Most species appear to be narrowly restricted in their feeding preferences, either to one or two species, or a genus of plants. Some subfamilies of Cicadellidae and many Delphacidae seem to be confined to feeding on species of Poaceae. Perhaps as few as 1% of the known species are pests of crop plants where their host range interacts with plants used for food. Sometimes it is clear that a wider range of hosts are utilised as feeding hosts than those on which nymphal development takes place. The majority of leafhopper species feed from phloem tissue, while some feed from xylem tissue and others from the contents of mesophyll cells. As far as is known most planthoppers feed from phloem tissue, but several families seem to feed as nymphs from fungal hyphae, especially those of the family Derbidae, species of which are frequently abundant in the tropics.