Try to make your search as simple as possible:
Stevia will find all items including that sequence of letters.
Search results for "Stevia":
"Stevia aristata var. villarice" - Stevia villaricensis
"Stevia oxylaena" - Stevia selloi
"Stevia purpurea" - Stevia eupatoria
"Stevia sphacelata" - Palafoxia sphacelata