The present invention relates to genetically-modified oleaginous yeasts for producing non-native sterols at commercially useful levels, especially for example in providing sterols individually or as a mixture in an artificial dietary composition for honeybees or other insects or animals. For this purpose, an oleaginous yeast, e.g. Yarrowia lipolytica, may be employed wherein the yeast has reduced production of ergosterol compared with a wild-type oleaginous yeast or is incapable of producing ergosterol and is provided with a sterol surrogate to aid growth. From such yeast, however, other yeast may be engineered which retain useful sterol production without need for a sterol surrogate, e.g. production of sterol mixtures in which 24-methylenecholesterol or campesterol is the dominant sterol.