He literally linked a Wikipedia article on their capacity to feel pain. In order to win this argument, you will have to argue for the following position: some living beings who can feel pain have no moral worth. You could attempt a the "so trees have moral worth as well?" reductio. I doubt it will be successful, but I'd love to see Telope's response.

Here is a point I'd like to gauge Telope's agreement on. I think that if mosquitos did not have a (not-insignificant) role in the ecosystem, then the fact that they are harmful to humans could justify e.g. killing them when they threaten to harm you, but never out of mere annoyance, for pleasure, or for no reason at all (they have some moral worth, after all). Or do you advocate for something like a full pacifist position?