Thrips simplex (Morison, 1930)

Thripinae, Thripidae, Terebrantia, Thysanoptera

Figures

Fig. 1: 8-segmented antenna, segments III and IV with forked sense cone, terminal segments VI-VIII
Fig. 2: Head dorsal with ocellar triangle
Fig. 3: Pronotum
Fig. 4: Meso- and metanotum
Fig. 5: Fore wing, fore wing distal region
Fig. 6: Sternites III-V
Fig. 7: Sternites VI and VII
Fig. 8: Tergites VIII and IX

Thrips simplex causes damage on flowers and leaves of gladiolus and related species such as walking iris (Neomarica sp.), and also carnation. Both sexes fully winged. Body and legs dark brown; tarsi and antennal segment III yellowish brown; fore wings brown with base paler. Antennae 8-segmented; segments III & IV slightly constricted at apex and with forked sense cone (Fig. 1). Head wider than long; with 2 pairs of ocellar setae, pair I absent, pair III small and arising just inside anterior margins of ocellar triangle; postocular setae I & III about as long as ocellar setae III, pair II minute (Fig. 2). Pronotum with 2 pairs of elongate posteroangular setae of which the external pair is slightly shorter than the inner pair; posterior margin with 3-4 pairs of setae; anterior margin with 4 pairs of setae (Fig. 3). Mesofurca with spinula. Metanotal median area with transverse reticulation at anterior, and irregular equiangular reticulation or longitudinal reticulations on posterior half and faint internal markings inside most reticles; median setae arising behind anterior margin; campaniform sensilla absent (Fig. 4). Mid and hind tarsi 2-segmented. Fore wing first vein with 6-10 (mostly 7) setae on distal half; second vein with a complete row of about 14 setae; clavus with 5 marginal setae, clavus terminal seta longer than subterminal seta (Fig. 5). Tergite II with 3 or 4 lateral marginal setae; tergites V-VIII with ctenidia present laterally, on VIII posteromedial to spiracles; posterior margin of VIII with complete but slightly irregular comb of small microtrichia (Fig. 8); pleurotergites without discal setae, sculptured with rows of coarsely ciliate microtrichia. Sternite II with 2 pairs of marginal setae, III-VII with 3 pairs (Fig. 6), the median pair on VII arising in front of posterior margin (Fig. 7); sternite II with 1 or 2 discal setae, III-VII with about 10-16 discal setae in single row.
Male similar to female in color and structure, but smaller.

The genus Thrips L., 1758
There are nearly 300 species currently recognized in the genus Thrips making this genus one of the largest taxa within the order Thysanoptera. The genus was redefined progressively during the 1970's (see Mound et al. 1976), to include many species previously placed in Taeniothrips. The genus Thrips now includes a range of species, some with the antennae 7-segmented, others 8-segmented, and a few with the number of segments varying between 7 and 8. Similarly, some species have few setae on the fore wing first vein, whereas others have a complete row of setae on this vein. The species with a complete setal row on the first vein were placed from some taxonomists in the genera Isothrips or Isoneurothrips. However, all of the species in Thrips have the following character states: antennal segments III & IV with forked sense cone, absence of ocellar setae I, pronotum with 2 pairs of elongate posteroangular setae, paired ctenidia laterally on the tergites V-VIII, tergite VIII ctenidium arising posterior to the spiracle (in contrast to species of the genus Frankliniella). Other character states, such as number of antennal segments, number of setae on the fore wing veins, and number of discal setae on the abdominal sternites are variable between species (Mound & Masumoto 2005; Nakahara 1994; Palmer 1992). Identification keys are available for the species of this genus from many parts of the world. Of particular importance is the published key by Mound (2010) for members of the genus Thrips from Afro-tropical region as well as previous Lucid keys from Moritz et al. (2001, 2004 and 2009).

Thrips simplex is very similar to some other Thrips species - like Thrips acaciae, Thrips brevisetosus, Thrips florum, Thrips gowdeyi and Thrips hawaiiensis. Thrips simplex has sternites III-VII with at least 1 pair of discal setae and pleurotergites without discal setae (Thrips australis, Thrips microchaetus, Thrips subnudula and Thrips tenellus, all of them have sternites III-VII with at least 1 pair of discal setae and pleurotergites with discal setae; Thrips orientalis and Thrips parvispinus with discal setae on sternites III-VI but not on sternite VII, and without discal setae on pleurotergites; Thrips nigropilosus, Thrips palmi, Thrips pusillus and Thrips tabaci, all of them have sternites and pleurotergites without discal setae).
Inside this group Thrips simplex is the only species that has ocellar setae III arising within ocellar triangle anterior to tangent of anterior margin of hind ocelli, and fore wing first vein with 6-10 setae on distal half (other five species with ocellar setae III arising on anterior margin of, or in front of, ocellar triangle, and fore wing first vein with less than 5, mostly 3, setae on distal half). Most of the species exhibit campaniform sensilla on metanotum (except for Thrips gowdeyi and Thrips simplex without metanotal campaniform sensilla), and 2 pairs of elongate pronotal posteroangular setae (except for Thrips brevisetosus without elongate pronotal setae). In Thrips simplex as well as Thrips acaciae, Thrips brevisetosus and Thrips gowdeyi the metanotal median setae arise behind anterior margin (in Thrips florum and Thrips hawaiiensis median setae arise at anterior margin). As in Thrips gowdeyi, the species has always 8-segmented antennae (in Thrips acaciae and Thrips brevisetosus antennae are always has 7-segmented; in Thrips florum and Thrips hawaiiensis antennae are 7- or 8-segmented). Compared to Thrips acaciae and Thrips brevisetosus with mainly equiangular reticulation on metanotal median area, in other species the metanotal median area exhibit transverse sculptured lines at anterior, but longitudinal or equiangular reticulations on posterior half (Thrips gowdeyi and Thrips simplex), always longitudinal and parallel sculptured lines on posterior half (Thrips florum), or longitudinal and parallel sculptured lines or equiangular reticulations on posterior half (Thrips hawaiiensis). Furthermore, Thrips simplex have rows of fine microtrichia on pleurotergites, situated on widely spaced lines of reticulation (like in Thrips acaciae and Thrips gowdeyi; in Thrips brevisetosus, Thrips florum and Thrips hawaiiensis these rows of microtrichia absent). Outside this group, also Thrips tabaci and Thrips subnudula have pleurotergites covered in rows of fine microtrichia.
Species of the genus Thrips are similar to species of Stenchaetothrips, Microcephalothrips abdominalis, Larothrips dentipes and Fulmekiola serrata because of tergites V-VIII bear a pair of ctenidia laterally, which placed on tergite VIII posteromedial to the spiracle, and all species have no ocellar setae I. In contrast to species with craspedum on tergites II-VII (Microcephalothrips abdominalis, Larothrips dentipes and Fulmekiola serrata), species of Thrips and Stenchaetothrips have no posteromarginal craspedum on tergites and sternites. Species of the genus Thrips as well as Fulmekiola serrata and species of Stenchaetothrips have 2 pairs of elongate posteroangular setae (Microcephalothrips abdominalis with 2 pairs of moderately elongate pronotal setae and Larothrips dentipes without elongate setae). Compared to the species of Thrips, Microcephalothrips abdominalis, and Larothrips dentipes, which have ocellar setae II much shorter than or about as long as III, Fulmekiola serrata and species of Stenchaetothrips have ocellar setae II much longer than III, and sternites always without discal setae.

Life history
As with other thrips species the life cycle from egg to adult is dependent on temperature. The full cycle can take about 15 days (Lewis 1973) to over a month. Adults may live for more than one month producing several generations in one year depending on seasons. With greenhouse temperatures the developmental time from egg to adult can decrease to about one week.

Host plants
Gladiolus sp. and related species of Iridaceae such as Calla sp., carnation (Dianthus sp.), Neomarica sp.

Vector capacity
None identified, but possible mechanical distribution of phytopathogenic fungi and bacteria.

Damage and symptoms
Thrips simplex is generally found on the flowers, leaves and corms of its host plants and causes deformities and discolorations of flowers, and corms become soft and are prone to decay (Denmark & Price 1989).

Detection and control strategies
The flowers of gladioli with a light colour (white "White Prosperity", yellow "Nova Lux") were less infested in comparison with cultivars with dark coloured flowers (orange "Hunting Song", red "Oscar"). Use of black polythene sheet resulted in increased infestation of gladioli (Milevoj et al. 2008).

Additional notes
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Biogeography

Worldwide: Asia, Australia, New Zealand, Central and South America, Europe, North America, Africa. Angola, Egypt, Ethiopia, Kenya, Lesotho (Maseru), Morocco, South Africa (Gauteng: Pretoria; Western Cape: Mossel Bay, Somerset West, Kirstenbosch), Sudan, Uganda, Zimbabwe.

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