Introduction

Integrated Pest Management (IPM) of greenhouse crops includes strategies such as monitoring, and cultural, chemical and biological controls. It also includes physical controls such as screening to exclude flying insect pests. While screening has been a textbook component of IPM programs for many years, until the mid-1990s it was rarely used in Ontario. Today, some growers have retrofitted (usually small) areas of their greenhouses with insect exclusion screening.

Screening removes an important variable from a grower's pest management program - the movement of pests into the greenhouse from outside. This includes common greenhouse pests such as thrips, aphids, and whiteflies, but also some less common pests such as tarnished plant bug and European corn borer. These can become major pest problems if you use biological control programs to reduce pesticide use. Benefits of using insect screening have been demonstrated in Israel and California. In Ontario, growers who installed screens report reduced pest levels and pesticide use, and improved effectiveness of pest control measures (especially biological control).

Target pests

First, you must determine which pests you need to exclude. The size of the pest determines the mesh size of the screen needed. Large mesh sizes of the type normally found in household screens are inadequate for excluding most major greenhouse pests. However, they can keep out occasional larger pests such as tarnished plant bug and lepidopteran (moth) pests. For the more common pests, smaller mesh sizes are needed (Table 1). When you have multiple pests of concern, choose a mesh size to exclude the smallest of these pests.

Table 1. Screen mesh sizes needed to exclude major greenhouse pest species
PestMinimum Mesh Size for Exclusion1 millimetre = 1000 microns (µ) (From Bethke, 1994)
Leafminer (Liriomyza trifolii)608µ
Green peach aphid (Myzus persicae)434µ
Melon aphid (Aphis gossypii)355µ
Greenhouse whitefly (Trialeurodes vaporariorum)288µ
Silverleaf whitefly (Bemisia argentifolii)239µ
Western flower thrips (Frankliniella occidentalis)215µ

Effectiveness

Data from trials in commercial greenhouses in the Niagara region show that screening can: (a) delay the onset of a thrips infestation, (b) significantly reduce thrips levels inside greenhouses, and (c) facilitate a sustained reduction in thrips populations in greenhouse crops. Figure 1 shows monitoring results from a commercial greenhouse growing cut roses in Ontario, and emphasizes the relationship between thrips entering the greenhouse from the outside, and the trap catches of thrips on the inside.

Thrips populations in a rose crop as monitored by sticky cards
Figure 1. Thrips populations in a rose crop as monitored by sticky cards; a) in the vent opening, b) in the greenhouse closer to the vents, and c) in the greenhouse away from the vents. (Data courtesy C. Teerling, Agriculture and Agri-Food Canada, Vineland).

Accessible description of Figure 1

Image shows the impact of screening on thrips populations in 2 cyclamen crops in adjacent areas of the same operation. The reduction in thrips numbers in the screened area was immediate and lasting.
Figure 2. Thrips populations in screened vs. non-screened greenhouses. (Data courtesy, Fernlea Flowers, Delhi, Ontario).

Accessible description of Figure 2

Image shows the effects of screening on thrips populations in 3 adjacent compartments, each of 20,000 sq. ft. containing potted and bedding plants. The middle compartment was screened and the ones on either side were unscreened. Thrips numbers in the unscreened compartments increased earlier than in the screened one, where thrips stayed at lower levels for approximately 2 months during late spring and summer.
Figure 3. Thrips populations on sticky cards in 1998 in a screened vs. 2 adjacent unscreened compartments. (Data courtesy Jeffery's Greenhouses, St. Catharines, Ontario).

Accessible description of Figure 3

Ventilation

A major concern of growers considering screening is the reduction in air flow. Reduced air flow can lead to overheating of the greenhouse, and stress on the fans (in the case of fan-ventilated greenhouses) that have to work harder to pull the same amount of air through the partially-blocked vent. Reduced ventilation is a valid concern, but it can be addressed by increasing the surface area of the vent. In many situations this is done successfully by building a screened framework around the vent opening. The goal is to ensure that the final surface area of the vents provides sufficient air exchange to allow adequate cooling of the greenhouse.

Several methods are available to determine the increase in surface area of the vents required for any given greenhouse. Factors such as screen mesh size, fan capacity, and static pressure drop (the difference in air pressure between inside and outside the greenhouse when the fans are running) are important. Although software programs exist, the calculations can be complicated and are generally best left to the screening manufacturers.

Screening structures

The type of screening structure you install depends on:

  • whether you need to increase surface area to maintain adequate ventilation, and
  • the type of ventilation and vent openings - passive vs. forced, positive vs. negative pressure, side vents vs. peak or gutter vents.

In general, passively-vented greenhouses are more difficult to screen and ventilation through them may not be as effective as in forced ventilation systems. Some options for growers are shown in the following photographs.

Screening of side vents

Fitting screens flush into vent opening
Figure 4. Fitting screens flush into vent opening.
Fitting screens flush into vent opening.
Figure 5. Fitting screens flush into vent opening.
Extending the gutters on a gutter-connected house and adding a screened bay to the existing structure. A door has been built into the screened area to allow access from the outside.
Figure 6. Extending the gutters on a gutter-connected house and adding a screened bay to the existing structure. A door has been built into the screened area to allow access from the outside.
Building a lean-to along the vent side of the greenhouse to fully enclose all the vents.
Figure 7. Building a lean-to along the vent side of the greenhouse to fully enclose all the vents. In Figure 7, note the polyethylene covering over the screen, protecting the screen from winter damage.
Building a lean-to along the vent side of the greenhouse to fully enclose all the vents.
Figure 8. Building a lean-to along the vent side of the greenhouse to fully enclose all the vents. 
Screen attached to the vent and the vent opening using poly-lock.
Figure 9. Screen attached to the vent and the vent opening using poly-lock.
Building an interior framework to screen the vents from the inside of the greenhouse.
Figure 10. Building an interior framework to screen the vents from the inside of the greenhouse.

Screening of fans

Intake fan with screened frame outside the greenhouse.
Figure 11. Intake fan with screened frame outside the greenhouse.

Screening of roof peak vents

Fitted 'accordion-style' screens can be individually installed in each vent opening of a Venlo-style peak vented greenhouse.
Figure 12. Fitted 'accordion-style' screens can be individually installed in each vent opening of a Venlo-style peak vented greenhouse.
A single screen across the whole peak (below the level of the vents) in a similar fashion to an energy curtain.
Figure 13. A single screen across the whole peak (below the level of the vents) in a similar fashion to an energy curtain. This can significantly reduce light levels and may be more useful in conditions where high light is not a priority. Such a screen may also be more difficult to clean.

Screening of gutter vents

Customized screens have been installed in some gutter-vented research greenhouses, using a system of weights to keep the screening taut and unobtrusive when the vents are closed.
Figure 14. Customized screens have been installed in some gutter-vented research greenhouses, using a system of weights to keep the screening taut and unobtrusive when the vents are closed.

Screening of doorways

Screened sliding door.
Figure 15. Screened sliding door.

Maintenance and cleaning of screens

The small pore size means screens are very prone to blockage with dust and other deposits, especially in the summer. Such blockage reduces air flow through the screens and may contribute to excessively high temperatures. It is therefore important to clean the screens regularly. It is also important, when designing your screens, to ensure easy access to them for cleaning. Wash screens from the inside with a high pressure hose.

Do not do this when the fans are operating, because the water will block the pore openings and completely stop air flow, leading to overheating of the greenhouse.

The effectiveness of the screens depends on their ability to exclude flying insects. Repair tears and holes as soon as possible. You can repair small tears by simply gluing a piece of screening over the hole.

Durability

When buying screening, consider the durability and longevity of the product. The longevity of the screen depends on how it is made, what it is made of, and how it is installed.

For example - see Figure 7 and Figure 10.

Consider how susceptible the screening will be to damage from equipment, lawn mowing debris, entry by mice or other animals.

Most commercial screens on the market are reported to have a life span from 3 to 5 years, based on manufacturer claims.

Costs

The cost of screening depends on several factors:

  • the final screening design
  • the increase in surface area needed to maintain adequate ventilation
  • the cost of the screening material
  • whether roof vents or side vents are to be screened
  • the frequency of replacement; frequency of cleaning.

The biggest variable is the difference in cost between installing screens in a force-ventilated, side-vented greenhouse, and a passively-vented greenhouse with roof vents. Nevertheless, reports from growers who have installed screens over side vents in force-ventilated greenhouses, suggest a rapid payback from reduced pesticide costs and improved pest management.

Additional reading

Baker J.R., Bell M.L. and Shearin E.A. (1997). Insect Screening. Information Note 104: Ornamentals and Turf, Integrated Pest Management. North Carolina Cooperative Extension Service. NCSU, 4 pp.

Bell M.L and Baker J.R. (1997). Choose a greenhouse screen based on its pest exclusion efficiency. North Carolina Flower Growers' Bulletin. 42(2):7-13.

Bethke J. A. (1994). Considering Installing Screening? This is what you need to know. Greenhouse Manager, April 1994, 34-36.

Hahn R.H. and Rosentreter E.E. (1989). Heating, Ventilating and Cooling Greenhouses. In: ASAE Standards, R.H. Hahn and E.E. Rosentreter (eds.), 452-455.

National Greenhouse Manufacturers Association. Insect Screening - Greenhouse Insect Screen Installation Considerations for Greenhouse Operators (Copyright Pending 1996).

Sase S. and Christianson L.L. (1990). Screening Greenhouses - Some Engineering Considerations. Proceedings of the 1990 Northeast Agricultural/Biological Engineering Conference; 1-13.

Willits, D.H. (1993). Greenhouse Cooling. North Carolina Flower Growers' Bulletin. 38(2): 15-18.

Accessible image descriptions

Figure 1. Thrips populations in a rose crop as monitored by sticky cards; a) in the vent opening, b) in the greenhouse closer to the vents, and c) in the greenhouse away from the vents.

Thrips populations in a rose crop as monitored by sticky cards in the vent opening peaked on the 22nd of September at 105 thrips per sticky card per vent.

Thrips populations in a rose crop as monitored by sticky cards in the greenhouse closer to the vents peaked on the 24th of September at 80 thrips per sticky card per vent

Thrips populations in a rose crop as monitored by sticky cards in the greenhouse away from the vents remained fairly constant. It was highest on the 11th of August with about 18 thrips per sticky cards per vent.

Figure 2. Thrips populations in screened vs. non-screened greenhouses.

The thrips population in an unscreened greenhouse peaked in early-May at 43 thrips counts per week. While in a screened greenhouse, the thrips peaked also in early-May but at 7 thrips counts per week.

Figure 3. Thrips populations on sticky cards in 1998 in a screened vs. 2 adjacent unscreened compartments.

In the first unscreened compartment, the thrips population peaked off the charts in early-August at above 300 thrips counts per week.

In the second unscreened compartment, the thrips population was high in mid-June with thrips populations at 140 thrips counts per week, and again in late-August at about 150 thrips counts per week.

In the screened compartment, the thrips populations peaked in late-August at about 80 thrips counts per week.