There's only one way to find out. Wikipedia said that they mainly eat grass and grain crops, so you shouldn't have much to worry about from phytotoxins in their gut contents or bodies. Just try feeding them to your least favourite axolotl first (I have loads, and I test new things out on my axolotls, preferably a juvenile, before I expose any animals that would be more difficult to replace, just in case it proves fatal). I tried feeding gooseberry sawfly larvae
to one of my axolotls, but it wasn't keen on them. My gooseberry bushes get ravaged by them every year.