We are not sure. Scientists have recently proven that one of the ingredients, Origanum (oil from the oregano plant), damages the cell wall of microbes and causes them to leak. Then, it seems, menthol can seep into the cell. Once inside the microbe, menthol wrecks its metabolism. How menthol works has been known for years, but microbes can resist almost any amount of menthol if their cell wall is in one piece.
This does not seem to happen with animal or plant cells. This is another way our products are unique.
As to how it kills insects, we simply don’t know.