The petunia cultivars Calypso, Super Blue Magic and Summer Madness have been shown to make good indicator plants for several reasons. First, these varieties are highly attractive to western flower thrips. Secondly, infected petunia plants do not serve as source of virus in the greenhouse because INSV does not become systemic within these petunias. Infected leaves can be picked off and discarded and petunia indicator plants can remain in the greenhouse to continue monitoring. Another reason is that the virus lesions show up very soon after thrips feed.
Thrips feeding injury on the foliage leave distinct white feeding scars.
If the thrips are carrying INSV, a brown rim and, later, a circular lesion can be seen around the white feeding scars as early as 2-3 days after exposure to thrips. It is important to attract and encourage thrips feeding on the foliage of indicator plants. To encourage thrips to feed on the foliage of indicator plants, keep the petunia flowers removed from the plants and place a blue non-sticky card in each pot at plant height. The blue card will attract thrips to the vicinity and increase the chances that they will actually land on the petunia plants. Blue plastic picnic plates cut in half work well for this purpose.