It is a good idea to use yellow Glue Traps to monitor / control whitefly alongside Encarsia. Yellow Glue Traps should be hung next to plants early in the season to monitor & control any early whitefly. These sticky Glue Traps can be left in position once the Encarsia has been introduced - place them next to the growing point of the plants and raise as the plant grows. This way the Encarsia will not be trapped as they tend to congregate nearer the lower leaves - the traps will only catch whitefly, which fly to the tops of the plants. For plants such as tomatoes, hang the traps with string from the greenhouse roof, so they can be raised as the plant grows. Whitefly are attracted by the bright colour and become stuck to the trap. They should be changed as they become dirty. Glue Traps are available in packs of 10.