Insect exclusion screens: the size of the holes from a three-dimensional perspective

Abstract

Insect-proof screens are considered as flat bodies. However, from a microscopic point of view a screen is not a flat body but, on the contrary, the spatial arrangement of the threads determines that the passage surface for an insect is larger than the one obtained in the measures made on images in orthogonal projection. This paper compares the effectiveness against Bemisia tabaci of different types of screens and perforated sheets. A perforated sheet has a negligible thickness and therefore, the comparison between these two types of physical barriers allows determining qualitatively the importance of the spatial arrangement of the threads on the efficacy of the screens. It also compares the efficacy between screens with similar hole widths to assess the validity of the “prison effect” as design criteria. The results show that if screens are used smaller holes (measured in orthogonal projection) are needed in comparison to the holes of the perforated sheets to obtain similar values of efficacy against the whitefly because the porous surface of the holes of the screens is not flat. The results also indicate that the geometry and the shape factor of the holes of both physical barriers must be similar to obtain a measure of the 3D surface of the holes of the screens. Screens with the same hole width have different efficacy against B. tabaci. The most critical hole region for the passage of the insects is the distance defined by the crossing of two consecutive warp threads. This separation divides the hole in two parts. Although this distance is sufficiently small so that the insect cannot pass if the hole is very elongated the insect could pass by one of the halves and for this reason the hole length of the holes also influences on the efficacy of the screens.