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Deformed mealworm beetles


I got 100 worms from the pet store back in the spring. Nursed them back to health, they grew and reproduced. Their offspring are now pupating and reproducing. Everything was fine until this week.

I just did a major overhaul of my system including cleaning, changing the feed, and trying a few different ideas for a more scalable pupae hatchery. During the process, I moved the pupae quite a few times.

The Problem

Ever since then, a lot of the new beetles are missing part or all of their shell. They seem to be getting stuck coming out of the pupa, and it doesn't fully come off like it should, so they can barely get around.

The possible causes I can think of:

  1. Stress from being moved.
  2. They are now lying on paper towel instead of plastic, maybe it's adhering to them. But I doubt it.
  3. There's a lot more pupae now than ever before, maybe they're too cramped and this is all coincidental.
  4. The pupae hatchery design itself. They're on a slope instead of flat ground now.

Is there anything I can do to save the remaining pupae from this fate?



New hatchery


P.S. Question of semantics... when they come out of the pupa, is it called "hatching" or is there a better word?


  • edited September 2017

    Hi EntoAaron, - Eclose is when larvae hatches from egg & also when adult emerges from pupae.

    There are phases of hormones at different stages in an insect life cycle. It is not as simple as one hormone switches off, while another switches on & then there is a change we can call a new form of the same bug.

    Pupae live under such phases of hormonal predominance & abeyance. Part of their transition involves forming incipient wings, another is preparing a new exterior underneath the old & loosening the bonds to the old exterior.

    When stressed pupae can pull back, or simply hold back, on commiting to the hormonal shifts that downstream cause their wing development. It seems very common to see in a normal group of pupating larvae a few beetles emerge (eclose) with defective wings & also a few whose exterior has not fully loosened from it's new exo-skeleton.

    My guess is that, what can be considered handling stress impacted their sensory signal capability to interpret that their environment was not safe to emerge into. Then their hormonal levels (titier) readjusted to delay the progression of body changes (ex: wings) that would put them past the point of no return but to eclose.

    The reason some emerged normal, some with apparent wing problems that became tolerable & yet some with permanent problems you suggest is probably due (in part) to where, as pupae, they were along the spectrum of hormonal concentrations. In the wild larvae wander off to pupate in a place they will not be bumped by living larvae. Pupae will writhe some to pump their metabolism, but a neighboring pupal writhing is presumably sensed as benign, as opposed to a disturbance.

    Fate of the weird looking adults will be a question of percentages; not all will survive, but some will. Possibly some you think are abnormally fused to their pupal shell or stumbling weakly will also be fine.

  • Interesting! That makes a lot of sense, thanks a bunch

  • Again EntoAaron, - Tenebrio molitor mealworm pupae are sensitive to touch, vibration, noise & magnetic field. The hormone ecdy-sterone (commonly referred to as the "moulting" hormone) & juvenile hormone ("growing" along hormone) have much higher levels of concentration swings than occur in the pupae than their surges seen at end of larval stage.

    Not all kinds of insect pupae have the same day segues of ecdy-sterone & juvenile hormones. Over 25 years ago published research noted that mealworm pupae have an ecdy-sterone peak during days 5-7.

    We humans (vertebrates) do not undergo metamorphosis & as such our cells' cycle goes from the "G1" stage to mitosis (division). Insect cells pass to mitosis after getting through "G2" cell cycle.

    It is elevated levels of ecdy-sterone hormone that let insect cells go from "G2" phase to mitotic cell division. If the level of ecdy-sterone hormone drops the cell cycling can pull back (so to speak) to existing in a "G2" stage until enough ecdy-sterone hormone builds up to allow mitosis.

    Unfortunately the above dynamic is not absolute in regard to all cell types & exceptions are not the same for all kinds of insects. In Tenebrio molitor elevated ecdy-sterone hormone levels will stall epi-dermal cells in the "G2" phase of their cell cycle & not enable cell mitosis. Laying down a new cuticle for the future adult is a process that starts during peaking ecdy-sterone hormone, but the proteins that will go into that cuticle are only able to be cobbled together (synthesized) a bit later (approximately during 2 days after ecdy-sterone has begun steep decrease).

    Juvenile hormone pupal level will not immediately see it's peak after the ecdy-sterone hormone peak starts to decrease, although will in a couple of days; their inter-play is not like taking turns where one is simply switched on maximum & the other switched completely off. In contrast, in the last instar transitioning to pupal stage both of these 2 hormones' maximum concentration do not raise as high as either hormone will peak in mid-pupation & are more synchronized with each other.

    The ecdy-sterone dynamic is exactly what dia-pausing (over-wintering) pupae live with (mealworms do not require diapause of course). Their cells are resting (so to speak) in "G2" phase of cell cycle & only when titier concentration of ecdy-sterone hormone rises again do they get out of dia-pause to complete meta-morphosis (mitotic division of new kind of reprogrammed daughter cells).

  • edited September 2017

    In the wild larvae wander off to pupate in a place they will not be bumped by living larvae.

    So based on this, do you suggest removing pupae from other larvae early in their pupal stage, to prevent being bumped by larvae? It is widely believed that we should remove prone pupae to prevent cannibalism, but this provides a second, more serious reason to remove them. a lot of farmers (like me) pick out pupae once a week, but maybe removing them 3-4 times a week will yield healthier beetles?

  • That's a good question, I think!

    I pick out my pupae daily so I doubt that the larvae caused the disturbance. Like I said, I had moved the pupae from container to container a couple times in one day just before it all started.

    UPDATE: All the new beetles I'm seeing now seem to be back to normal. Crisis is over. Which is especially fortunate given the timing; they're going through a pupation spurt. I was getting 5-15 pupae a day when this started but now I'm getting 50-70 a day, just a few weeks later.

  • I have noticed that the first beetles to emerge in each try seem to be the most deformed, but the majority of them afterwards emerge just fine.

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