The life history of thrips belonging to both suborders Terebrantia and Tubulifera are unique. Eggs are laid either within the plant tissue (Terebrantia) or often on the surface of the plants (Tubulifera). There are two distinct larval stages (Larva I & Larva II) which are vermiform in nature. Both the larval stages move actively and feed on the host plants. The number of prepupal/pupal or resting stages are varied, two in Terebrantia and three in Tubulifera. The pupal stages are found either in the soil or on the plant litter near to the soil and they are inactive non feeding stages. Thrips reproduce by haplodiploidy. The most common form of reproduction is arrhenotoky, where male progeny arises from unfertilized (haploid) eggs and female offspring from fertilized (diploid) eggs. In some species, such as Heliothrips haemorrhoidalis, Hercinothrips bicinctus, Helionothrips errans, Parthenothrips dracaenae, Scirtothrips longipennis, Leucothrips nigripennis, and Chaetanaphothrips orchidii, males are very rare (Lewis 1973). These species reproduce by thelytoky, whereby individual females produce exclusively female progeny by a process that does not involve fertilization. Other species, such as Thrips tabaci or Taeniothrips inconsequens can switch between thely- and arrhenotoky. A screening for the presence of Wolbachia using 16S rDNA and ftsZ gene primers produced interesting results. The arrhenotokous reproducing species Echinothrips americanus and Gynaikothrips ficorum tested positive for Wolbachia, while the bacterium was not detected in Frankliniella occidentalis and in the thelytokous population of Thrips tabaci. Wolbachia was found in the thelytokous reproducing species Hercinothrips femoralis and Parthenothrips dracaenae (Kumm & Moritz 2008).

The ability of thrips to transmit tospoviruses is subject of a dynamic process and depends on the vector status and the genetic determinants of the tospovirus (Sin et al. 2005; Ullman et al. 2005). Presently there are about 11 species of more than 6000 thrips known to vector Tospoviruses to a wide range of plants belonging to more than 1000 species in about 90 families (German et al. 1992; Mound 1996; Ullman 1996) (Fig. 1 and Tab. 1). Only adult thrips that have acquired a virus during their first or early second larval stage can transmit tospoviruses (van de Wetering et al. 1996; Amin et al. 1981, German et al. 1992, Ullman et al. 1991, 1992, 1995). Usually high titers of viruses can be measured within the midgut epithel cells, the visceral muscle cells of the anterior midgut region, the lobed salivary glands and the malpighian tubules, but not within the haemocoel (Fig.2). Such results emphasis the point made earlier that virus particles must move from the visceral muscle tissue to the membrane of the lobed salivary gland cells to avoid the hemocoel. This is only possible during a very short time window of a temporary association of the cell membranes of midgut epithel cells, visceral muscle cells and cells of the lobed salivary gland during the first larval stage. This complex is the result of displacement of the brain into the prothoracic region by enlarged cibarial muscle groups. However, the examinations of morphogenetic movements during ontogenic development has provided a totally new view of the likely virus pathway through a vector (Moritz et al. 2004).
Apart from the tospoviruses, thrips species like Frankliniella williamsi are known to vector Machlomovirus like maize chlorotic mottle machlomovirus (MCMV) in Poaceaous crops such as Maize. Unlike with the tospoviruses, where transmission is circulative and persistant, MCMV is transmitted in a non-persistent manner. Some populations of typical vector species, such as Thrips tabaci, are known to have lost their ability to transmit, whereas populations of other species may increase their vector potential. Such phenomenon needs to be considered in future research efforts to undestand factors influencing thrips-tospovirus interaction, such as host plants, geographic area, climatic conditions, population and reproduction status of the vector etc.

Tab. 1: Tospovirus-Vectors, origin and transmitted tospoviruses (CaCV: Capsicum chlorosis virus, GRSV: Groundnut ringspot virus, INSV: Impatiens necrotic spot virus, TCSV: Tomato chlorotic spot virus, TSWV: Tomato spotted wilt virus, WSMV: Watermelon silver mottle tospovirus, CSNV: Chrysanthemum stem necrosis virus, IYSV: Iris yellow spot virus, TYRV: Tomato yellow ring virus, GBNV: Groundnut bud necrosis virus, CLCSV: Calla lily chlorotic spot virus, MYSV: Melon yellow spot virus, PCFV: Peanut chlorotic fan-spot virus, PYSV: Peanut yellow spot virus, ZLCV: Zucchini lethal chlorosis virus, PRSV: Polygonum ring spot virus) (Riley et al. 2011)

Fig. 2: Frankliniella occidentalis: Larval anatomy with a closely contact between salivary gland and anterior part of the midgut.