© 2008 The Authors Journal compilation © 2008 The Royal Entomological Society Agricultural and Forest Entomology (2008), 10, 61–67 Introduction Many begomoviruses have high transmission rates and even a small population of the vector Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) may be responsible for very rapid disease spread. For this reason, disease control has often proved difficult except were crop genotypes resistant to dis- ease are available. Various forms of protected cultivation are increasingly used, most commonly greenhouses constructed from insect-proof mesh or polythene (Hanafi & Schnitzler, 2004) or other materials. These have frequently proved to be effective in reducing virus disease (Hanafi et al., 2003) but such structures are costly and may not be economically feasi- ble, especially in developing countries. Instead, insecticides are often applied heavily and frequently and, in some countries (e.g. south India), up to 90% of farmers rely primarily on pesticides for control of tomato yellow leaf curl virus (TYLCV) disease (Nagaraju et al. , 2002). In Israel, B. tabaci exclusion screens have been adopted nationwide and have proved a cost-effective disease control solution for tomato production (Taylor et al., 2001). Barriers of vegetation have been used in an attempt to con- trol whitefly-borne virus diseases in the field but with mixed Insect-screened cultivation to reduce the invasion of tomato crops by Bemisia tabaci: modelling the impact on virus disease and vector J. Holt, C. Pavis*, M. Marquier*, T. C. B. Chancellor, C. Urbino* and N. Boissot* Natural Resources Institute, University of Greenwich, Central Avenue, Chatham Maritime, Kent ME4 4TB, U.K. and *Unité de Recherche en Productions Végétales, INRA-Cirad, Domaine Duclos, 97170 Petit-Bourg, France Abstract 1 In two experiments carried out in Guadeloupe, barriers were used to reduce the entry of the virus vector Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) to tomato plots. The barriers erected around the crop were of insect-proof cloth fences (<50 mesh), 1.5 m in height, in the first experiment with a deltamethrin- treated, insect-attracting strip facing inwards, and, in the second, with the barrier but no insecticide-treated strip. 2 A mathematical model of epidemic development was fitted to the symptom data from the treated and control (unprotected) tomato plots. There were two viruses present, tomato yellow leaf curl and potato yellow mosaic; specific detection confirmed that symptoms gave an accurate indication of infection and that the two virus diseases had similar progress curves. 3 Parameter estimates obtained by model-fitting suggested that the barriers reduced vector immigration by approximately 12-fold but that B. tabaci retention within the plots was also increased slightly despite the mortality caused by the insecticide-treated strips. Disease establishment was delayed by approximately 2 weeks. The results obtained in the second experiment involving barriers deployed without insecticide-treated strips could be explained by a large increase in B. tabaci retention within the barriers resulting in more rapid virus disease progress than in controls. The results of mathematical modelling indicate that partial insect barriers can be worse than none because sufficient whiteflies can enter to establish a population and, at the same time, large numbers are retained in the barrier plot, with the net effect being a more rapid population increase than in the absence of barriers. Keywords Caribbean, geminivirus, mathematical model, plant disease, PYMV, sheltered cultivation, TYLCV. Correspondence: John Holt. Tel.: +44 (0) 1634 883225; fax: +44 (0) 1635 883379; e-mail: j.holt@gre.ac.uk