Localization of Tomato Spotted Wilt Tospovirus in Larvae and Pupae of the Insect VectorThrips setosus

Changes in the accumulation of tomato spotted wilt tospovirus (TSWV) nucleocapsid (N) protein in Thrips setosus, an insect vector, from eclosion to 17 days were revealed by the double-antibody sandwich-enzyme-linked immunosorbent assay (DAS-ELISA) system with individual thrips. First-instar larvae were fed on TSWV-infected Datura stramonium for 2 h, and then kept on healthy Vigna susquipedaris. Individuals were subsampled for 17 days from first-instar larval to adult stage. The N protein concentration increased gradually from the first- to second-instar larval stage after acquisition feeding, peaking on day 5. Viral concentration then declined drastically from the second-instar larval to pupal stage. During the adult stage after ecdysis, persistently low N protein titers were demonstrated by DAS-ELISA, but adult thrips could transmit the virus. N protein was localized in first- and second-instar larvae and pupae by indirect immunofluorescence. On the second to fourth days after acquisition, specific fluorescence signals were detected within the anterior midgut that then spread to the whole midgut during the second-instar larval stage. As time elapsed, N protein was detected throughout the larval midgut and possibly within the salivary glands in pupae at 6 days after acquisition. Fluorescence signals within the pupal midgut were observed, but were not as intense as in larvae.