Poinsettias are susceptible to many root rot organisms, but injury from
fungus gnat feeding is frequently misidentified as a root rot. Potato
slices pressed into the surface of the growing mix can be used to attract
and more easily detect the larvae. The flies may be monitored with yellow
sticky cards. Geraniums, poinsettias, vinca (Catharanthus) and ivy seem
to be especially prone to infestation.