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The Western Flower Thrips (Frankliniella occidentalis) is an invasive species and the most economically important of the approximately 5500 described species of thrips. This thrips causes enormous damage by feeding on greenhouse vegetable and ornamental crops and by transmitting plant-pathogenic tospoviruses. It is native to western North America, west of the Rocky Mountains from Mexico to Alaska, but since the 1970s has established across North America and invaded most countries in the world. It is now present on every continent but Antarctica. Brunner & Frey  (2010) identified two ecotypes corresponding to different climate regimes, possibly representing two cryptic species between which reproductive isolation has evolved, as suggested by Rugman-Jones et al. (2010).

Morphological identification of thrips is difficult because of their small size and similar appearance. Zhang et al. (2012) developed a simple PCR-based test to distinguish invasive Western Flower Thrips in China (where the species was first reported in 2003 and was established as a serious pest within five years). Such molecular tests are especially useful for the identification of eggs, first-stage larvae, pre-pupae, and pupae.

Western Flower Thrips is a major worldwide pest of a great diversity of agricultural and horticultural crops. It is highly polyphagous and causes direct feeding damage to fruits, leaves, and flowers. It also acts as a major vector of tospoviruses, most notably tomato spotted wilt virus (family Bunyaviridae, genus Tospovirus,TSWV). In The Netherlands alone, estimates indicate annual losses from direct damage by Western Flower thrips of $30 million, with additional losses from TSWV adding an additional $19 million. Western Flower Thrips has become a major invasive pest of vegetable and ornamental crops grown in open fields, shadehouses, and glasshouses (greenhouses) only since the late 1970s, although  it has long been regarded as a pest in its native range. As early as 1934, Western Flower Thrips was detected outside its native range, on naturalized California lupins in wild areas of New Zealand (they are abundant on introduced tree lupin, Lupinus arboreus, in sand dunes). However, this New Zealand population caused no apparent problems on crops and has traditionally been recognized as a distinct “lupin strain" (however, the pest strain apparently reached New Zealand in the early 1990s).

These distributions were apparently stable until the early 1980s, when Western Flower Thrips began to appear throughout the United States and adjacent Canada. In 1983, they colonized Dutch glasshouses and from there quickly spread across Europe and into northern Africa. They invaded South Africa and much of Asia. Globally, Western Flower Thrips is now probably the most important insect pest of commercial glasshouses and also has established in many outdoor crops. The discovery that there are apparently two reproductively isolated cryptic species of "Western Flower Thrips", that have been inadvertently lumped under the name Frankliniella occidentalis (Rugman-Jones et al. 2010) has important implications for interpreting past studies and planning future ones.

Western Flower Thrips are highly variable in size and color. Although adult males are all pale, female color ranges from nearly white through yellowish to black. The "glasshouse strain" that has spread around the world varies from pale to intermediate, and dark forms are rare.

In its native distribution, the Western Flower Thrips has a broad ecological range, being found from sea level to subalpine altitudes and from wet to arid habitats. In California, it occurs across many vegetation types from the Pacific coast to the interior mountain ranges at elevations up to 3,000m. Although most thrips are host-plant specific, the Western Flower Thrips is highly polyphagous, having been reported to feed on more than 240 plant species in 62 different plant families; these thrips can also be important mite predators under some circumstances (Mound 2005).

Kirk and Terry (2003) attempted to reconstruct the geographic spread of Western Flower Thrips.

Morse and Hoddle reviewed the invasion biology of thrips in general (Morse and Hoddle 2006).

(Kirk and Terry 2003; Brunner and Frey 2010 and references therein; Rugman-Jones et al. 2010 and references therein)


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