The Beekeeper’s Companion Since 1861
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The Curious Beekeeper

The Puzzling Pupa and its Cryptic Cocoon

- October 1, 2022 - Rusty Burlew - (excerpt)

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 the 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.

They 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 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 ….