Another hypothesis has emerged regarding my Oryctes nasicornis and Cetonia aurata. Heard it from an Latvian entomologist.
Supposedly since I collected larvae in the winter (around 0 degrees C), warm winter for Estonia but still.. the larvae were in a hibernative state. And since I brought them in a warm room (23-24 degrees C) so quickly (after a few hours), the larvae experienced a shock because they could not adapt to this temperature rise that quickly. But some pathogens or nematodes (that are much smaller than the larvae) in the substrate become active much quicker. And while the immune system of the larvae was weakened, the nematodes or something started damaging the larvae, causing the black spots to appear and eventually the death of all of them. This could also explain why the C. aurata collected in the early fall (much warmer outside and no hibernation yet) had no "black spot".
Don´t know whether this is true but is seems a plausible hypothesis alt least. In other cases of "black spot" something else could have weakened the larvae (bad substrate?).
To me this seems to support the idea that the visible black spots are the reaction of the larvae against some pathogens or parasites.
Supposedly since I collected larvae in the winter (around 0 degrees C), warm winter for Estonia but still.. the larvae were in a hibernative state. And since I brought them in a warm room (23-24 degrees C) so quickly (after a few hours), the larvae experienced a shock because they could not adapt to this temperature rise that quickly. But some pathogens or nematodes (that are much smaller than the larvae) in the substrate become active much quicker. And while the immune system of the larvae was weakened, the nematodes or something started damaging the larvae, causing the black spots to appear and eventually the death of all of them. This could also explain why the C. aurata collected in the early fall (much warmer outside and no hibernation yet) had no "black spot".
Don´t know whether this is true but is seems a plausible hypothesis alt least. In other cases of "black spot" something else could have weakened the larvae (bad substrate?).
To me this seems to support the idea that the visible black spots are the reaction of the larvae against some pathogens or parasites.