All About Pupae

Pupae are the middle stage between mealworms (larva) and darkling beetles (adults). When a mealworm turns into a pupa it’s called pupating.

Pupae are squishy and nearly helpless. They don’t eat. They can wiggle if disturbed. They have little ridges down their sides which I think may be defensive because when they wiggle they poke at things (I've had a few in my hand that were somewhat pokey when they wriggled). There was a study regarding pupae wiggling which showed that the wiggling is a survival trait. When they cut the nerve that allowed pupae to wiggle, very many of them were eaten by mealworms while those with the nerve intact were generally not. Beetles, however, are not generally dissuaded from snacking on pupae by their wiggling so if you want a higher rate of survival you should keep them separate from beetles.

It takes about 7-9 days for pupae to develop into beetles. As they develop you will start to see darkening of the "eyes" and what looks like their legs. If the pupa starts to pulse rhythmically it's nearly developed and should become a beetle within a few hours.

Pupae Pulsing

Keeping Pupae Safe

Mealworms and darkling beetles are always on the look out for moisture-filled foods, and that's exactly what pupae are. Mealworms will try to snack on the pupae but are generally dissuaded by a pupae's thrashing about. Beetles are not so easily turned away and will eat a pupae without any regard to its wiggles. If you see black spots on otherwise healthy pupae, that’s a clear sign that the mealworms have taken a few nibbles at your pupae.

If you don't care about maximum production, you can take your chances and generally enough pupae survive to turn into more beetles to keep your cycle going, but if you want to prevent pupae snacking, you must separate them from the mealworms and the beetles. There are two major ways of handling this.

  1. Separate container for pupae. Just about any container will do, but you'll want something plastic so that new beetles can't escape. You'll need to pick out new beetles once or twice per day and place them into your beetle bin by hand.

  2. Pupae bridge, table, or boat. This is a separate container for your pupae that lives in your beetle box. A pupae bridge or table is simply a little raised platform (generally plastic) that the pupae sit on. It generally has a slightly raised edge so the pupae can't easily roll themselves off. New beetles wander off the edge and can't get off. A "boat" like container, is a shallow container with plastic walls that has a cardstock "plank" coming out of it. The new beetles "walk the plank" and again, fall into the beetle bin and can't get back.

I haven't noticed any interest in pupae-snacking from new (white or light tan) beetles. They seem to take about a day or two to come to terms with their new state in life before they seem to bother eating or drinking anything.

Although you can buy pupae bridges online, you really don't need to spend money on this if you have any plastic containers, maybe left over from food that comes in any reusable plastic storage container. You can take any plastic container with a reasonably flat bottom or top, turn it flat-side up, and set your pupae on top. You don’t need a fancy pupae bridge. You just need something plastic that the beetles can’t climb up after they crawl off it that’s big enough the pupae don’t roll themselves off too easily. I did make myself a couple of little acrylic pupae tables, but I still frequently just use the lid of a deli container as my pupae platform. Eventually I will take pictures of my acrylic pupae table and offer it for sale, but if you're interested, contact me and I can probably make you something.

Pro tip, if using an plastic container, cut the walls of your container to make a little “cave” for the beetles underneath. They do like the dark and cover from above.

Dead Pupae

Pupae turn a kind of gray-ish color when they die, and then eventually turn black. If you poke at a pupae and it's a weird gray color and it doesn't move, it's probably dead.

I feed "fresh" (gray but not black yet) dead pupae to my chickens who absolutely love them.

Environmental Factors for Survival

There are lots of thoughts about pupa survivability, much of which comes down to moisture. It is believed that if a mealworm doesn’t have enough moisture before pupating it may die in the pupal stage or it will have deformities as a beetle. I've seen one individual claim that they separated their deformed beetles and let them breed, and found that they had a higher rate of deformities in their next generation than from their non-deformed beetles. Thankfully deformed beetles make excellent chicken snacks, especially if you feed them when they’re still white and squishy. I feed all deformities to my chickens so I can’t say for certain whether deformities will be passed on.

Other ideas include -

However it is frequently discussed that all that matters is moisture intake before pupation, so there may be little we can do to improve viability once pupation has occurred. I would assume that excessive heat or dryness would still kill them, as would trauma or squishing. I'm also under the impression that the refrigerator will kill them but I have my doubts.

Finding Answers

I am currently running some experiments to see if I can alter/improve pupae viability. I will report my findings when I have some but so far I haven’t received any statistically significant results.

At this time it seems like increasing humidity from 30% to 70-90% has no statistically significant impact.

I also attempted putting pupae in the fridge for up to two weeks and have not received a statistically significant result. It seems that refrigeration for up to two weeks doesn’t materially affect pupae.

See more information on my experiments here.