Using the correct type of screening on greenhouses will reduce pest infestation levels and the need to apply pesticides. With Australia' s relatively warm climate, many greenhouses in Australia make use of screening for climate control. Unfortunately most materials sold for this purpose are not suitable for keeping out pests. Ask yourself first; 

Where do most of my pests come from?

If most pests enter from outside to seasonally reinfest the crop, screening will have a very real benefit. If, on the other hand, they mostly come in on plant stock imported from other greenhouse operations, screening may keep them from spreading to other greenhouses and outdoor vegetation on your own property. However it will also keep them inside the structure and require that you have a good pest control program in place within the screened house. 

Which pests am I trying to exclude?

Presuming a weed- and vegetation-free area is maintained around the greenhouse perimeter to deter walking insects, the screen will mainly keep out flying insects such as moths, aphids, thrips, leafminer flies and whiteflies. The smaller the pest, the more expensive the screen and the cost of installing it. If the only pests of concern are moths, a large screen hole size is adequate. Keep in mind that pests such as western flower thrips (WFT), melon aphid, and silverleaf whitefly (SLW) may not have reached your area yet, but probably will so plan for the future if you grow sensitive crops.

Screens will protect valuable crops

Do I need to screen out 100% of the pests? or can I live with reduced populations?

This will depend upon the type of crop being grown and its value. For instance, a propagator who markets on the basis of selling virus- or disease-free stock to other producers at a premium price can often justify the expense of exclusion screening when the alternative is intensive spraying of pesticides. If total exclusion is necessary consider all the areas where the target pest could gain entry. These may include vents, doorways, greenhouse seals, and material and personnel entering the greenhouse. If reduction of the population is adequate, a larger hole size may suit. Marketers of screening to keep out thrips often claim close to 100% exclusion. We have yet to see good data supporting this claim, but it would be much more difficult to test on a field scale as opposed to laboratory tests.

What type of screening is available? 

A number of international suppliers are selling screening for the purpose of excluding pests. While all the screens will reduce entry, some are far more effective than others. Three basic types are promoted - a multi-layered fibre, a polyester sheet and single-layer material:

Increasing the surface area increases air flow

1. Single-layer materials of regular weave may be composed of polyester or other man-made fibre, or of metal. Although metal is much more durable and has a longer life than the other materials, it is also the most expensive and difficult to install. Cloth materials are the least durable but relatively easy to install and are the only type commonly available in Australia.
2. Multi-layered fibres comes in various thicknesses and densities. Hole size is very irregular. Those similar to air filter screens allow reasonable air movement but fairly easy passage of thrips and similar sized pests.
3. Polyester sheet has holes punched into it from one direction so that there is a rough side and a smooth side. The smooth side should be the outer side. Air movement may be severely restricted.

The hole size depends on the smallest pest to be excluded. Suggested hole sizes for three major pests are:

• 0.192mm (western flower thrips), 
• 0.340mm (melon aphid), and 
• 0.462mm (silverleaf whitefly). 

For example, the average width of the body of a western flower thrip (presumably the much larger female), is given as 0.215mm and a hole diameter of 0.192mm is recommended for exclusion. Small scale laboratory trials carried out by in 1990 in Canada indicated that over 80% of WFT females were able to escape through wire mesh screen with 0.149mm diameter holes, a seemingly impossible feat. Unless Canadian WFT are unusually small, clearly more research remains to be done. Other factors also come into play here. Thrips exclusion trials in Israel found that screening that is an attractive colour to thrips, such as white, may actually bring thrips to the screening, making it more likely that they will attempt to penetrate it. Aluminium-coloured screening repels thrips (also aphids and whitefly), so there is a dual effect. It would be worth experimenting with some of the aluminium-coloured shading/thermal materials on the market.

What else do I need to consider? 

Reduced air flow is a major problem when screening vents, particularly with fine screens. If your greenhouses rely on passive ventilation they may require replacement of plastic or fibreglass side walls with the screening. For all greenhouses, particularly those dependent upon normal air flow, you will need to consult with your local greenhouse engineer, screen manufacturer and fan supplier.

Air flow can also be seriously impeded by dirt collected on the screens. Regular cleaning is therefore very important. The finer the screen, the more frequently you will need to clean it. Protection from rain will also be necessary. Screens may also affect light transmission, so unless they are being installed on a south wall, a clear material would be an advantage.

The Bottom Line
Using the correct type of screening on greenhouses will reduce pest infestation levels and the need to apply pesticides. The spread of pesticide resistant WFT and SLW will make their use essential for some crops. Single-layer materials of regular weave are the only type commonly available in Australia. Disadvantages are that they cost around $7 to $9 per square metre and reduce airflow.

Acknowledgements
The nursery industry pot levy and HRDC are part funding the national IPM in Greenhouses project centred in Gosford at NSW Agricultures Horticultural Research and Advisory Station. Photos supplied by Living Shade Pty. Ltd., Hornsby NSW. 

*Entomologists, NSW Agriculture, PO Box 581, Gosford NSW 2250. Ph:(043) 481 900 Fax:(043) 481 910