Leeks: Enemy #1 Onion Fly and Leek Moth.
Carrots: Enemy #1 Carrot Fly.
Here's the Matchmaker Magic: Once paired together the smell of the Leeks repel the Carrot Fly AND the smell of the Carrots repels the Onion and Leek Moth. I told you it was magic! No chemicals necessary. But how do you know how to make a Successful Match? Below I've attached a starter's guide to Matchmaking. While the guide is very basic you might find it helpful when questioning which plants will make a successful match. (Y for yes N for No)