Screen Test

Insect screens could be the solution to propagations pest problems

Most Nurseries use some form of integrated pest management (IPM). Basically, IPM involves pest prevention and monitoring as part of the management program. IPM aspects are control, sanitation/exclusion, environmental/cultural management and pesticide use. Pest management includes control via mechanical, cultural, biological and chemical means.

Many nurseries use greenhouses for a higher level of environmental control and, subsequently, to boost the productivity of propagation areas.  Propagation is high-density production, and there are typically large dollar volumes of inventory on propagation house floors and benches.  These can be very vulnerable to pest infestation.

Screening is a relatively low-investment mechanical control that can be applied to prevent pests from attacking from the outside.  The beauty of screening is that it is highly efficient and works around the clock.  Its performance is consistent over time (think no resistance buildup).

If screening is selected and installed properly, it can pay for itself in a short period and work for years with minimal maintenance. Various industry sources quote that screening gives greenhouse growers 30-70 percent reduction in pest problems, and as high as 90-percent reduction in pesticide costs.

Keep ‘em out

Screening for insects in the simplest form of filtration.  The screen is a filter that collects unwanted particles – insects in our case – from a moving air stream based on particle size.

The screen is a grid of connected holes that allow air and insects smaller than the holes to enter.  Insects larger than the hole sizes are kept outside.

The screen must be placed in areas where insects are able to creep or fly in: all openings such as doors, roof vents, sidewall vents, fan louvers, etc.  It’s safe to assume that if air is coming in, then insects can come in unless there are screens.

Also, a screen is only effective when it is secured, so a screened vestibule may be necessary to prevent open doors from inviting in pests past the screen.  Some nurseries install double-door systems on propagations houses to solve this problem.

Screen Variety

The same insects do not attack all crops.  For this reason, many insect screen meshes are available.  Insect screens are classified by hole size or mesh.  Mesh size will be given in terms such as 38 x 44, which is the number of threads per inch in each direction.  Since the diameter of the threads varies from screen to screen, it is safest to buy screen material by hole size.

It makes sense to choose the screen with the smallest hole possible. Holes small enough would, technically, keep all insects from entering.  However, just as small holes restrict insect movement, a screen with small holes can restrict the amount of ventilation air from entering the propagation house. This can result in high temperatures during warm weather due to inadequate airflow.  However, if some simple design guidelines are followed, the appropriate insects can be excluded with tolerable impact on ventilation.

All insect screens discourage the entry of insects.  To restrict insects with a high level of certainty, the steps to choose the right screen are:

Determine the smallest insects that cause damage.

Determine the smallest dimension for this insect. For example, for Thrips you would choose the diameter of the thorax rather than the length of the insect.

Choose a screen that has a hole size smaller than this smallest dimension of the insect. Choose a different screen depending on whether you wish to exclude thrips, whiteflies, aphids, or leafhoppers.

Size to fit

Now half of the work is done. The only remaining steps involve choosing the amount of screen necessary and determining your ventilation needs. This is where the consideration of airflow restriction enters the picture.

Every screen or filter can be evaluated for its effect on airflow. For insect screens on the market, this evaluation is typically performed in a wind tunnel at a university or independent laboratory. This evaluation can be plotted on a pressure0velocity curve. This evaluation tells us that the faster air flows through the screen (the higher the velocity), the more resistance it generates or the more pressure is required to get the air through the screen.

Let’s use a tennis racquet as an illustration. The strings represent an insect screen. When we swing the racquet slowly, there is very little resistance and air flows easily thought the strings.  Now imagine driving down the road at 55 mph and sticking the racquet out of the window with the face of the racquet perpendicular with the air stream.  Tendonitis would be the least of your problems.

You can see that the resistance is much greater at higher air velocities. The same goes for greenhouse screens.  It’s easy to tell structures that have undersized insect screens because when all the fans are on, it’s difficult to pull the door open.

Size Comparison

This resistance can be overcome by increasing the area of the screen.  Air filters in cars and in newer vacuum cleaners are pleated. They’re pleated so air can pass through a larger surface area of the filter. This effectively slows down the air and reduces resistance.

Most mechanically ventilated greenhouses have a fixed maximum airflow volume with all of the fans turned on and a fixed vent-window area and a minimum sustained wind velocity that helps ventilate the greenhouse.

Using these values and the pressure-velocity curve, you can choose the amount of screen area required to have an acceptable impact to airflow.  Most greenhouse fans are designed to move a high volume of air at low pressure.  Pressure restrictions come from vents that are too small of from air passing through cooling pads or through poorly designed screens.  If the restriction is too great, the fan becomes choked and won’t move air sufficiently.

Seek help

Ask you local extension engineer or a representative of the screen or ventilation-equipment manufacturer to help make sizing calculations.  Choose a screen that is ultraviolet-light stabilized so it will have a long life and will not degrade in direct sunlight.

Are insect screens right for your operation? Make a quick investment calculation. Try to think of the impact that insects have on your propagation.  Is production or quality impacted? How often and how much pesticides are you applying?  How does this affect labor costs, and how could a loss of a crop affect your company?

Divide the installed cost of the screen by the estimated annual savings and you will find out how long it would take to pay yourself back for the investment.

Kurt Parbst is President of Ludvig Svensson Inc.
(704) 357-0457 phone
(704) 357-0460 fax
kurt@svensonamericas.com
www.svensonamericas.com