Microbes in flowers are crucial to bees' diets, in that sense, changes in the microbiome could be starving insects.
Ask an entomologist what makes a bee a bee, and you'll probably get some version of "bees are just wasps that became vegetarians." Bees are actually omnivores, and their meat is microbes.
This finding may open a new window on why bees are in trouble: Anything that disrupts the microbial community in a bee's food, be it high heat linked to climate change, fungicides, or another stressor, could be causing let developing bees starve.
Bees are supreme pollinators for what their babies eat. Most animals visit flowers to steal nectar, and they may or may not brush off the pollen and carry it to the next flower. Female bees, in contrast, deliberately collect pollen, along with nectar, to feed their babies. This choice of larval food is part of what defines a bee.
Scientists have known for decades that fermenting microbes are present in pollen, but no one seemed to consider whether they were also an important food for bees. The microbes function as an “external rumen” that breaks down parts of the pollen. It's logical that bees can ingest some microbes, but two researchers decided to investigate whether they eat enough to make them omnivores, and whether bees really need those microbes to thrive.
Prarthana Dharampal of the University of Wisconsin - Madison and Shawn Steffan, working jointly at the university and the Agricultural Research Service (ARS) of the United States Department of Agriculture, evaluated 14 different species of bees in six of the seven families of bees They found that bees eat substantial amounts of microbes, enough to change the way they fit within food webs.
Scientists use a scale to classify where organisms in that network belong: those that produce their own food, such as plants, register at the so-called trophic position 1 (TP 1), herbivores register at TP 2, and carnivores do so. They do at TP 3, or even more if they eat other carnivores. The average TP in all the bees studied was 2.6, placing them directly in the omnivorous place, halfway between herbivores and carnivores. Interestingly, the trophic position varies between families, ranging from slightly above herbivores (2.11) to solid carnivores (3.09). Now that the TP is known, Dharampal says he wonders if the bees are really looking for pollen or if they are looking for the microbes associated with pollen.
For most people, the idea that microorganisms can qualify as meat is radical. In the past four years, Steffan and his colleagues, including Dharampal, have published a series of papers showing their evidence that microbes are an important part of a variety of food webs, including those involving bees. Their findings confirm that fungi, bacteria and other microscopic players can fit anywhere on the food web, altering our view of predator and prey, carnivore and herbivore, and what makes a bee a bee.
Steffan and his colleagues have also shown that microbial meat is a necessary part of the bees' diet. The researchers tested a species of mason bee that lays eggs in tubes on the ground for easy access and transport. In each tube, the mason bee lays a series of eggs, each in its own wad of pollen and nectar. The researchers had a Utah beekeeper send them a batch of tubes immediately after the bees filled them. They then took the eggs out of the tacos and separated the males from the females and used only the larvae of male bees, divided into seven groups of 12.
The scientists sterilized half the pollen and then fed different mixtures of sterilized and non-sterilized pollen to the groups. As the percentage of sterilized pollen in food increased, the probability of death of the larvae also increased. The larvae also weighed less and took longer to mature. "Microbes are a very important source of nutrients for these bees," says Dharampal. "If you remove this critical source, or portion, from their diet, they suffer greatly."
The idea that bees are vegetarian is ingrained in entomology, and Steffan admits that he and his colleagues ran into headwinds when they tried to get them to accept his documents relating to omnivory in bees. Finally, they were published in the American Naturalist and Proceedings of the Royal Society B, respectively. Gloria Degrandi-Hoffman, who works for ARS and has researched the honey bee microbiome but was not part of the work, says the scientific community is always skeptical. When a new finding goes against a widely accepted perception, people are convincing.
The results from mason bees suggest that bees could starve or starve if certain microbes disappear from their diet. Scientists have attributed the decline in both managed and wild bees to various combinations of habitat loss and degradation, pests and pathogens, pesticide exposure, and climate change. They have largely focused on how these factors impact bees directly. The next step is to see if stressors can affect pollen-borne microbes. Steffan says that any stressor that throws the external rumen out of control could be "an indirect, but no less lethal way" to kill bees.
One of those factors is the heat of climate change. "It may not be that the heat is directly lethal to the development of bees," says Steffan. "But it could very well be that the high heat kills the microbial symbionts in the pollen, and then the bee suffers from the lack of microbes." Steffan and Dharampal are investigating this possibility.
Fungicides could also be the culprit. Although more research needs to be done, Steffan says, "We have ample evidence, at this point, that fungicides drastically alter the microbial community of fermenting pollen." And he adds, "agricultural use of fungicides is most likely a primary stressor: the main stressor, for the decline of bees."
Of course, failed bee populations can cripple crops and wild plants that help pollinate. About three-quarters of the earth's flowering plants and crops benefit from animal pollinators, including 87 of the world's 115 most important food crops. The 20,000 species of bees in the world are not the only animals that pollinate, but they are the best pollinators for many staple foods.
Knowing the role of microbes in pollen can help solve conservation challenges by, for example, directing flower choices for habitat restoration.
Sandra Rehan of York University in Toronto, who studies the microbial life associated with wild bees and was not part of the recent papers, says the findings “will have long-term conservation applications once we associate the flowers, the landscape and microbes ”.
In a 2017 study, she and her co-authors wrote: “Pollinator habitat restoration efforts may need to consider flower plantings that increase the presence of core bacteria found in flowers, adults, and arrangements. pollen, such as Lactobacillus and Saccharibacter. Future work is needed to determine the role of these core bacteria in restoring healthy pollinator communities. "
The new take on pollen microbes is just the latest example of how important the microbiome is in all walks of life, which we may have ignored at our risk. “We, as animals and flowering plants, have flourished, to the extent that we are able to cooperate, co-opt and capture microbial services,” says Steffan.
He adds that we see food webs through the lens of vertebrates and mammals, but microbes have been on the planet much longer than animals or plants.
Steffan, Dharampal and others call for a radical overhaul of how we view life on earth. At the end of their article on the American Naturalist, they write: "Considering the symbiosis of bees and microbes from a microbial perspective, microbes can be seen as avid beekeepers, facilitating and assisting their faunal symbionts in the annual pollen harvest."