Integrated Control of Plant-Feeding Mites IOBC/wprs Bulletin Vol. 50, 2009 pp. 65-68 65 Mutual interference of Scolothrips longicornis Priesner (Thysanoptera: Thripidae) on Tetranychus urticae Koch (Acari: Tetranychidae) Hajar Pakyari 1 , Yaghoub Fathipour 2 1 Department of Plant Protection, Islamic Azad University, Takestan Branch, Iran, Hajar.Pakyari@gmail.com - Member of Young Researchers Club of Takestan Branch-Iran; 2 Department of Entomology, Faculty of Agriculture, Tarbiat Modares University, P.O. Box 14115–336; Tehran, Iran, fathi@modares.ac.ir Abstract: The mutual interference of the predatory thrips Scolothrips longicornis Priesner on the two- spotted spider mite, Tetranychus urticae Koch was studied in 24 h time period at 26 ±1°C, 60 ±5% RH and a photoperiod of 16:8 L:D h. Nicholson's model and linear regression were used to determine per capita searching efficiency and interference coefficient, respectively. The per capita searching efficiency decreased significantly from 0.0397 to 0.0247 as the predator densities increased from one to sixteen. Therefore, different densities of the predator (intraspecific competition) could affect the efficacy of Scolothrips longicornis. Key words: two-spotted spider mite, predatory thrips, Scolothrips longicornis, Tetranychus urticae Introduction Spider mites (Acari: Tetranychidae) are widespread agricultural pests, which often cause severe damage to various crops (Gerlach & Sengonca, 1985). The use of fertilizers and chemicals seems to enhance their intrinsic powers of increase (Van de Vrie et al., 1972). Bio- logical control is an alternative to their chemical control. Predatory thrips (Thysanoptera) are among the natural enemies of spider mites. All species of the genus Scolothrips appear to be specialized predators of spider mites (Priesner, 1950). Scolothrips longicornis Priesner is a native beneficial thrips in Iran. It is common in bean, cucumber and tomato fields and is considered to be an important predator of numerous spider mite species (Pakyari et al., 2008). The phenomenon of mutual interference was initially modelled by Hassell and Varely (1969), who found an inverse relationship between a predator's searching efficiency and and its density. Understanding the mutual interference of predators is necessary for the success of biological control programs, as it assists with mass rearing efforts and can facilitate the explanation of observed outcomes in the field (Fathipour et al., 2006). Therefore, in this study we determined the mutual interference (intraspecific competition) of Scolothrips longicornis on Tetranychus urticae under laboratory conditions. Material and methods Mite and thrips colony The colony of T. urticae was initiated using individuals collected from cucumber fields of Varamin (Tehran province). The mites were maintained on detached cucumber leaves placed upside down on a layer of wet cotton inside Petri dishes (150 mm in diameter). The lids of the Petri dishes had a 30 mm diameter hole covered with fine nylon mesh to allow ventilation.