Reasons to Consider Screening Acquisition of resistance to pesticides has made insect control a real struggle in greenhouses. Western flower thrips, silverleaf whiteflies and aphids have become noticeably more resistant to pesticides over the past few years. Although pesticides remain an important tool for pest management, other methods of pest control, especially screening, must be used to slow the build up of pesticide resistance and to conserve the usefulness of pesticides.

Environmental and health problems associated with pesticides have sensitized the public and greenhouse workers to pesticide issues. Because of dermal and respiratory exposure to pesticides, greenhouse workers have greater risk associated with pesticides than any other group of agricultural workers. An obvious way of reducing risks is to reduce exposure. Screening should reduce the numbers of pests entering a greenhouse so fewer pesticide treatments are needed.

Viruses are a third problem confronting greenhouse operators. As tomato spotted wilt virus becomes established in perennial weeds outdoors, some thrips sucked into greenhouses in hot weather may carry the common or lettuce strain of tomato spotted wilt virus. Other thrips may acquire impatiens necrotic spot virus from infested plants in the greenhouse and start spreading this disease around inside. It is much easier to screen out thrips than to eradicate thrips-vectored viruses on susceptible crops!

Screening Structures - Pest exclusion structures do not need to be constructed of heavy timbers nor be elaborate. Screening fabrics are relatively light but strong and they do not require much support. Most retrofitted screens are placed outside the ventilation windows and entry doors. Provide easy access to the inside of the screen so that it can be washed from the inside if necessary. Exclusion will be much more effective if workers can be trained to keep the doors closed as much as practical especially during ventilation.

Static Pressure - When exhaust fans are running. the air pressure drops inside the greenhouse. Doors are harder to open, and gusts of air whoosh through opened doors as the pressure equalizes with the pressure outdoors. The air pressure inside a greenhouse is called static pressure. If one end of a manometer tube is inserted into the greenhouse, the level of the liquid inside rises as the fans come on and static pressure drops. You must make sure that

1. the flexible tubes are free of any drops of liquid,
2. the tubes are open and not kinked and
3. the tube connections are tight.
4. Make sure the manometer is level.
5. Be sure to make your final measurements when the outside air is relatively still.

Otherwise, you may obtain irratic results! (Manometers at reasonable prices are available from Dwyer Instruments and other vendors).

Static pressure is usually measured in inches of water. If the static pressure drop is too great, the fans will not be able to move enough air to properly ventilate the greenhouse, the fans will use excessive power, and the greenhouse will overheat during hot summer days. Johnson (1990) suggests using only screening materials that create a static pressure drop of less than 0.05" of water at 250 feet per minute air velocity. Sase and Christianson (1990) recommend a 0.032" pressure drop for clean screening materials and a total pressure drop not exceeding 0.1" with dirty screening. We recommend a maximum pressure drop of 0.1 " for a screened greenhouse with clean screening.

Adequate Ventilation - To check that a greenhouse is ventilated adequately, estimate the amount of air moving through the ventilation windows with all fans running and other doors and windows closed. Using a manometer, measure the static pressure drop by subtracting the pressure with the fans shut off from that with the fans running. Use that pressure drop when consulting a fan specification chart given in various supply catalogs. For each fan model, look across to the volume of air moved by that fan at your static pressure drop. Interpolate between the 0.0", 0.05" and 0.1" volumes given for the various fans and motors. (For example, if the pressure drop is 0.025", the volume of air moved by the fans would be about half way between the volumes given for 0" and 0.05"). Then add together the volumes of each fan. Upon dividing the total volume by the square footage of the greenhouse, the quotient should equal an air exchange between 11 and 17 ft³ [cubic foot] per minute per ft² [square foot] (recommended by Willits, 1993). This recommendation is higher than Nelson's (1985) of 8 ft³ per minute per ft². If the volume of air exchange is below 8, the structure is likely to overheat during hot, bright weather and the selection of screening fabrics may be limited. If total volume of air exchange is well above 17 ft³ per minute per ft², transpiration and evaporation may be excessive.

Screening Materials - Non-woven or spunbonded fabrics are materials that are extruded and stuck together to form a fabric. FlyBarr is a spunbonded fabric with a plastic mesh that gives it strength. It is highly resistant to airflow and difficult to clean. FlyBarr is available from .Hydro-Gardens, P.O. Box 9707, Colorado Springs, CO 80932. Typar is a spunbonded polypropylene material that is very resistant to ultra violet light degradation. It is also resistant to air, so growers using Typar must use a relatively large screen area. Reemay is spunbonded polyethylene and breaks down in a matter of months outdoors. Compared to Typar, Reemay is less resistant to airflow and so may function well with a much smaller area. Typar and Reemay are manufactured by Reemay, Inc., 70 Old Hickory Blvd., Old Hickory, TN 37138 and are sold in various greenhouse supply catalogs. These two fabrics are by far the least expensive of all screening materials available.

Woven materials we tested included Pak 32x32, 44x44 and 52xS2 fabrics, manufactured by Pak Unlimited, 3300 Holcomb Bridge Road, Suite 215, Norcross, GA 30092. Lumite 32x32, 42x42 and 52xS2 fabrics are manufactured by Synthetic Industries, 6525 The Corners Pkwy., Suite 115, Norcross, GA 30092. Kontrol 45000series products are available from Baycor Products Group, 3500 Parkway Ln., Suite 500, Norcross, GA 30092. Econet fabrics are manufactured by LS Americas, 1813-E Associates Ln., P.O. Box 19548, Charlotte, NC 28219. BugBed fabrics are available from Green Thumb Group, 1-800-240-3371. NoThrips, AntiVirus and Insecta 500 screens are available from Green-Tek, 407 N. Main St., Edgerton, WI 53534. Durascreen is available from DuraGreen Marketing, P.O. Box 1486, 2600 Britt Rd., Mount Dora, FL 32757. Many of the above materials are carried by various greenhouse suppliers. These woven materials vary widely in their airflow resistance and exclusion capabilities.

The effectiveness of various screening materials for excluding pests has been investigated by Bethke (1994) in the laboratory and Bell and Baker (1997) using uniform approach velocities of 300 feet per minute. According to Bell and Baker (1997), presently NoThrips and BugBed 123 appear to be most effective in screening out thrips with BugBed ll0UV and Econet S also giving good (>70%) thrips exclusion. BugBed 123, BugBed l l0UV, BugBed 85, Econet S, Econet T, Pak 52xS2 and Protex I are the most effective in excluding whiteflies, though a few other screens provide good whitefly exclusion.

Resistance of Screening Materials - Materials with small holes are often better for pest exclusion but are usually more resistant to airflow. If a grower decides to apply a fabric with a very small hole size but under-estimates the area of material needed, the ensuing screen may block airflow resulting in high static pressure drop, inadequate air exchange, higher energy consumption by the fans, excessive wear and tear on the fans, and high greenhouse temperatures (Figure 1).

When building an exclusion structure, be sure to incorporate easy access to the inside to facilitate cleaning of the screening material. Clean fabrics have less resistance to airflow. It is a good idea to have a manometer convenient to check static pressure in each screened greenhouse on a regular basis especially in hot, dry weather when screening is likely to be rapidly fouled by dust.

Do Not clean the screening material while the ventilation fans are running! Clean the screens early in the morning or wait until evening. Water can fill the holes in the material by capillary action and completely stop airflow. On a sunny day, greenhouse temperatures would rise swiftly, and unscreened windows and door would have to be opened to prevent heat damage to plants before the water m the screen evaporates. Opening the windows and doors defeats the whole exclusion effort.

How to Retrofit Screening on a Greenhouse -Caution: Any screening retrofitted to a greenhouse without other changes will decrease airflow and increase greenhouse temperatures. The following procedure should help avoid serious ventilation problems. Another consideration: If you start with pests already inside or bring pests in on plants or clothing, screening will keep them inside.

Step 1. Measure the pressure drop inside the greenhouse by using a manometer: _________ inches. (If the pressure drop is close to 0.1 " without screening, consider enlarging the ventilation window. If the fans stay the same, enlarging the ventilation windows will reduce static pressure.)

Step 2. Subtract the pressure drop in Step 1 from 0.1": ________ inches. This difference is a guide to how much additional resistance to air movement can be tolerated. For example if your pressure drop in Step 1 is 0.025", then you can use a screen that adds up to an additional 0.075" of pressure drop without exceeding our maximum recommended pressure drop.

Step 3. Now calculate an estimated total air movement at 0.1" by consulting a fan specification chart given in various greenhouse supply catalogs. Look across the line from the model of each fan at the volume of air moved by that fan at that static pressure drop. Then add all the volumes of each fan together: ________ cfm.

(Now you can check if the volume of air that will move through the house after screening exceeds Nelson's (1985) or Willits' (1993) recommendations. If not, your houses may become too hot during July and August. Consider using larger motors on the fans or adding additional fans.

Suggested Air Exchange per minute:

Willits' Low:

Area of greenhouse X l l cfm = ________cfm.

Willits' High:

Area of greenhouse X 17 cfm = _______ cfm.

Step 4. Calculate the area of the ventilation windows.

Total Ventilation Window Area (length X width): ________ ft2.

Step 5. Calculate the approach velocity of the air moving through the ventilation windows (Total air volume after screening from Step 3 divided by ventilation window area from Step 4):_________ ft/mm.

Step 6. Examine Figure l and find the approach velocity from Step 5 on the horizontal axis. Those fabrics whose curves do not exceed the pressure drop level calculated in Step 2 at the approach velocity from Step 5 can be used directly over the ventilation window. If the resistance curve for the fabric you wish to use exceeds the pressure drop level from Step 2, then move to the left along the velocity axis until you reach a velocity at which the resistance does not exceed Step 2 pressure drop. Then divide the velocity through the ventilation window in Step 5 by the lower velocity on the chart and the quotient is the number you must multiply the area of the ventilation window by to arrive at the area of the screening material: Multiplication Factor: ___________

Figure 1. Resistance factors for eight screening materials shown as functions of velocity in feet per minute and static pressure in inches of water.

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Figure 1. Resistance factors for eight screening materials shown as functions of velocity in feet per minute and static pressure in inches of water.

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• Comparison of Greenhouse Screening Materials for Excluding Whitefly and Thrips. Bell, M. and J. R. Baker. 2000, J. Econ. Entomology 93(3):800-804.
• Screening of Greenhouses for Insect Exclusion. Murphy, G. 2009 (revised). Ontario Ministry of Agriculture, Food and Rural Affairs Factsheet.
• Select greenhouse screening materials for their ability to exclude insect pests. N.C. Flower Growers' Bull. Bell, M. and J.R. Baker. 1997.
• Extension Plant Pathology Publications and Factsheets
• Horticultural Science Publications
• North Carolina Agricultural Chemicals Manual

For assistance with a specific problem, contact your local N.C. Cooperative Extension Center.