Peppermint leaves were washed but not visually checked. They were then added to lemon syrup which was cooked and strained. Can one use the syrup?
It seems that the syrup can be used.
I don’t believe that the leaves are a level of muchzak be-tolaim.
Even if they are, if it is usual that rinsing with water gets rid of bugs, the chazakah will not carry over after rinsing, and because the leaves were cooked there is a safek sefeika (see Shulchan Aruch 84:9 and Shach 39).
Therefore, although the leaves must be checked, bedieved the syrup is fine.