96 PHYTOPATHOLOGY Virology Thrips Resistance in Pepper and Its Consequences for the Acquisition and Inoculation of Tomato spotted wilt virus by the Western Flower Thrips P. C. Maris, N. N. Joosten, D. Peters, and R. W. Goldbach Laboratory of Virology, Wageningen University, Binnenhaven 11, 6709 PD Wageningen, the Netherlands. Accepted for publication 3 September 2002. ABSTRACT Maris, P. C., Joosten, N. N., Peters, D., and Goldbach, R. W. 2003. Thrips resistance in pepper and its consequences for the acquisition and inoculation of Tomato spotted wilt virus by the western flower thrips. Phytopathology 93:96-101. Different levels of thrips resistance were found in seven Capsicum accessions. Based on the level of feeding damage, host preference, and host suitability for reproduction, a thrips susceptible and a resistant acces- sion were selected to study their performance as Tomato spotted wilt virus (TSWV) sources and targets during thrips-mediated virus trans- mission. Vector resistance did not affect the virus acquisition efficiency in a broad range of acquisition access periods. Inoculation efficiency was also not affected in short inoculation periods, but was significantly lower on plants of the thrips resistant accession during longer inoculation access periods. Under the experimental conditions used, the results obtained show that transmission of TSWV is little affected by vector resistance. However, due to a lower reproduction rate on resistant plants and a lower preference of thrips for these plants, beneficial effects of vector resistance might be expected under field conditions. Additional keywords: Capsicum annuum, Frankliniella occidentalis, vector resistance, virus transmission. Thrips (Thysanoptera: Thripidae) cause serious problems in the cultivation of a wide range of greenhouse and field crops. They create major damage on plants by causing reduction in plant growth, deformation of plant organs, and cosmetic damage in the form of silver scars on leaves and flowers. Moreover, during feeding, thrips may transmit tospoviruses (family Bunyaviridae) (27,31,39), of which Tomato spotted wilt virus (TSWV) is economically the most important representative (13). Several thrips species, belonging to the genera Thrips and Frankliniella transmit TSWV. The western flower thrips (Frankliniella occiden- talis) is thought to be the most predominant and effective vector in many parts of the world (10,32,36). Attempts to impede tospovirus spread by chemical control of its vectors has proven to be difficult due to the development of vector resistance to insecticides (9,25,41), the polyphagous behavior of thrips, and their often hidden way of life inside flowers where they can escape from insecticides sprayed. In addition, TSWV cannot efficiently be controlled in some crops by application of insecticides as the virus often spreads by incoming adults. These thrips may transmit the virus before they acquire a deadly dose of insecticides (17,30). Moreover, there is increasing public demand to develop alternative control measures, as the practical use of most of the potentially effective pesticides is no longer allowed in an increasing number of countries. The use of virus and/or vector resistant cultivars is another option to control TSWV. Virus resistance has been reported in accessions of pepper and tomato (5,14,15,26,29), and a single dominant TSWV resistance trait has been described for pepper (6). This gene, designated Tsw, has been introduced in a number of cultivated Capsicum hybrids (7). An operational level of thrips resistance (toward F. schultzei) has been reported in the groundnut cv. Robut 33-1 (2). The incidence of Groundnut bud necrosis virus (GBNV) was 30 to 60% lower in this cultivar than in the widely used cv. TMV2, although both cultivars were equally susceptible to this tospovirus by mechanical inoculation. Since the acquisition and transmission rate by thrips did not differ between these cultivars, the lower incidence in the field was explained by a lower preference of F. schultzei for ‘Robut 33-1’. This type of resistance, restricting the spread of a given virus and often designated “field resistance”, was also observed for TSWV in tomato (15,16). Significantly fewer plants of four Lycopersicon cultivars became infected with TSWV by thrips inoculations than by mechanical inoculation (15), indicating that vector-mediated components were involved. Partial resistance against F. occiden- talis has also been reported for five pepper accessions (12). How- ever, this resistance was not evaluated with respect to its effect on the spread of TSWV in the field or on the virus acquisition and inoculation efficiencies of the thrips. Hence, despite several reports, the effect of plant resistance to thrips on transmission of TSWV is not fully understood and requires more investigation. The objective of this study was to identify and define thrips resistance among a number of pepper accessions, and to analyze the potential effects of this type of resistance on the acquisition and inoculation efficiency of a population of F. occidentalis for TSWV isolate BR01. MATERIALS AND METHODS Pepper accessions, thrips population, and virus isolate. Seven Capsicum accessions (Pikante Reuzen, Perla RZ, Mazurka RZ, CPRO-1, CPRO-2, PI 152225, and PI 159236) were used in this study. Accessions PI 159236, PI 152225 (5,6), and Perla RZ (J. Haanstra, personal communication) were known to be resistant to TSWV. A population of F. occidentalis ‘IS2’ (34), which originated from an infestation on mango in Israel, was used in the trans- mission studies. This virus-free population was reared on Phaseo- Corresponding author: D. Peters; E-mail address: dick.peters@wur.nl Publication no. P-2002-1106-01R © 2003 The American Phytopathological Society