Among the four entomophthoraceous species, Entomophthora coronata, E. apiculata, E. thaxteriana, and E. virulenta, only the first two species produced culture filtrates that were toxic when inoculated into the hemocoels of the larvae of seven insect species. The culture filtrates, however, produced no apparent toxicity when fed to the larvae of Galleria mellonella. The fungi produced mycotoxins in all media suitable for their growth in vitro and in vivo. The mycotoxins produced by the two fungi had similar properties and appeared to be identical, but E. coronata produced much more toxin than E. apiculata in the same period of time. More toxin were produced by both fungi in shake than in stationary cultures. The median effective doses (ED₅₀) of the culture filtrates (96-hour cultures) were estimated from data.

The mycotoxin was not chemically identified, but tests indicated that it was a protein of small molecular weight. The mycotoxin caused a characteristic blackening sign in inoculated larva of Galleria mellonella; it also inhibited metamorphosis and affected larval feeding. These signs and symptoms varied among the seven insect species. The mycotoxin also inhibited bacterial growth in dead G. mellonella larva.

In Galleria mellonlela larva, the mycotoxin apparently altered the staining properties of the hemolymph and caused the clumping and coagulation of the hemocytes. Silk gland cells also showed pathologies, but other tissues and organs appeared to be unaffected. Blood color changes and clumping of the hemocytes did not occur in fungus-infected larvae.