Prohibiting pests

Environmental concerns and greater regulation on pesticide use are making the grower’s job of pest control more difficult. When designed and installed properly, screens can help to provide exclusion. This can reduce pesticide use, lessen employee exposure to chemicals and provide higher quality plants.

Screens work best with exhaust fan ventilation systems. The positive airflow from exhaust fans gives a constant pressure difference. With a natural ventilation system, the airflow is dependent on buoyancy and wind for movement and varies considerably. This requires a greater amount of screen area.

Mesh

A common method of identifying insect screening is by mesh. The U.S. system is to count the number of openings in one linear inch. For example, a 50 mesh would have 50 openings. The screening may have different mesh between the horizontal and vertical direction. The greater the mesh, the smaller the openings and the smaller the insects that can be excluded. Depending on the material and thread diameter, the porosity and therefore air movement will vary.

Airflow

The airflow through screen material varies with the size of the openings. This is determined by the smallest insect you are trying to exclude. The openings for thrips are less than 0.006 of an inch, whereas openings for flies can be 0.040 of an inch. The smaller the openings, the greater the amount of screen that is needed to provide adequate airflow. Table 1 shows approximate insect dimensions, typical insect screen openings and air reduction.

To provide adequate airflow for ventilation, the area of the screen that covers the vent/shutter opening has to be increased. This is usually done by providing a frame or structure over the intake that will support additional material. For a greenhouse with fans on one endwall and a vent or shutters on the opposite endwall, adding an extra hoop covered with screen material to the intake end will provide the extra area. For greenhouses with fans along one sidewall and vent on the opposite side, a lean-to structure or box frame over the vent will usually be adequate. Pleated screens are also available that attach to existing motorized vent windows.

Sizing

The calculations for determining the amount of screen material needed is fairly complex and best determined by computer. Contact the manufacturer or supplier to get the needed size. Factors that affect the amount of screen area include:

• Location: Greenhouses in the south require more ventilation due to greater thermal load. The solar load may be 25% greater in Arizona as compared to a location in Michigan. Clouds and smog also affect this.
• Existing greenhouse shading: Less ventilation and therefore less screen material is needed if a thermal screen, exterior shade material, whitewash or evaporative cooling is in place.
• Existing ventilation system: This includes the number and size of fans, their output and air inlet size. After closing all openings except the intake shutters or vents, a monometer should be used to get the pressure drop (inches of static pressure) of the existing system before the screen is installed.
• Size of the greenhouse: This includes floor area and height.
• Resistance of screen material: The opening size for thrips is much smaller. The screen area needed can be from two to 10 times the shutter or vent opening.
• Shrinkage: Allow for 1-2% shrinkage when installing the screen.

Maintenance

Keeping the screen system in good condition requires proper maintenance. All openings (doors, gaps under baseboards) other than the screened vents should be closed to reduce access to insects.

Dust, dirt and pollen can plug up the tiny screen openings quickly. Washing with water from a hose and nozzle from the inside out works well. Do it on a warm day so that the screen dries quickly. Do not use high pressure from a leaf blower or water nozzle, as it may tear the material or alter the hole size.

A check on the operation of the system can be made by monitoring and recording the static pressure drop across the screen. When it increases above 0.4 of an inch of water, it probably should be cleaned.