Being the solution is a contact type of spray, it will be very unlikely than many if any earthworms would be affected by it. It does kill the slugs and snails on contact (they melt away) and (I think it acts as a deterrent and not an attractant). I have lots of earthworms and I have never noticed any dead ones after spraying.
I would certainly try the product you bought and see how it works, all the reviews I read on it seemed positive.
Then post back what you think of it.
