Mealworm pupae optimum environment

I'm experiencing a bit more pupae die off than I'd expect. I keep my growing environment between 80-85 degrees F and 70-80% humidity. I separate the pupae every other day and keep them separated from mealworms/beetles.

Should they be covered with wheat bran or left exposed without any bran on them?

Any other suggestions on what I could do to reduce the die off?

Thank you!


  • Hi oskizzle, - I get the impression you have a heap of pupae; possibly in a room where the temperature gauge is not in their midst. For now see if the following is a guide.

    Mealworm Tenebrio molitor pupae during their early days keep pressure in their circulatory haemolymph down. They still have metabolic generated CO2 & since pupal haemolymph pH is naturally low they have limited CO2 neurtralization capability inside

    Mealworm pupa uses their tracheal system to let CO2 out whether actively pumping their circulatory system or resting via both heart pumping & abdominal pressure pumping of their haemolymph. In early pupal stage the pumping activity pattern is less (sometimes only every 1-5 hours) than when the pupae goes pharate, when pumping becomes regularly needed.

    To get the internal CO2 out mealworm pupae (not all insects do this) do not fully close their pupal spiracles (tracheal system contact structures with outside air). Nor do they sip air through those spiracles intermittently.

    This feature keeps pupal interior pressure low & the result is moisture in the air can condense on the walls of a trachea. It an important way they keep from drying out.

    As temperature goes up the volume of CO2 metabolically produced (internal enzyme activity rises with temperature) also goes up & this needs to gas out from pupal spiracles in order to keep interior cell pH from going too low. Once the pupae are dealing with elevated temperature (both from internal metabolic "heat" & external environment mass of pupae generates) there is some upper limit of CO2 volume pupae exudes.

    The problem is that beyond the upper limit of CO2 volume a pupae can deal with their spiracles fail to cope. As more & more spiracles lose function the volume of CO2 the pupae can dissipate goes down; depending on how much this happens the volume of CO2 gotten rid off crashes so low it afflicts the internal cells with too low pH for cellular functioning.

    Once a pupae dies it gasses out CO2, but this is not from life's metabolism; rather downstream events due to dying cells mitochondrial membrane losing their integrity & protons from the mitochondrial energy generation system let loose. Of course that CO2 goes out among the general pupal population until dissipates & adds external gas challenges to the live pupae spiracles.

    The fact that circulatory pulses are not relate to pupae spiracle valve ventilation may indicate your pupal conditions are too dense. Remember larvae naturally wander away before pupation seeking a different type of location.

    A bit of bedding should give them more insulation from rising temperatures (day's & cohort's) plus some spiracle clearance. I would suggest trying a shorter light period (10 hours?) as well, although their pupation duration will probably be longer.

  • Very thorough as always, thank you much.

    I have my mealworms in an enclosed space, temp and humidity controlled. No light at all.

    Yes, I have a boatload of pupae in a container with wheat bran, about 1.5 to 2 inches deep.

    Would it be better to spread then out and not have them too deep?

  • The preceeding & following is speculation of your issue. If it is actually relevant (& can not think off other issue) then giving them more bran beeding would be worth trying.

    Although I am not sure your transfer of pupae is an issue predisposing some to vulnerability down the way.(Quote): "... a portion ... 13-15% ... highly sensitive to handling ... lost significantly more water ...." From (1998) Harak, et al. "Calorimetric investigations on physiological stress in Tenebrio molitor ... pupae", journal Thermochimica Acta, Vol.309(1-2)

    If you can try to get a few different times (in 24 hours) of temperature readings down where pupae lay. Technically your ambient temperature (& relative humidity, as well as darkness) are fine for pupae. If pupal bunch interior often high in sectors consider lowering room temperature.

    Still you may find giving the pupae a little different environment than they had as larvae might lower pupal mortality rate. Tenebrio molitor pupae do not diapause & in the wild mealworm larvae pupate in the spring.

    In other words in a natural setting there are both temperature & photo-period changes related to pupation. Since your larval herd is entrained to their "ideal" growth environment & you don't want to go hotter/lighter then possibly the pupae would respond better by going lower in temperature & lower in light.

    Again, I would try 10 hours of diffuse ambient light with 14 hours of dark "no light at all" cycles (I assume larvae are getting 12 hours or more of gentle light during development). And similarly, get temperature down just a little bit to 25°C (77°F). Consider this a combination photo-period + temperature option; although expect pupation to last longer.

    By the way, the contractions of muscles between pupal abdominal segments do help them move CO2 out. As opposed to heart pump "circulatory pulses" mentioned above, in case was vague.

  • I have lowered the temperature in my enclosed raising area to 78 F. I currently have zero light at all, as they are in an enclosed box basically that I open to feed/clean/maintain. I will attempt to add ambient light for them.

  • Ambient light and spreading the pupae out appear to be successful. Less die off than before, will continue to monitor.

  • Bumping this because refered to it recently.

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