and corn pollen provide a highly unbalanced diet because of the scarcity of several elements. Therefore, in the neighborhoods of monoculture plantations, plant species whose pollen provides scarce atoms, such as various species of clover, should be planted to allow for stoichiometric balancing of the diet. Using flower strips composed of plants that produce large quantities of pollen or nectar of poor or unknown nutritional value is not a good conservation strategy. Native plants producing pollen that allows for stoichiometric balance of the O. bicornis diet include buckwheat, rose, common gorse and broad bean. Common camellia and fuzzy kiwifruit (and probably Rosaceae and Fabaceae in general) produce stoichiometrically balanced pollen for both O. bicornis and A. mellifera. A list of plants that produce nutritionally balanced pollen for the honey bee is presented here.
The honey bee has a high demand for Na and frequently forages for minerals in “dirty water,” swimming pools and seawater. Recently, two interesting works were published on this topic: here and here. In addition, beekeepers say it is good to prepare salt water for honey bees (approx. 1 g per liter).
Notwithstanding the above observations, we know little about the nutritional needs of wild bee species, of which there are thousands in the world. Different species have specific and homeostatic body stoichiometries; i.e., they each have specific “elemental recipes” for their newly emerged adult bodies, which are produced from nutrients available during larval development (changes in body chemical composition during subsequent adult life are a different story). The concept of the stoichiometric niche predicts that different bee species should collect different pollen species in various proportions to stoichiometrically balance their larval diets. Unfortunately, there is almost no knowledge on the nutritional suitabilities of different plants to various wild bee species.
Decreasing plant diversity is thought to be one of the causes of the dwindling number of pollinators worldwide. It was recently shown that various plant species act synergistically to influence honey bee nutrition (here and here) and O. bicornis nutrition (here and here) and that the foraging strategies of bumblebees can be shaped by the ratios of macronutrients in pollen (here and here). However, we don’t know the degree to which wild bee populations are limited by the availability of various pollen species or how changes in floral composition and diversity impact the nutritional balance of bee diets. Pooling knowledge on the nutritional needs of bees with data on the chemical compositions of various pollen species would allow us to identify key plant species that can help bees compose balanced diets. However, first, we need to gain knowledge on the nutritional needs of wild bees. There are resources to start with, such as a list of the best garden flowers for bumblebees, a list of plant species used by honey bees, a list of pollen sources for oligolectic solitary bees, and a compilation of literature data on the elemental compositions of pollen.
Many species of pollinators are needed for full ecosystem functioning, and they need resources to thrive and prosper. It wouldn’t be wise to ignore the dietary requirements of bees, and failing to understand their biology can really blow up in our faces.
Author biography: I use ecological stoichiometry to study the limitations imposed on organisms due to mismatches between their nutritional demands and the supply of nutrients in their environment. My work focuses on two examples of herbivore life histories shaped by the stoichiometries of their food, representing the extremes of feeding strategies: (1) nutrient-deficient dead wood exploited by xylophages and (2) nutrient-rich pollen used by bees. My current research activities concern sexual dimorphism in the nutritional needs of bees and the effects of taxonomically diverse floral resources on bees.
Affiliation: Institute of Environmental Sciences, Jagiellonian University, Kraków, Poland
- Lavender field of Provance. Lavender attracts a variety of pollinating insects and is considered one of the most bee-friendly plants. It offers plenty of nectar (energy for pollinating insects), but its pollen is nutritionally imbalanced for bees. Author: Andyblind / Wikimedia Commons
- Sunflower field in Spain. Sunflowers produce pollen that is nutritionally imbalanced for bees. Author: Diego Delso, delso.photo, License CC-BY-SA / Wikimedia Commons
- Osmia bicornis (male) – wild mason bee. Author: Orangeaurochs from Sandy, Bedfordshire, United Kingdom / Wikimedia Commons