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, marigolds are highly attractive to thrips and can serve as an indicator plant drawing them out of the crop. When predatory mites are released unto their foliage, the marigolds serve as a trap plant where thrips are attracted and killed. Because marigolds provide a source of pollen and thrips for the predatory mites they are a habitat/banker plant, enhancing mite survival and reproduction.
See the fact sheet: Plant –Mediated IPM Systems Explained by University of Vermont