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Vector Relations

A Midgut Barrier to Tomato Spotted Wilt Virus Acquisition by Adult Western Flower Thrips. Diane E. Ullman, Department of Entomology, University of Hawaii, 3050 Maile Way, Room 310, Honolulu 96822; John J. Cho(2), Ronald F. L. Mau(3), Daphne M. Westcot(4), and Diana M. Custer(5). (2)(5)Department of Plant Pathology, Hawaii Institute of Tropical Agriculture and Human Resources (HITAHR), Maui Research Station, P.O. Box 269, Kula 96790; (3)(4)Department of Entomology, University of Hawaii, 3050 Maile Way, Room 310, Honolulu 96822. Phytopathology 82:1333-1342. Accepted for publication 8 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1333.

Ultrastructural and serological analyses provide evidence that a midgut barrier underlies the inability of adult western flower thrips (WFT), Frankliniella occidentalis, to acquire tomato spotted wilt virus (TSWV) through adult feeding on infected plants. As demonstrated for other thrips vector species, only WFT that acquire TSWV as larvae can successfully inoculate plants with the virus as adults. Transmission electron microscopy (TEM) observations and serological results demonstrate that larval and adult WFT ingest TSWV particles during feeding on infected plants, after which virions can be found within the midgut lumen and epithelial cells. Among adult thrips that ingest TSWV as adults and not as larvae, TSWV particles accumulate within masses of amorphous electron-dense material in the cytoplasm of midgut epithelial cells, frequently lack a visible surrounding membrane, and are sometimes associated with multivesicular bodies and rough endoplasmic reticulum. Dissemination of virus particles beyond the midgut epithelia could not be detected serologically or with TEM observation in adult thrips given acquisition feeding as adults. Serological evidence and TEM observations also show that although TSWV is ingested by adult WFT, it is not retained. In contrast, among larval thrips given acquisition feeding on virus-infected plants, TSWV was detected serologically and with TEM observation in the midgut epithelia and hemocoel. Furthermore, when TSWV was acquired by larvae, the virus was transtadially passed and persisted through adulthood. From these data we conclude that TSWV ingested by adult WFT is apparently degraded or altered in the midgut lumen and/or epithelial cells, such that events leading to dissemination to the hemocoel and secondary target organs in the insect cannot occur. Consequently, these adult WFT do not become infective unless virus acquisition feeding occurs during the larval stages. These data represent an important first step toward investigating molecular and biochemical mechanisms governing vector specificity, transmission efficiency, and use of thrips serology to determine the epidemiological significance of thrips populations.