The use of anti-insect screens placed on greenhouse vents has become generalized in warm climate areas, where there is a big pressure from insect pests which transmit virus diseases. Increasing the efficiency of the screens to reduce the entrance of pests into the greenhouse generally involves a reduction of the ventilation rate as the screen porosity is lower. This can, in part, be compensated by using screens with lower diameter of the threads which allows a higher porosity for the same efficiency to exclude insects. In the present work the characteristics of the most commonly used anti-insect screens in Almeria greenhouses, including their porosity, hole size and thread diameter were measured. Equations were developed which determined the geometric parameters of the screens accounting for their three dimensional nature. These equations enabled the effectiveness of screens in excluding insects from the greenhouse to be quantified. This allows a comparison between screens, and therefore selecting the most efficient giving maximum insect exclusion with maximum porosity. INTRODUCTION The horticultural crops in warm climate regions suffer from agronomic and climatic conditions which favours pest and disease pressure (Cabello et al., 1990; Alcazar et al., 2000). More importantly insect vectors of virus diseases are the most serious concern of greenhouse farmers in warm regions (Franco et al., 1999; Lacasa Plasencia and Sanchez Sanchez, 1999; Hanafi, 2005). In order to fight these insects in the greenhouses different methods can be used such as chemical methods, use of photoselective covering materials, integrated pest management, biological control or physical methods (Antignus, 1999; Hanafi et al., 2003), amongst these are the use of antiinsect screens. The European Union is continuously decreasing the maximum residue limits of pesticides permitted, making it necessary to search for alternative solutions to chemical control such as integrated pest management including among others the use of photoselective plastic films and physical control. Physical control consists of placing denser anti-insect screens on the greenhouse vents which may limit the entry of virus vectors such as Frankliniella occidentalis and Bemisia tabaci. However, the use of denser screens, has the disadvantage of reducing greenhouse ventilation (Perez-Parra et al., 2003). Despite the decrease in the ventilation capacity and in the transmission of solar radiation, the use of anti-insect screens is widespread as a physical control method to fight vectors (Bethke et al., 1994; Baker and Shearin, 1994; Bell, 1997; Bell and Weatherley, 1999; Teitel et al., 2000; Critten and Bailey, 2002; Diaz Perez et al., 2003). There have been a large number of studies dealing with the geometrical characterization and calculation of the porosity, of commercial anti-insect screens (Ross and Gill, 1994; Bell and Baker, 1997; Teitel, 2001; Bartzanas et al., 2002; Cabrera et al., 2002; Valera et al., 2003). Field experiments to test the exclusion of different types of anti-insect screens have been performed (Bell and Weatherley, 1999; Bell and Baker, 2000; Taylor et al., 2001; Berlinger et al., 2002; Giordano et al., 2003; Hanafi et al., 2003; Camacho Ferre et al., 2004), The important parameters are the uniformity in hole size and the three dimensional effect which increases the maximum diameter inscribed in the hole (Cabrera et al., 2002). Therefore, it is necessary to characterize the screens and to determine the most efficient way to limit the penetration of pest vectors with the least negative impact on greenhouse ventilation. MATERIAL AND METHODS Geometrical characterization and exclusion of pest insects Twenty one commercial anti-insect screens were studied (S01-S21) (Table 1). Six 1 cm samples of each screen were digitised with a scanner (HP PhotoSmart S20, Hewlett-Packard Company, USA) at a resolution of 2400 ppp which provided a precision of 10.6 μm pixel, in order to analyse their holes (Figs. 1 and 2). In addition, two samples of 0.2 cm of each screen were also digitised, to measure the thread diameter (Fig. 3) and to correct the hole size measurements, using an electronic microscope MIC-D (Olympus Corporation, Japan), with a precision of 3.8 μm pixel. The digitised samples were converted from greyscale to black and white images and analyzed using software (UTHSCSA ImageTool v3.0, University of Texas Health Science Center at San Antonio, USA) to finally obtain the size of all the holes, the maximum diameter inscribed in each hole, the thread diameter and the screen uniformity. Considering that the anti-insect screen (Fig. 4a) is made with only one type of thread of D (mm) diameter, and knowing the number of threads cm in the X and Y directions (NX, NY), the average size of the hole in both directions LX (mm) and LY (mm) can be calculated: