J. Biol. Control, 23(2): 2009 Physical structures of host plants affect preference behaviour of predaceous thrips, Scolothrips longicornis (Thysanoptera: Thripidae) NEDA KHERADPIR 1 , MOHAMMADREZA REZAPANAH 2 , KARIM KAMALI 1 and YAGHOUB FATHIPOUR 3 1 Department of Entomology, Faculty of Agriculture and Natural Resources, Islamic Azad University, Science and Research Branch, P.O. Box 14155-4933. Tehran, Iran. 2 Department of Biological Control Research, Plant Protection Research Institute of Iran, P.O. Box 19395-1454, Tehran, Iran. 3 Department of Entomology, Fac. Agriculture, Tarbiat Modares University, P.O. Box 14115-336, Tehran, Iran. E-mail: n.kheradpir@gmail.com ______________________________________________________________________ ABSTRACT: The aim of this study was to determine the effect of plant physical structures of leaf surface on the first step of host plant preference behavior of predacious thrips, Scolothrips longicornis. The experiment was carried out using individual predators on two sets of leaf disks belonged to four host plants (cucumber, tomato, sweet pepper and eggplant) and the couple units of the plant leaf disks (tomato + cucumber, cucumber + eggplant and eggplant + tomato). Time taken for selection process was set to 5 minutes. The number of selecting each plant and time needed to make decision were recorded and analyzed. Final results revealed the predator had no significant preference among four plant disks, except in couple combination tests, eggplant with some physical features such as trichomes type and density had the highest rate of selection. Predacious thrips preferred host plants with rational dense trichomes, which are arranged in clusters and leave some space for the predator to walk, search prey and also oviposition. Plants such as sweet pepper with low trichomes density were not favorable for the predator. Time needed showed that predator could make decision between host plant within a family with similar leaf surface structure sooner than plant of different plant families that confirms the effect of host plant structure on predator foraging behavior. KEY WORDS: Host plant, preference, Scolothrips longicornis. ______________________________________________________________________ INTRODUCTION Spider mites (Acari: Tetranychidae) are widespread agricultural pests which often cause severe damage to various crops (Gerlach and Sengonca, 1985). Biological control would be a good alternative to the other management methods, especially chemical control. Among the natural enemies of spider mites are predacious thrips with varying degree of specialization in mites. All species in the genus Scolothrips appear to be specialized predators of spider mites (Priesner, 1950). Scolothrips longicornis Priesner is native predator in Iran in many plant fields such as cucumber, bean, tomato and sugar cane. In combining different management methods such as natural enemies and host plants, their interaction should be studied to predict the final output of the program (Pakyari et al., 2008). Although major attention is focused on the host plant volatiles induced by herbivores affecting the natural enemies’ reaction toward host plants, in some cases it is seen that plant physical structures such as color,