Hi to everyone

I would like to ask about a kind of problem we are reporting in our laboratory colony of Ostrinia nubilalis. Larvae have started showing these blackish thoracic legs, and eventually "necrotic" spots appear along their body. However, they are performing well in the artificial diet, and no mortality is observed. We do not know whether these symptons may be compatible with a virus/fungal/bacteria infection.

Any idea about what is happening?

Thanks in advance