The effect of screened openings on greenhouse microclimate

The use of screens to reduce insect entry into greenhouses has become a common practice in many countries. The screens act as a mechanical barrier that prevents migratory insects from reaching the plants, and thus reduce the incidence of direct crop damage and of insect-transmitted virus diseases. As a consequence, the need for pesticide application is reduced; growers can follow international mandatory regulations, and can use biological control agents as well as insect pollinators. The exclusion of very small insects is achieved by installing fine-mesh screens across the greenhouse openings. Since the porosity (ratio between open and total areas) of these screens is usually low, they impede ventilation and reduce light transmission. Therefore, it is important to determine their resistance to airflow and their optical characteristics, in addition to characterizing their effectiveness against insect entry. Studies in wind tunnels have shown that screens that generated a higher pressure drop for a given incident air velocity caused higher temperature and humidity within a greenhouse. The fine-mesh insect-proof screens reduce the discharge coefficient of the openings, and thus reduce the mean air velocity and turbulence level at the openings. The screens generate small eddies and increase the spectral decay rate of the turbulent flow, and, furthermore, they reduce the air velocities within the greenhouse, which results in higher temperature and humidity gradients there. These problems may be addressed by increasing the area of the openings in naturally ventilated greenhouses or by using forced ventilation. When ventilation does not provide the desired air temperature, artificial cooling systems, such as fan and pad or fogging systems, need to be applied to maintain conditions favorable to plant growth. The present review examines the current literature on the use of insect-proof screens in greenhouses. It summarizes methods for characterizing the pressure drop through screens and screened openings, and examines the effect of screens on the microclimates of greenhouses and screenhouses. Differences among the various studies are discussed and directions for further research are suggested. (c) 2007 Elsevier B.V. All rights reserved.