Children's or middle grade, fantasy. Female protagonist with a ladybug "guide"

For some reason I had it strongly associated with the phantom tollbooth and Momo in my head. In fact I was SURE it was Momo until I started rereading THAT book.

I remember the ladybug being vaguely grumpy and I remember that being where I learned about ladybugs and aphids...

I remember there being some dark power that for some reason the girl was going to have to fight it?

Help!