Fig. 1. The pads of this greenhouse are covered with insect screen, using a lean-to arrangement of the screen.
Photo courtesy of Christopher J. Currey

Controlling insect pests is one of the most important priorities for greenhouse food crop producers. One of the challenges in controlling insects on food crops in greenhouses is a lack of pesticides labeled for use on edible plants grown in controlled environments. To reduce the potential for insect pest problems to develop, preventing them from entering and establishing in the greenhouse is essential. Insect screens are an important tool in excluding pests, and this article will highlight important considerations for their use.

Effective screening

The first consideration for installing insect screens is to determine what part(s) of the greenhouse will be covered. For crops that are less susceptible to pests, screening the air inlets (such as pads) can provide sufficient exclusion to make a difference. However, for facilities looking for exceptional exclusion from screens, more comprehensive coverage is required. In addition to the air inlet, exhaust fans and vents also need screening to maximize exclusion. If screening is installed, be sure to use air lock doors and seal gaps and leaks. Without doing this, the screens are rendered less effective with these opportunities for insect infiltration.

Selecting screens

The first step to take when selecting an insect screen to install is to consider the target pest or pests that are to be excluded. Screen selection is directly related to insect size, which varies among the most common pests. For example, a leafminer is over 600 µm wide, while thrips are around 200 µm wide. The smaller the insect size, the smaller screen opening required. For example, a screen that is sufficient at excluding whiteflies will not be sufficient for screening thrips. However, if whiteflies are the primary pest, then do not use screen intended for excluding thrips; this will reduce airflow or increase the screening area to maintain adequate airflow. In addition to differing in hole sizes, the materials used differ. Most screens are made of nylon, polyethylene or a blend of polyethylene and acrylic. These materials can be used to create screens that are woven or knit with different hole sizes, or a film with holes perforating it. When selecting screens, the investment cost can be related to the durability. Note that longer-lasting products initially cost more.

To reduce the potential for insect pest problems to develop, preventing them from entering and establishing in the greenhouse is essential.

Maintaining air movement

When considering insect screen, maintaining adequate airflow in the greenhouse is as important as effectively excluding insects. Whenever screens are installed in a greenhouse, they restrict the airflow. In an actively cooled greenhouse, the reduced airflow can cause fans to work harder and potentially burn out. When screens are installed, in order to maintain airflow, the area of the openings in the screen must equal the area that is being screened. A screen simply installed flush over an air inlet would not provide adequate ventilation. However, there are several ways to install screens to accommodate the extra material needed to maintain airflow. A lean-to, as seen in Fig. 1, or other enclosure can be built around the openings to be screened. Alternatively, screens can be pleated, like an accordion, to increase the surface area of the screen to maintain airflow. Screens are best suited for greenhouses using some type of active ventilation. However, screens can be used in passively ventilated greenhouses, but care must be taken to minimize the effect of increased temperatures in the greenhouse on crops.

Screen maintenance

Once screens are installed, don’t forget to maintain them in order to sustain sufficient air flow. As screens collect dust and dirt, airflow will be reduced from blockages resulting in openings getting clogged. Regular monitoring will indicate when cleaning is needed. Screens should be washed at a time when venting is not necessary, since the water on screens from washing will reduce airflow and increase the temperature in the greenhouse. Avoid using brushes or pressure washers, as these tools can increase the openings in the screen which, as a result, will render them less effective at excluding pests.

The take-home message

Non-chemical approaches to insect pest control are an important tool in protected greenhouse-grown food crops. Insect screens can be an effective tool when the right equipment is covered, appropriate screen is selected and maintained and adequate greenhouse airflow is ensured. Whether including screens as part of a new build or retrofitting old facilities, consider insect screens for improving pest control.

The author is an associate professor in the Department of Horticulture at Iowa State University. Reach him at ccurrey@iastate.edu.