Plesiothrips perplexus (Beach, 1896)

Thripinae, Thripidae, Terebrantia, Thysanoptera

Figures

Fig. 1: 7-segmented antenna, segments III and IV with forked sense cone (female), 7-segmented antenna (male)
Fig. 2: Head dorsal with ocellar triangle
Fig. 3: Pronotum
Fig. 4: Meso- and metanotum
Fig. 5: Fore- and hind wing, fore wing distal region
Fig. 6: Meso- and metasternum
Fig. 7: Sternites VI and VII
Fig. 8: Tergites VII-IX
Fig. 9: Tergites VIII-XI

Plesiothrips perplexus develops within florets of several grasses like maize, rice, and sugarcane. Both sexes macropterous. Body typically bicolored with head and thorax brown but abdomen light brown or yellow with apex dark; legs and antennal segments III & IV yellow; fore wings light brown with basal quarter clear. Antennae 7-segmented; segments III & IV with sense cone forked, IV longer than III with apex produced into a neck, segment I with a pair of small dorso-apical setae (Fig. 1). Head longer than wide and projecting in front of eyes; 2 pairs of ocellar setae present, setae I absent, setae III about as long as side of ocellar triangle and arise just in front of hind ocelli within ocellar triangle; postocular setae small (Fig. 2). Pronotum with 2 pairs of long posteroangular setae; posterior margin with 3 pairs of small setae (Fig. 3). Metanotum with scarcely equiangular reticulation or no sculpture medially; median setae small and arising well behind anterior margin; campaniform sensilla absent (Fig. 4). Mesofurca with spinula (Fig. 6). Mid and hind tarsi 2-segmented. Fore wing first vein with 8-13 setae, and then 2 setae near wing apex; second vein with a complete row of about 10 setae (Fig. 5). Tergites without sculpture medially; usually without ctenidia, but rudimentary ctenidia sometimes present on VI & VII; tergite VIII posterior margin with a few small microtrichia laterally; ovipositor valves very weak and without teeth (Fig. 8 and 9). Sternites without discal setae; median pair of marginal setae on sternite VII arise at posterior margin (Fig. 7).
Male similar to female but smaller; antennal segment III very small, IV-VI each with several rings of long setae, VII very small (Fig. 1); tergite IX posterior margin with 2 stout thorn-like setae each arising from prominent tubercle; sternites III & IV each with one pair of small circular glandular areas placed laterally.

The genus Plesiothrips Hood, 1915
Currently, 19 grass-living species are listed in this genus, all except one from the New World. The presence and position of the rudimentary ctenidia on tergites VI & VII, also the lack of ocellar setae I, suggest that this genus might be related to the genus Thrips. However, the presence of a pair of setae at the dorsal apex of antennal segment I, and the remarkable sexual dimorphism of the antennae, suggest that genus Plesiothrips is more likely to be only distantly related to the genus Thrips. Members of the genus have a weakly developed ovipositor and 7-segmented antennae with forked sense cone on segments III & IV. In females antennal segment III is smaller than IV, and in males segment III is very small but IV-VI are greatly enlarged (Mound & Kibby 1998; Mound & Marullo 1996).

Compared to other species of Thripinae, in Plesiothrips perplexus antennal segment III is considerably shorter than segment IV. The species has an intermediate position between the complex of the genera Tenothrips and Megalurothrips which lacking tergal ctenidia on tergites V-VIII, and the Thrips, which have paired ctenidia laterally on tergites V-VIII. Plesiothrips perplexus has sometimes one pair of (even though only rudimentary and weakly developed) ctenidia on tergite VI & VII, but no ctenidia on tergite VIII. Like members of the genus Thrips, Plesiothrips perplexus has only 2 pairs of ocellar setae on the head (ocellar setae I absent), whereas species of Tenothrips and Megalurothrips have 3 pairs of ocellar setae. Plesiothrips perplexus as well as Tenothrips frici and Megalurothrips sjostedti have 2 pairs of elongate posteroangular setae, and tergite VIII posterior margin with a few short and irregular microtrichia only laterally. But compared to both other species with 8-segmented antennae, in Plesiothrips perplexus those are 7-segmented. As in Diarthrothrips coffeae, the species possesses antennal segments IV & V with a clear ring near the base which separated off narrow darker ring at the base, but Diarthrothrips coffeae has 8-segmented antennae, 3 pairs of ocellar steae, a metafurca with a well developed median spinula, and no comb of microtrichia on tergite VIII.

Life history
As with other thrips species the life cycle from egg to adult is dependent on temperature. The full cycle can take less than one week to over a month and adults may live for more than one month producing several generations in one year depending on seasonal weather (Lewis 1973).

Host plants
Various Poaceae including maize, rice and sugar cane.

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

Damage and symptoms
-

Detection and control strategies
-

Additional notes
Breeding on grasses, the adult females have a very weak ovipositor that is probably not capable of inserting eggs into plant tissue. Presumably the eggs are deposited within the flowering heads of grasses and developing within grass florets.

Biogeography

Beach AM (1896). Contribution to a knowledge of the Thripidae of Iowa. Proceedings of the Iowa Academy of Sciences. 3: 214-227

Hinds WE (1903). Contribution to a monograph of the insects of the order Thysanoptera inhabiting North America. Proceedings of the United States National Museum. 26 (1310): 79-242

Hood JD (1915). An interesting case of antennal antigeny in Thysanoptera. Proceedings of the Entomological Society of Washington. 17: 128-132

Karny H (1925). On some tropical Thysanoptera. Bulletin of Entomological Research. 16 (2): 125-142

Lewis T (1973). Thrips: their biology, ecology and economic importance. Academic Press Inc., London Ltd., 349 pp.

Moritz G, Morris DC & Mound LA (2001). ThripsID - Pest thrips of the world. ACIAR and CSIRO Publishing Collingwood, Victoria, Australia, CDROM ISBN 1 86320 296 X

Moritz G, Mound LA, Morris DC & Goldarazena A (2004). Pest thrips of the world - an identification and information system using molecular and microscopical methods. Centre for Biological Information Technology, University of Queensland, Australia, CDROM ISBN 1 86499 781 8

Moulton D (1929). New Thysanoptera from Cuba. Florida Entomologist. 13 (4): 61-66

Mound LA & Kibby G (1998). Thysanoptera: An identification guide, (2nd edition). CAB International, Wallingford and New York, 70 pp

Mound LA & Marullo R (1996). The thrips of Central and South America: An introduction (Insecta: Thysanoptera). Memoirs on Entomology, International, Vol. 6. Associated Publishers, Gainsville, 487 pp

Palmer JM, Mound LA & du Heaume GJ (1989). 2. Thysanoptera, 73 pp. In Betts CR [ed.], CIE Guides to insects of importance to man. CAB International, Wallingford, Oxon, UK

zur Strassen R (1983). Thysanopterologische Notizen (6) (Insecta: Thysanoptera). Senckenbergiana Biologica. 63 (3-4): 191-209

zur Strassen R (2003). Die terebranten Thysanopteren Europas und des Mittelmeer-Gebietes. Die Tierwelt Deutschlands und der angrenzenden Meeresteile nach ihren Merkmalen und nach ihrer Lebensweise, 74. Teil. Goecke & Evers, Keltern, Germany, 277 pp