bee biology

The Detritus of Brood Rearing: Reduce, Reuse, Recycle

Raising a batch of brood requires a treasure trove of resources and generates a heap of waste. Some folks have estimated that a honey bee larva increases in weight up to 1,500 times during the five-and-a-half-day period from hatching to spinning a cocoon.1 If you had an eight-pound newborn growing at the same rate, it would weigh 12,000 pounds at the end of the five-and-a-half days. Imagine the food. Imagine the diapers. Multiply by thousands.

This article first appeared in American Bee Journal, Volume 158 No 6, June 2018, pp. 683-685.

When you think about the throughput—food in, feces out—you begin to wonder where it all goes. Then there are the eggs, the cocoons, and the brood cappings. Where do they go? And why doesn’t a brood frame smell like a cesspool and look like a dump? In truth, a marvel of biology and waste management is required to make the system the work.

Edible eggs

I began to think about waste management recently when a beekeeper asked, “What happens to the egg shells?” I immediately remembered the restaurant where I worked as a student. The breakfast cooks cracked eggs and stacked the shells, nestled one inside the other, until they towered two feet above the counter, all glossy and stinky with that peculiar empty-eggshell aroma. I can still smell those sticky turrets.

But honey bee eggs don’t have shells like chicken eggs do. Instead of a hard casing, the eggs of most bees are soft and pliable. They are covered with a membranous, translucent, and flexible material called chorion. Although we say bee eggs “hatch,” the chorion doesn’t rupture in the usual sense. Instead, the chorion dissolves and is absorbed by the larva as it grows.2

Compared to other species, honey bees have very small eggs. Bees that lay one egg on a pollen ball tend to have large eggs, whereas social species that lay many eggs and feed the larvae continually have tiny eggs. The fate of the nursery is also relevant. Species that reuse the nursery, such as honey bees, are more likely to reabsorb the eggs completely, while bees that abandon the nest are more apt to leave the “shells” behind.3

Bee eggs don’t have brittle shells but are covered with a translucent, flexible chorion that gets absorbed by the larva as it grows.

Bee eggs don’t have brittle shells but are covered with a translucent, flexible chorion that gets absorbed by the larva as it grows. Photo © Christopher Wren.

Fun facts about feces

Think about a honey bee larva. It floats luxuriantly in a pool of brood food that is continually replenished by the nurse bees. To achieve the rate of growth that honey bee larvae experience, they need to be fed constantly. In fact, honey bee larvae are fed hundreds of times per day. With all that eating going on, why isn’t their brood food contaminated with their own feces? It’s not like they leave their cells to use the facilities, so what gives?

The system used to control fecal deposition in the nest is similar in nearly all bee species, whether they live in a social community or in a solitary cavity. Basically, the reason they don’t deposit excrement in their food is they can’t. When bees are in the early larval stages, the front part of the digestive system—the part that eats—is not connected to the back half of the system—the part that excretes.4 With such a failsafe system, mistakes don’t happen.

Connecting the parts

It works like this. You can imagine the digestive system as a series of tubes, each connected on one or both ends. In adults, the bee ingests food into its mouth and then the food proceeds through the alimentary canal to the midgut where most digestion and nutrient absorption takes place. From there, the waste travels to the hindgut where it collects before being excreted.5 In larvae, however, the connection between the midgut and the hindgut is missing. There is no connection whatsoever, so the waste accumulates in the midgut.

The two sections of the gut are not connected until just before the first cocoon is spun. In other words, the larva has finished eating and is about to enter the pupal stage in which no eating occurs. As soon as the attachment is made, the waste travels the rest of the way through the bee and is expelled through the anus. This stage is called the prepupa or sometimes the “defecated larval” stage.

In honey bees, most of the feces is captured by the strands of the cocoon and woven right into it. The feces remains sequestered within the fabric of the cocoon such that it is not loose inside the brood cell nor is it inside the cocoon with the developing honey bee. Everything is kept separate, clean, and neat.

In solitary bees, it works much the same way. As soon as the larva completely consumes the pollen ball, the digestive system connects and voids. In bee species that don’t reuse their brood chambers, the feces may be left in the chamber instead of being incorporated into the cocoon, and in a few specialized bees, larval defecation may begin before pupation. But regardless of where the feces is deposited, the food is always kept free of contamination.

Fixing brood rearing mistakes

Occasionally, some feces may escape being captured by the cocoon of honey bees. In that case, the worker bees handle the problem. A passage by M. Lindauer explains his observations: “In a very few cases I was also able to observe that a thin film of wax was laid over any spot of larval excreta which the larva had failed to cover with the cocoon.”6 He goes on to explain that once the feces was sealed in wax, the cell interior was polished and made ready for the next use.

Brood frames get darker and darker with age, and at least part of the color is due to excrement embedded in the cocoons or covered by a wax coating as described above. Other materials, including dirt, pollen, propolis, and parasite debris also contribute to the darker color of used combs.

Anyone who has rendered beeswax from old brood combs is familiar with the large proportion of miscellaneous debris, known as slumgum, which accumulates over time. All the embedded materials appear to add to the strength and rigidity of old combs. Unfortunately, some disease organisms or their spores can remain viable within the accumulation of debris, and certain pesticides may accumulate over time. But the normal detritus of the hive is not a problem for honey bees.

Tenerals are young bees that have just emerged. They have a pale color that darkens as their exoskeleton hardens.

Tenerals are young bees that have just emerged. They have a pale color that darkens as their exoskeleton hardens. Photo © Christopher Wren.

Brood cappings

According to Mark Winston, workers even reuse the wax cappings that cover the brood cells. He writes, “To emerge, the tenerals7 begin by using their mandibles to perforate the cell capping with small holes as they rotate within the cell…. The wax cappings are manipulated with the mandibles and fastened to the cell wall, where adult workers pick them up and reuse them to cap other cells.”8

When you think about this, it makes a lot of sense. Beeswax is biologically expensive for the bees to make, so from a natural-economy standpoint, it makes sense that the colony reuse as many materials as possible.

A detailed description by Bruce Lineburg suggests the cappings are sometimes taken off in pieces, and placed in a recycling bin of sorts. “A worker aiding a bee to emerge simply places the material it removes at convenient points about the comb where it may be readily obtained for capping the same or other cells at some future time.” In addition, he describes a slightly different sequence when the worker bee emerges by herself. “In case the emerging bee cuts its way out unaided, then the frayed edges of the cap, for the most part, remain attached to the rim of the cell.” These are later removed by workers and reused where needed.8

Much indirect evidence points to the fact that brood cappings are routinely reused. For example:

  • When brood cappings are analyzed, propolis, pollen, and bits of cocoon are found embedded in the beeswax. These added ingredients give wax cappings their slightly bubbly appearance and pebbly texture.
  • Darker cappings are found on darker brood combs, while the cappings on new brood comb are as light as the comb itself. This suggests that the cappings are made from similarly-aged material.9
  • Brood raised during the winter or very early spring is capped at a time when bees are not secreting beeswax, so the wax must come from a preexisting source.
  • In observational experiments, brood cappings are routinely collected from the bottom board by worker bees.10
Brood rearing: The cappings of brood cells have a pebbly texture and are very close in color to the comb they are attached to. In contrast, honey cappings (upper right) have a smoother, more uniform surface and may have a lighter color.

The cappings of brood cells have a pebbly texture and are very close in color to the comb they are attached to. In contrast, honey cappings (upper right) have a smoother, more uniform surface and may have a lighter color. Photo © Christopher Wren.

The three Rs

Although brood rearing is biologically expensive, honey bees never forget the three Rs of waste management. The size of the eggs is reduced, the chorion and brood cells are reused, and the cappings wax is recycled. In addition, fecal management is incorporated into cocoon disposal to the advantage of the colony, strengthening combs and sequestering many pathogens.

The honey bee’s system of waste management conserves resources, limits disease transmission, and allows combs to be used through many generations. And best, it’s all done without landfills, septic tanks, compost heaps, or waste treatment facilities. Maybe it’s time we take a lesson from our bee friends.

Honey Bee Suite

Notes and References

1. Traynor KS, Traynor MJ. 2015. Simple, Smart Beekeeping. Middletown MD. Image Design Publishing.
2. Mattingly RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland, Oregon. Beargrass Press.
3. Michener CD. 2007. The Bees of the World. Baltimore. The Johns Hopkins University Press.
4. O’Toole C, Raw A. 2004. Bees of the World. New York. Facts on File Press.
5. Snodgrss RE, Erickson EH, Fahrbach SE. 2015. The anatomy of the honey bee in JM Graham (Ed) The Hive and the Honey Bee (pp. 111-165) Hamilton IL. Dadant & Sons, Inc.
6. Lindauer M, Watkin B. 2015. Division of Labour in the Honeybee Colony. Bee World 34:4, 63-73.
7. Teneral honey bees have just emerged from their brood cells and are soft-bodied with a pale buff color. They will darken as their exoskeletons harden.
8. Winston ML. 1987. The Biology of the Honey Bee. Cambridge. Harvard University Press.
9. Lineburg B. 1923. What do bees do with brood cappings? American Bee Journal 63:235-236.
10. Lineburg B. 1923. Conservation of wax by the bees. American Bee Journal 63:615-616.

Note: A special thanks goes to Christopher Wren for his great photos. You can see more of his work at TrogTrogBlog and TrogTrogBee.

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  • Rusty, Excellent article and a subject that is constantly being brought up in beginner classes. I believe we need to discuss this subject, probably more at the Journeyman level than certified, but never the less it needs to be discussed. Might be a good subject for one of your great quizzes. Walt

  • Rusty,
    How do larvae eat – anatomically speaking – do larvae have an anatomical mouth or do they absorb what they are fed? That’s a question I get sometimes doing hive demos.