Mealworm Pathogens

What are some common pathogens in mealworm colonies?

I have a medium-sized mealworm operation, but have recently been struggling with some form of pathogen in the colony. I am unsure of whether is it fungal, bacterial or parasitic, or perhaps due to the rearing conditions. It began with the majority of the pupae developing black spots and strange, dark, lumpy growths on their heads, backs and near their legs. Some of them would die and turn black, however, the ones that do complete pupation (~30%), the beetles emerge with wing and leg deformities. The beetle mortality rate is quite low, and the beetle fecundity does not seem to be impacted. More recently, I have noticed that some worms in their later instars have bubble-like growths on their legs - as if the tissue has bubbled up around the leg, as well as black lumpy bits on their 'tails'. I have been feeding my worms apples from an organic orchard, so that could be a potential source of transmission. My growing conditions are pretty consistent: 25-27 degrees celsius, and 65-70% humidity. My rearing density is likely higher than ideal, and pupae in which I first started noticing this issue were from old beetles (~2 months).

Has anyone had similar issues or have any ideas of what this may be, or potential avenues to investigate?



  • I've not seen this before, do you have pictures of what you're describing?

  • Strange indeed. Here are a few photos:





  • Hi SJohnston, - Yes, there are pathogens that mealworms can get. My inclination is that more likely the specific “organic” orchard’s apples should be looked into.

    Your last picture looks a little bit to me like an earwig latched onto a mealworm pupae for (presumably) feeding on pupal hemolymph. There are many varieties of earwigs & some as small as 5 mm (~1/3 inch); in orchards they feed on other insects (1 kind of larger earwig is even sometimes introduced as a biological control of aphids in apple orchards).

    As regards growths and deformities: these sound like out-of-phase hormone transitions (segues of hormones at transitional growth phases is discussed elsewhere in the Forum; try search box function with phrase “juvenile hormone”). In agriculture insect hormone interrupters are used to impair bug colony levels & there are also some “organic” products that work by disturbing normal progression(s) of hormones in different insects.

  • I have connected with an entomologist who works with USDA and he suggested it was cannibalism - the lumpy growths being dry hemolymph that has oozed out of wounds where larvae have eaten the pupae. We tried picking the pupae out of the larvae bins much more frequently and reducing out larvae densities and that seemed to improve the problem. Thanks for the suggestions!

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