Just as humans need a variety of foods to remain healthy, a honey bee colony needs a variety of pollen types. Pollen is the sole source of amino acids in the honey bee diet, save for trace amounts in honey. A shortage of pollen can mean diminished life spans, less resistance to disease, or poor foraging ability, among other things.
The Importance of Pollen Variety
Variety is important because not all the amino acids are found in a single type of pollen. Some have a greater assortment than others, so eating a variety of pollen types is the ticket to good colony nutrition. In nature, this would not be difficult. But in many modern settings, especially those containing a small number of flowering species, bees may come up short in one or more of the essential amino acids.
Pollen and Brood Rearing
As beekeepers, we know that pollen is necessary for brood rearing. The youngest honey bee larvae do not eat pollen directly. Instead, the nurse bees eat the pollen in the form of bee bread. Such a protein-rich diet stimulates their hypopharyngeal glands to secrete royal jelly, which is then fed to the young larvae. After about three days, small amounts of pollen and diluted honey are mixed into the brood food of both workers and drones, while young queens continue on a diet of pure royal jelly.
Adult workers eat energy-rich honey almost exclusively. Because foragers don’t eat bee bread or pollen directly, when they do need protein, they beg the nurse bees for it. Nurse bees feed all members of the colony from time to time.
Foragers and Nurse Bees: Who Knows Best?
Since the foragers that collect pollen don’t eat it, they are sometimes not too picky about what they collect. Sometimes foragers will bring home other stuff-sawdust or coffee grounds, for example-that have a powdery consistency and the right particle size. Some researchers believe that honey bees cannot determine the food value of pollen: if it looks like pollen, it must be good.
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Although foragers sometimes collect inferior pollen or non-pollen, the nurse bees-the ones that actually have to eat the stuff-are much more selective. In fact, nurse bees often discard some of the treasures their sisters bring home from the field, especially things with no food value, or even that expensive pollen substitute.
But recent research shows that when it comes to actual pollen, even the nurse bees cannot determine the quality. Or, even if they recognize poor quality, they are unable to communicate that information to the foragers who are hauling it in. In short, since we cannot rely on the colony to adequately balance its own diet, a variety of pollen choices is the best solution.
Pollen Needs Vary with Brood Rearing
When you understand how pollen is used in the hive, you can see why a colony doesn’t need a large supply during winter. In late autumn through mid-winter, when there is little brood rearing, a colony can get by with a minimum of pollen. However, heaps of good-quality pollen are needed throughout the major brood-rearing periods, especially in late winter and early spring.
When pollen is scarce, a colony may be forced to live off protein reserves stored as vitellogenin in the workers bees’ fat bodies. But that supply is limited, so beekeepers often opt for pollen substitutes before the major flows begin.
A Changing Environment and Bee Nutrition
It is easy to fall into the trap of thinking that since our fathers, grandfathers, and great uncles never worried about bee nutrition, we shouldn’t either. Back then, the bees gathered pollen on their own and thrived. No one gave two thoughts about its source or amino acid content.
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While that may be true, we have substantially changed our environment in the intervening years. No longer are urban areas separated by large swaths of natural vegetation. No longer do farms grow an impressive array of crops. And no longer do roadsides provide a sparkling display of wildflowers. Bee nourishment? A shortage of high-quality pollen is a relatively new concern for beekeepers. Today, a bountiful and diverse supply of pollen is often lacking due to habitat loss, invasive plants, monoculture farming, and herbicides.
Monoculture crops fall short
For bees, pollen from flowering plants is virtually the only source of protein, lipids, vitamins, and minerals. But in modern farmlands variety is suppressed. Bee colonies that pollinate large-acreage monocultures-such as almonds-have a severe lack of variability in their diets. Just as one fruit or vegetable doesn’t satisfy all your nutritional needs, one type of pollen is not enough for bees. Pollinating these crops with honey bee colonies is fine, as long as the beekeeper understands the nutritional stress a monoculture can inflict.
Pollen from different plants varies tremendously in both the quantity and quality of protein. Researchers have found that protein content can range from about 2 to 61% by dry weight, depending on the species. Furthermore, depending on protein source, it may completely lack some of the amino acids necessary for proper growth and development.
An example of a mediocre pollen source is the common dandelion, Taraxacum. Bees love dandelions, and they flit from blossom to blossom in large numbers. But dandelions are missing some of the essential amino acids. Research has shown that a diet of pure dandelion pollen will hinder larval development in mason bees, prevent brood production in honey bees, and cause 100% larval rejection in bumble bees.
Honey bees evolved on diverse vegetation and that is still what serves them best. In a natural environment, monocultures aren’t much of a problem. A bee would seldom-if ever-run into an endless monoculture of dandelions, so they don’t cause an issue. But bees plunked down in the middle of acres and acres of a single crop will have trouble nourishing the next generation. And even if the young bees survive and mature, their immunity to diseases, parasites, and even pesticides may be compromised. Just like any other animal, bees need healthy immune systems to survive, and adequate immunity depends on proper nutrition.
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Invasive species make a poor honey bee diet
Many beekeepers welcome the vast acreages of invasive plants that are swallowing the landscape. The reason is nectar. Some of the invasive plants, including star thistle, Japanese knotweed, Chinese tallow, and kudzu are excellent honey producers that fill the supers and pay the bills. But although they are a bonus for the beekeeper, they can be hard on the bees.
Invasive species are simply another form of monoculture. Like the planted crop, the invasive weed reduces biodiversity. Instead of having a vast array of flowers to choose from, the bees have one main entree. Instead of forty different types of pollen, each with a unique amino acid profile, the bees are left with one. Instead of a kaleidoscope of flowers blooming one after the other across the seasons, there is one ginormous bloom followed by dearth.
Whenever the diversity of plant life is compromised, all those that depend on it are weakened as well. After foraging on invasive plants, managed honey bees may require pollen supplements to keep them healthy. But the others-the native bees, beneficial insects, and other random invertebrates-are simply out of luck. And when the insects disappear, the birds, frogs, and mammals that ate them go hungry as well. In any biological system, nothing is as toxic as uniformity.
Use of Herbicides
Herbicides hurt bees in multiple ways. Although the direct effect of herbicides on honey bees is inconclusive, some research has shown that the ingestion of glyphosate is linked to a reduced ability to navigate. Perhaps more problematic is the fact that herbicides kill natural bee forage, such as flowering weeds, while simultaneously promoting the spread of invasive plants.
Roadsides, croplands, cityscapes, and playgrounds that are stripped of natural vegetation are magnets for invasive species. Just as farmers prepare the soil for their monoculture crops by spraying the weeds, other users of herbicides prepare the soil by spraying the native vegetation, thus making it more attractive to invasive weeds.
Climate Change
Although we don’t know much about the long-term effects of climate change, a few disturbing papers have suggested subtle problems. One recent study examined the protein content of goldenrod pollen in comparison to goldenrod pollen obtained from herbarium collections where the plants were collected years ago. The scientists found that the amount of protein in goldenrod pollen dropped by about one-third between 1842 and 2014.
Similar studies have shown that the protein content of grains used for human food has also dropped during this period, and the drop is believed to be due to higher carbon dioxide levels in the atmosphere. Apparently, an abundance of carbon dioxide causes plants to grow faster and bigger, but the increased growth in the vegetative parts reduces the amount of protein stored in seeds and pollen.
Warmer spring temperatures are also causing some plants to bloom out of sync with the native bees that pollinate them. This occurs when the plants bloom earlier in the year, while the bees emerge at the regular time. In monolectic relationships where one bee species pollinates one plant species, both bee and plant can go extinct.
What Can Beekeepers Do?
Beekeepers can no longer ignore the protein needs of their colonies. Instead, we must keep in mind that, depending on location, the pollen supply may not be as plentiful, as diverse, or as nutritious as it was in the past. We shouldn’t hesitate to use pollen supplements if there is a chance our bees might need them. Especially when we see disease, poor overwintering, or lackluster performance, we need to remember that well-fed colonies will outperform their malnourished counterparts.
Looking at the larger picture, planting a diverse assortment of flowers and flowering trees may be our best long-term solution. Some groups have worked to alter the spraying regimens in their local communities-simply changing the timing of maintenance can make a big difference. Others are working to build community awareness by planting pollinator gardens, utility easements, median strips, and parks with bee-friendly flowering species. Planting flowers may seem like a small thing, but if enough people understand the consequences of a diversity-starved environment, perhaps we could make a difference.
Wild Bees Strategically Target Flowers for Balanced Nutrition
Wild bees aren’t just flitting from flower to flower, collecting pollen at random. Instead, they are strategically targeting flowers that enable them to carefully balance their protein, fat and carbs. In the first long-term, community-level field study of wild bumblebee nutrition, a team of ecologists led by Northwestern University and the Chicago Botanic Garden discovered that wild bees aren’t just flitting from flower to flower, collecting pollen at random. Focusing on pollen consumption, the study revealed that coexisting bee species occupy two distinct nutrient niches. Larger bodied bees with longer tongues prefer pollen that’s high in protein but lower in sugars and fats. Bees with shorter tongues, however, tend to gather pollen that’s richer in carbs and fats.
Nutritional Needs of Wild Pollinators
“Despite the general importance of wild pollinators, especially bees, we know very little about their nutritional needs,” said Northwestern’s Paul CaraDonna, the study’s senior author. “Given widespread pollinator declines that have been observed around the globe, this knowledge gap is surprising and concerning. Our research provides some of the best information yet on the availability of nutritional resources found in wildflowers and how pollinators use these resources.
In the wild, bumblebees mainly consume two floral-based foods: sweet, syrupy nectar and fat- and protein-packed pollen. While adult bees sip nectar for a quick burst of energy, they also collect pollen for their babies, or larvae, to help them grow. “We know that bees forage exclusively from flowers for pollen and nectar,” CaraDonna said. “Beyond that, we are in the dark. That is like humans shopping at a grocery store and assuming that all food items in the entire store have similar nutritional value.
Comprehensive Nutritional Map
While other researchers have conducted short-term, lab-based studies on nutrition for single species of bees, the Northwestern and Chicago Botanic Garden team aimed to develop a more comprehensive nutritional map for how things play out in the wild. To do this, the researchers observed eight different species of wild bumblebees at a field site in the Colorado Rockies. The team took the pollen samples back into the lab, where they measured the macronutrient content of each pollen sample, specifically calculating the concentrations of protein, fat and carbohydrates.
“All pollen contains protein, fats and carbs,” Bain said. “But each type of pollen has a different mixture of these macronutrients. Some are very high protein like a steak. Others are more like a salad. After determining the macros for each pollen sample, the researchers compared each bee species’ diet with their physical traits (like tongue length) and with seasonal shifts in flower availability.
Seasonal Shifts in Flower Availability
Not only did pollen’s nutrient content vary substantially among plants, but it also changed throughout the season. Spring flowers, for example, have more protein-rich pollen, while late-summer flowers are richer in fats and carbs. “Queen bees emerge in the spring to establish their colonies,” Bain said. “They forage when the snow first melts, collecting protein-rich pollen for themselves and their first brood. Later in the summer, worker bees take over foraging, and half of the species shifted toward pollen with less protein and more fats.
The researchers also noticed the eight bumble bee species naturally divided into two diet groups. Long-tongued species collected pollen with higher protein and lower fat and sugar. Shorter-tongued species collected pollen with lower protein and higher sugar and fat. In another surprise, the protein differences from flower to flower are larger than expected. In some flowers, protein only made up 17% of the pollen.