The thing about the withermoth is, that it only showns itself when you get close, too close.
Well, that is folly. You can see where a Whithermoth is if you look closely. You see a moth-shaped pattern in the grass that almost has the same colour as its surroundings, but there's always a shimmer of blue that you can see.
So if you're scared to death by these things, just take caution in the Highlands and have a keen eye.
I hope this might save some Hardcore Characters' lives :)