Wheat seedlings germinating in the presence of the systemic fungicide fenpropimorph accumulate 9beta,19-cyclopropylsterols (95% of total sterols) in place of Delta(5)-sterols, which are normally produced in these plants. Adult females of the phytophagous insect Locusta migratoria show a dramatic decrease in their cholesterol content when reared on fenpropimorph-treated wheat. These females lay eggs with the ecdysteroid concentration reduced by up to 80% as compared to controls. Injection of fenpropimorph to the insects or feeding them on wheat coated with the fungicide (normal sterol composition) does not affect their sterol or ecdysteroid profiles; addition of cholesterol to fenpropimorph-treated wheat prior to feeding restores normal ecdysteroid titers in the insects. The severe reduction of the ecdysteroid content in eggs laid by females reared on fenpropimorph-treated wheat is associated with a series of developmental arrests and/or abnormalities. The results show that the dietary 9beta,19-cyclopropylsterols cannot be used by Locusta in place of Delta(5)-sterols for ecdysteroid biosynthesis. They suggest that the selective inhibition of specific enzymes in the sterol biosynthetic pathway of the plants can be used as a strategy to control insect development.