The number of thrips species that have been inadvertently introduced to California, presumably through human activities, is now at least 60. Some of these will have been introduced many years ago, such as the greenhouse thrips, Heliothrips haemorrhoidalis, and the fern thrips Leucothrips nigripennis, both of which presumably came from Southern Brazil (Mound & Monteiro, 1997; Mound, 1999). In contrast, Scirtothrips perseae and Neohydatothrips burungae have both been found in California only in the past 10 years, both of them in association with the avocado orchards (Hoddle, 2002). Moreover, the Oriental gall-invading species, Androthrips ramachandrai, has been recorded from California only very recently, together with Gynaikothrips uzeli, the leaf-rolling thrips on Ficus benjiamina (Boyd & Held, 2006). Such invasions are continuing, largely as a result of the expanding international horticultural trade involving transport by air, rather than by sea. One Oriental species, Liothrips floridensis on Cinnamomum camphora leaves, has been known in Florida for many years, but was recognised in California only in 2011. Similarly, a healthy population of an Australian species, Dendrothrips howei, was discovered at a Los Angeles plant nursery in 2010, and by 2012 was considered established and causing damage to its host plant. The difference between "introduction" and "establishment" is emphasised by the records of the crop virus vector species Frankliniella schultzei. Only two females of this species have been identified from California, both from areas of native vegetation just north and northeast of San Bernadino, but by March 2012 no evidence was available of any substantial population of this pest.
It is worth remembering that, during the peak marketing seasons in North America, up to 35 flights have been known to leave Colombia daily, fully laden with plants and flowers for Miami and Los Angeles. Because of this massive trade in horticultural produce, the identification and information system presented here includes nearly 40 species that are currently not established in California, but that are considered potential immigrants, particularly from Hawaii. For example, the citrus pest, Pezothrips kellyanus, and a Pacific flower thrips, Thrips maculicollis, have both been intercepted many times at Sacramento in floral leis arriving from Hawaii.
There is limited biological or geographical pattern concerning the origin of the immigrant species. Early European settlement involving sailing ships presumably introduced many thrips, such as grass-living species associated with hay (Limothrips cerealium) or within grass seeds (Chirothrips manicatus), and several fungus-feeding species arrived presumably on dead wood (Mound, 1983). One species, Cartomothrips browni, has been widely distributed around the world from Australia in the seed capsules of various Eucalyptus species. Another, Amynothrips andersoni, has been introduced to California deliberately, as a biological control agent against a weedy plant. However, most of the introduced species are phytophages that have presumably been transported accidentally on live plants.
The species listed here as potentially invasive include a range of widespread pests in warmer parts of the world, such as the red-banded cocoa thrips, Selenothrips rubrocinctus, and the melon thrips, Thrips palmi, as well as the Southeast Asian species, Thrips parvispinus, that is now a pest in Hawaii. Also included are species from the Caribbean area, Frankliniella bispinosa, F. cephalica, F. invasor and F. parvula. The horticultural trade in Ficus species has introduced the Oriental species Anascirtothrips arorai into Florida, and several other species that are common in the Caribbean must be considered potential immigrants to California, including Haplothrips gowdeyi and Anaphothrips sudanensis. Again, there is little pattern to the species selected for this list except that all of them have been found in association with crops in other parts of the world. Two potential immigrants are the rice thrips, Stenchaetothrips biformis, and some of the thrips associated with sugar cane such as Podothrips lucasseni. The inclusion here of one particular species, Oxythrips divisus, requires explanation; there is an old but unsubstantiated record of a European species of this genus from California, thus it is possible that one or more species of Oxythrips will be found eventually in California, and O. divisus is included here as a reference point.