The puzzling guide to bee cocoons & how to see them

Inside: Because honey bee cocoons are spun inside a brood cell, we seldom see them. Here is a detailed look at where and how they are formed.

We all know honey bees spin cocoons, right? It’s a fact backed by every beekeeping book ever published. But talk to enough beekeepers, and you will soon learn most of them have never seen a bee cocoon, at least not one they remember.

Intrigued by this anomaly, I read everything I could about cocoons and pupation. The first thing I learned was a surprise: Not all bees spin cocoons. In fact, most species do not.

Which bees build cocoons?

The only two bee families that have plentiful cocoon spinners are the Apidae and the Megachilidae. These two families comprise the long-tongued bees. Throughout the remaining five bee families — those comprising the short-tongued bees — cocoon-building is rare. Taxonomists consider it a primitive behavior in solitary bees, passed down from their ancestors, the hunting wasps. Over time, cocooning disappeared from most of the solitary bees.

Many species In the Apidae family spin cocoons, except for the Anthophorinae (robust digger bees), Xylocopinae (carpenter bees), and most of the cleptoparasites. That means all the honey bees, stingless bees, orchid bees, bumble bees, and many others spin cocoons.

Every bee species in the Megachilidae family builds cocoons.¹ This group includes all the mason bees, leafcutters, woolcarders, pebble bees, and many others. They make strong, dark-colored cocoons that resemble Jelly Belly beans. They are durable enough to harvest, store, wash in bleach, and ship through the U.S. Postal Service. And that’s saying something.

Protecting pupae from danger

Instead of spinning cocoons, many “modern” solitary bees line their brood cells with watertight and pathogen-resistant substances. Most use secretions from the female’s Dufour’s gland or floral oils collected in the field to coat the cell walls. These durable finishes are painted on by Mom when she’s not foraging or laying eggs. The treated brood chambers are protective enough that cocoon-spinning — an energy-intensive endeavor — is simply unnecessary.²

All bees have a Dufour’s gland near the stinger, but the function of its secretion changes from species to species. Some species use it for nest building, as stated above. But In other species, the secretions are used to enrich larval food or increase pheromone production.

In honey bees, the Dufour’s gland appears to stimulate retinue formation. Queens secrete more than laying workers and laying workers secrete more than regular workers. Some researchers believe the queen paints her eggs with Dufour’s secretions so the workers can tell real queen eggs from laying worker eggs.³

It seems counterintuitive that pupae developing in well-protected cavities, such as mason bees and honey bees, would need a cocoon while bees in a soil cavity, such as ground bees and digger bees, would not. Some scientists think the primary purpose of the cocoon is to protect the young bee from its own feces, but that doesn’t explain why most bee species don’t spin them.

A closer look at the cocoon spinners

Regardless of whether they spin cocoons, all bees are holometabolous insects. Loosely translated, that means a bee goes through all four life stages of complete metamorphosis: egg, larva, pupa, and adult.

As with similar insects, cocoon spinning occurs when the bee transitions from a larva to a pupa, during the fifth larval instar. You may have seen the firm, brown cocoons of mason bees. Or perhaps you’ve seen the curiously shaped cocoon of a moth or the chrysalis of a butterfly. In all cases, the cocoon safely seals the pupa from the outside world.

In bees especially, the pupal stage is relatively short, usually lasting only a week or two. During this brief time, the organism is redesigned. The internal organs of the larval stage are dismantled and reassembled into those needed for the adult stage.⁴ This is no small chore.

Because so much can go wrong during the rebuild, the faster it proceeds the better. Long pupations provide more time for predators, diseases, inclement weather, or other misadventures to befall the developing insect. It’s best to get it done, and done fast. In some bees, like masons and leafcutters, the pupation happens quickly, but the newly emerged insect stays within the cocoon for long intervals, perhaps months, as a fully grown adult.

Bees eat, and then they don’t

When you analyze the four life stages of complete metamorphosis, you see eating stages alternate with fasting stages. The completed egg is a fasting stage in which stored food is used to build a larva. The larval stage is an aggressive eating stage, characterized by uncontrolled appetites and tremendous growth. Next, the pupal stage is the fasting stage made possible by the larval stage. And finally, the adult is another eater. Clearly, the stages alternate between eating and using what was previously eaten.

Egg production in honey bees

The egg is the link between generations. Although the egg is in a fasting stage, the queen produced it. She needed copious amounts of food to create and nourish the eggs that — without supplementary food — would produce the next generation of larvae.

An egg cell originates in the ovary and matures as it travels through one of the queen’s ovarioles. An ovariole is simply a long tube. A queen has many ovarioles in each ovary, running parallel to each other. Although the queen has lots of eggs, they don’t develop all at once. Instead, they mature in small batches as they pass through the ovarioles.

These immature egg cells are called oocytes, and the first thing they do is divide repeatedly. Eventually, 48 nurse cells surround each oocyte. The sole purpose of the nurse cells is to nourish the developing egg.

As the oocyte matures, the queen tops it with yolk precursors that she absorbs from her own hemolymph, much like a squirt of mustard, yellow and gloppy. The oocyte absorbs nutrition from both the nurse cells and yolk precursors to form a large (comparatively speaking) yolk as it continues to travel through the ovariole. At some point, a layer of chorion (soft eggshell) coats the egg.⁵

After the mature egg enters the oviduct, it becomes fertilized (or not) before the queen deposits it in a brood cell. Later, the yolk will feed the embryo as it develops into a larva.

Larvae eat themselves silly

To say larvae are aggressive eaters is an understatement. The larvae of butterflies and moths can mow plants to the ground, leaving nothing but skeletal veins. Right now, for example, cabbageworms, the larvae of the cabbage white butterfly, are devouring cole crops across the continent.

Incredibly, about 85 percent of the worst crop pests are larval stages of some innocuous-looking insect. On the bright side, however, the larvae of the introduced cinnabar moth are now chewing their way across pastures of tansy ragwort, an invasive plant dangerous to livestock.

Honey bee larvae have equally voracious appetites. Some sources estimate that a honey bee larva increases its weight 1500 times in 5.5 days. That kind of growth requires a serious commitment and plenty of munchies.⁶

The larvae store the food in fat bodies for use during the pupal stage when the bee transforms into an adult. Cocoon spinning is the last activity of the larval stage.

Rolling into a protective cocoon

Inside the newly capped brood cell, the larva somersaults head over tail in a move similar to the forward rolls you learned in elementary school. As the bee rolls, it produces four substances. The silk glands, near the mouth, secrete a clear liquid in long strands. In addition, bees excrete three substances from the anus: one is clear, one is yellowish, and one is feces.⁷

As the larva somersaults within the brood cell, the strands continue to exude from the bee’s body. The movement continues until two or more layers of cocoon cover the entire larva, top to bottom and all around. Although the cocoon material must be water-resistant, it also must protect the bee from drying, pathogens, and parasites. At the same time, it must be permeable to the gases of respiration, including oxygen, carbon dioxide, and water vapor.

In honey bees, the newly spun cocoon looks like a cellophane sack. Unlike a moth cocoon, a butterfly chrysalis, or even a mason bee cocoon, we never see an empty, abandoned cocoon. But why not?

Honey bee cocoons stay behind

After the pupa develops into an adult, the bee emerges from her natal cell, leaving the cocoon behind. Adhering to the cell like wallpaper, it will not release. And the feces remains sequestered, nature’s way of keeping the brood cell clean for the next generation.

Since cocoons stick forever inside the comb, we seldom see them. What we do see is the darkening color of brood combs after successive generations of bees. The people who measure such things say the insides of brood cells get smaller as multiple cocoons and waste deposits become part of the comb structure.

The pupal stage of bees

Most articles gloss over the pupal stage of bee development. Within a paragraph or two, they explain that during the pupal stage, a bee transitions from a larva into a fully formed adult. But they seldom emphasize that the transformation is an incredible test of the imagination.

Safely secured in its cocoon or waterproof cavity with no additions of food or water, the larva completely deconstructs itself. The larva’s internal organs that were used for eating and processing brood food are dismantled. The tubercles that separated the larva from the cell wall disappear, and the spinnerets are decommissioned.

The bee uses the parts inventory generated by the deconstruction to build an entirely new body. Outside of the brood nest, the adult form requires different organs and additional accessories. The new bee needs a tough exoskeleton and glands for producing pheromones, brood food, and wax. The silk-producing glands, now unnecessary, become salivary glands.

Besides the inner workings, the bee needs tools for navigating the outside world. The pupa develops legs, wings, antennae, hairs, and eyes — none of which were necessary for life in a tiny dark cubicle. The bee needs structures for finding and carrying water, pollen, nectar, and plant resins.⁸

We’ve all seen pupae pulled from their cells in various stages of growth, but they always appear static and white. In truth, the changes are rapid and non-stop, controlled almost completely by hormones. In honey bees, the transition requires roughly 7 to 10 days, depending on caste and sex. Like butterflies, the creature that crawls from a cocoon bears little resemblance to the creature that built it.

Cocoon placement: where it goes

Many references insist that the larvae glue the cocoon to the cell wall as they spin it. However, if you study the pictures, you can clearly see the cocoon wrapped around the larva’s body.

Only when the pupa begins to develop into a full-sized bee does it expand to fill the entire cell. At that point, the cocoon becomes pressed against the cell wall, where it becomes stuck. 

This is easy to imagine in an environment where everything is sticky and gooey. By the time the adult emerges from the natal cell, it is a cocoon-free callow bee, soft and pale. In time, the exoskeleton hardens, the wings expand, and the bee readies for life as a fully mature adult. 

So many kinds of cocoons

Honey bee cocoons are unique in appearance. Throughout the bee species, cocoons may be papery, rough, or soft. They may be opaque, mesh-like, translucent, or clear as plastic wrap. And they may be white or various shades of yellow, tan, or brown. Even the layer count varies, making each one a surprise.

I’m always impressed by honey bees because of the unique adaptations they’ve made throughout the millennia. The honey bee cocoon is another surprising feature, similar to other bee cocoons yet honed to perfection for the specific job it must do.

Rusty
Honey Bee Suite

Acknowledgment

Since I was beeless at the time, I asked my friend and fellow beekeeper, Naomi Price, if she would photograph honey bee cocoons. I didn’t know what to expect, but what she achieved blew me away. Thank you so much for your perseverance in a tedious task. Now we can better understand why finding a honey bee cocoon is an unusual occurrence.

References

1. Danforth BN, Minckley RL, and Neff, JL. 2019. The Solitary Bees. Princeton, NJ. Princeton University Press.
2. Michener CD. 2007. The Bees of the World, Second Edition. The Johns Hopkins University Press, Baltimore, MD.
3. Mitra A. 2013. Function of the Dufour’s gland in solitary and social Hymenoptera. Journal of Hymenoptera Research 35: 33-58. https://doi.org/10.3897/jhr.35.4783
4. Houston T. 2018. A Guide to Native Bees of Australia. CSIRO Publishing, Clayton South, Victoria.
5. Snodgrass RE, Erickson EH, Fahrbach SE. 2015. The anatomy of the honey bee in JM Graham (Ed) The Hive and the Honey Bee. Hamilton IL. Dadant & Sons, Inc.
6. Traynor KS, Traynor MJ. 2015. Simple, Smart Beekeeping. Middletown MD. Image Design Publishing.
7. Jay S. 1964. The Cocoon of the Honey Bee, Apis mellifera L. The Canadian Entomologist, 96(5), 784-792. doi:10.4039/Ent96784-5
8. Winston ML. 1987. The Biology of the Honey Bee. Harvard University Press, Cambridge MA.