Guardian plants combine multiple interactions between plants, pests and their natural enemies. They attract the pest and beneficials and are used to monitor for pests and assess natural enemy effectiveness. They provide a sustained food source which ensures the survival of natural enemies. For example in this photo, Sweet Alyssum (Lobularia maritima) is being used as a guardian plant for this lettuce crop to manage aphids. Alyssum enhances the activity of syrphid flies to suppress aphids in lettuce.
See the fact sheet: Plant –Mediated IPM Systems Explained by University of Vermont