Any process that stops, prevents, or reduces the frequency, rate or extent of the chemical reactions and pathways resulting in the formation of ecdysteroids.
Synonyms:
exact_synonym:
down regulation of ecdysteroid biosynthetic process; downregulation of ecdysteroid biosynthetic process; negative regulation of ecdysteroid anabolism; negative regulation of ecdysteroid biosynthesis; negative regulation of ecdysteroid formation; negative regulation of ecdysteroid synthesis