Structure Basics, Part II: Using Insect Screening
What are the advantages of insect screening?
Properly installed insect screening restricts the entry ofinsects and pests and reduces pesticide use.
What are the disadvantages of insect screening?
The disadvantages of insect screening include increasingsizing and fastening problems, decreased ventilation, reduced access to thegreenhouse and added maintenance. In addition, screens can keep insects in aswell as out.
What negative effects do screens have on airflow?
Screens with small holes are more effective in excludingpests but are more resistant to airflow. A screen with too much restriction ofairflow can cause higher static pressure drops, inadequate air exchange,increased energy consumption by the fans, excessive wear on the fan motors andhigher greenhouse temperatures.
Can airflow problems be avoided?
Yes, correctly installing screens, which have been properlychosen for new construction or retrofitted to existing greenhouses, can excludeinsects and pests while still allowing for adequate airflow.
How many screen varieties are there?
There are many different screens for reduction of almost anytype of insect or pest. The challenge lies in matching the type of screen tothe insect or pest you wish to control. For crops that suffer from pests duringa limited part of the growing season, lighter-duty, less expensive screens willwork. For handling multiple pests at different points in the growing season(e.g., aphids in spring, thrips in summer and whiteflies in fall) select a morerestrictive screen.
Lighter screens can also be used for short-term, interiorzones and for periodic use when pests appear in larger numbers. Heavier, morerigid screens can provide protection against sun, wind, rain, hail, snow andwear and tear from equipment and workers brushing against it.
What about thrips; are they too small for screening?
Even though thrips are small enough to fit through mostscreens with good airflow, it has been shown in many cases that they can bedramatically reduced with the white screens designed for whiteflies. It istheorized that these screens are effective due to the color of the screens andthe thrips’ inability to recognize the material as something to feed on.
What is the basic range of insect sizes?
Insects range in size from 215 micrometers (western flowerthrips), which is barely visible to the naked eye, to 608 micrometers(serpentine leaf miner), which is easily diverted by screens.
What are insect screens made of and how do they hold up?
The most common screen, most often seen in homes, is made ofstainless steel and brass. While being the longest-lasting, it is the mostexpensive.
There are two types of polyethylene screens. One ismonofilament, woven with solid strands similar in appearance to fishing line,which is very rigid and strong. The other type is made of film that is punchedfull of “micro holes” and used as a crude, but low-priced, insectbarrier. Drawbacks include weak construction and low UV protection as well asvery restricted airflow.
A third type is polyethylene/acrylic. This material is madeof many fibers, “multi-filament,” which causes resistance to smoothyarns sliding together and thereby maintains the integrity of the holes.
A fourth type is nylon. This type is good for shorter-term,low-cost and light-duty applications; it is more restrictive to air flow.
Does the manner in which it is constructed affect thescreen?
There are three manners in which screens are constructed.Weave, the most common, provides a trade-off between hole size and airflow. Besure to always check the tightness of the weave and apply lateral tension tosee if the hole distorts. Knit means that each thread is tied around the next,creating a durable network resistant to tearing and raveling. Extra loops andknots may cause greater air restriction. Film can be punched full of microholes creating an insect barrier that is very restrictive to airflow and mustbe applied with the correct side out.
What is considered adequate ventilation?
Willits (1993) recommends an air exchange of 11-17 cubicfeet per minute per square foot. These low rates are based on a study conductedusing no alternate cooling devices, e.g., cooling pads, shade cloth, whitewash, etc.
How can I maintain adequate ventilation while retrofitting?
The first step in retrofitting with insect screen is tocheck the current ventilation system. Measure the difference in static pressurein the structure with all the fans off and then with all the fans running. Usethat pressure drop when consulting the manufacturer’s specification chartto estimate the total amount of air moving through the greenhouse. Interpolatebetween 0.0-inch, 0.05-inch and 0.01-inch volumes given for the various fansand motors. (For example, a 0.025-inch pressure drop is halfway between the0-inch and the 0.05-inch. Thus, the volume of air moved would be halfwaybetween the volumes given for 0 and 0.05.) Then add up all the volumes of thefans together. Divide this total by the number of square feet of thegreenhouse; the quotient should equal an air exchange of 11-17 cubic feet perminute per square foot. Certainly if the volume of air exchanged is below 8cubic feet per minute, the structure is likely to overheat during hot, brightweather. If the total volume of air exchange is well above 17 cubic feet persquare foot, the selection of screening fabric may be limited and transpirationand evaporation will be excessive.
What if my greenhouse is naturally ventilated?
In a naturally ventilated greenhouse, the speed of the airis neither as rapid nor as constant as that of greenhouses ventilated withfans. Therefore, there is no formula for determining how the greenhouse willfunction when screened. Naturally ventilated greenhouses can be successfullyscreened if the following guidelines are considered.
When does the crop suffer from insect damage? Is it at atime when heat loads are critically high for the crop? You may consider notscreening all vents, monitoring temperatures closely and removing the screenwhen the pest threat is past and the weather grows warmer.
If the greenhouse is already at its upper limit fortemperature, you have three options. One, increase the open area of your ventsand replace solid poly walls with walls made of insect screen and covered byroll-up poly film. If you are unsure of heat gain, experiment with one sectionat a time until you are comfortable with the application. Second, you couldscreen only the side that faces the wind, since most insects are carried bywind; this method has been shown to reduce insect populations. Three, considerthe color of shade cloth used to shade the greenhouse. Black shade cloths,although they have a long UV life, tend to create excessive heat transfer,radiating heat into the house. By using an aluminized shade cloth ,you cannegate the additional heat gain associated with the insect screen.
Are there specific things I need to do to maintain insectscreens?
Yes, the screens need to be cleaned or dirt and dust willalter the static pressure in the greenhouse. Although you should check with themanufacturer for proper cleaning guidelines, the following suggestions may beused for most screens. Clean the screens from the inside out with a hose and nozzle pressure. Never use high-pressure cleaners or brushes as they will alter the holes’ size and make the screening useless. Screening should not becleaned during ventilation, as the water can fill the openings and completelystop airflow. The best time to clean is in the evening when ventilation isusually over.
How do I attach insect screens to my greenhouse?
There are three options when fastening your screen: polyfastener, spring lock or lath. Remember that keeping the screen snug andavoiding abrasion are your main goals when attaching the screen. Contact yourNGMA greenhouse manufacturer for recommendations, as they may have ready-madesolutions available.
What kind of results should I expect when screening mygreenhouse?
Average grower: The average grower, one that grows and sellsto market or end users, usually wants good control of insects and will becontent with a 70-90-percent decrease in pests. These results can be achievedby screening the air inlet only.
Primary propagator and research facilities: Propagators havea high demand from their customers to provide insect-free plants, and researchgreenhouses need full control of the environment. In these two instances, thefollowing things need to be screened: the air inlets, vents and fans. In additionto screening, the doors need to be air-locked and all leaks or gaps in thehouse need to be sealed.
How advanced are screening applications?
The need for insect screens will continue to increase due toreduced availability of insecticides and the demand for high-quality,insect-free plants. With this increase, we will see more applications forinsect screens. As the NGMA members would like to encourage their successfuluse, they have written a considerations document specific to insect screens.Contact the NGMA office for a free copy or download it off the web atwww.ngma.com
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