Greenhouse insect screens - making the right selection A three year study conducted at North Carolina State University (USA) compared the effectiveness of various screens for excluding whiteflies and thrips under summer ventilation conditions resembling those of a commercial greenhouse. Each commercial screening material was compared with ordinary fibreglass window (fly wire) screening. Of the 26 screening materials tested only two were considered best for thrips exclusion and six were considered best for whitefly exclusion. Fly wire was as good as many commercial products! Role of insect screens In an integrated pest management system, exclusion of pests should be one of the first tactics considered to reduce the need for other control measures. Excluding pests using greenhouse screens is now more cost-effective than in the past due to the rising cost of pesticides and increased worker protection regulations. Reductions in pest populations, lower incidences of insect transmitted diseases and fewer pesticide applications have been documented when screens are used. Screen considerations Airflow A foremost consideration when designing a screen installation is the effect screen materials have on airflow. Airflow resistance, primarily a function of hole or mesh size, varies widely among screen products. In research carried out at North Carolina State University, Dr Bell and her team investigated the airflow resistance of several screening materials and developed resistance curves
for each product. Entomologist James Baker incorporated this airflow resistance data, along with greenhouse airflow recommendations, into a step-by-step guide for calculating the total screen area required for different materials. The information has been developed as a computer program and can be obtained from James Baker, Department of Entomology, North Carolina State University, Raleigh, NC 27695. Fax: +1 919 515 7746. Cost Costs vary depending not only on the price of the screen but also on type and size of the frame required and screen longevity or replacement considerations. Effectiveness The overriding concern should be one of effectiveness. The unfortunate fact is that many screens on the market do not effectively exclude common greenhouse pests at the air-approach velocities required for adequate ventilation. Furthermore, choosing a screen with small holes or high resistance to airflow does not ensure adequate exclusion. Comparing screen effectiveness This study compared the effectiveness of various screens for excluding whiteflies and thrips under summer ventilation conditions resembling those of a commercial greenhouse. Small wood-framed and polyethylene plastic covered cages were used. Each cage was constructed with the front open to allow covering with the screening material under test. The air-approach velocity used for each material was 300 feet per minute, an airflow velocity recommended for well designed production greenhouses. Yellow sticky traps were placed inside cages before the installation of screening materials and were used to sample whiteflies and thrips entering cages through the screens. Each commercial screening material was compared with ordinary fibreglass window (fly wire) screening which was used as the control to determine the degree of exclusion. Screen ratings Of the 26 screening materials tested, only five screens gave greater thrips exclusion than the fly wire window screen. Only two were considered optimal for thrips exclusion. (See accompanying table of Dr Bell s results.) Fifteen screens gave greater SLW exclusion than the fly wire screen and, of them, the six screens rated *** (three stars) against SLW can all be considered the best for exclusion of that pest. Screens with a similar percentage exclusion for thrips and/or whitefly to the fly wire screen were given a * (one star) rating. Only one product, BugBed 123, was rated *** for both SLW and thrips, providing exclusion of both at more than 93 percent compared with the fly wire screen. From this data, BugBed 123 can be considered the best overall screen for thrips and whitefly exclusion. No-Thrips was rated as *** for thrips exclusion and ** for whitefly exclusion where it achieved 87 percent, still a good result.
Where exclusion of thrips and/or thrips-transmitted disease is of paramount importance but where whiteflies may also be pests, BugBed 123 and No-Thrips should be strongly considered. An advantage of BugBed 123 over No-Thrips, depending on the greenhouse, is that BugBed 123 is only moderately resistant to airflow whereas No-Thrips is very highly resistant to airflow. Where whiteflies are the major pest problem in the greenhouse, four other screening materials - BugBed 110UV, BugBed 85, Econet S and Pak 52 x 52 - are likely to give excellent whitefly exclusion as well reasonable thrips exclusion. In the tests, several screens that did not exclude thrips still provided satisfactory exclusion of whiteflies. Screen longevity The top two screens may differ in longevity. On a demonstration and research greenhouse installed with BugBed 123, the screen barely lasted one season before tearing resulting from weathering occurred. Under similar conditions, No-Thrips appeared more durable. BugBed 110UV, also known in the US as BugShield, is unique among greenhouse screens in that it is dark colored and therefore may be less attractive to insects. It also has enhanced protection from UV degradation, thus possibly prolonging the life of the screen. Woven polyester insect screens inside open roof vents allow good circulation while excluding pests Effect of airflow resistance In general, the higher the airflow resistance of a screen, the smaller the mesh or hole size through which insects must pass. However, the exclusion capability varied widely for the screens exhibiting high resistance. While No-Thrips and Econet S (both exhibiting high resistance) were rated as *** screens for thrips and whitefly exclusion respectively, two other high resistance screens failed to exclude thrips and whiteflies any better than the fly wire window screen. In addition to hole size, Bethke and Paine found that hole geometry may play a role in insect penetration through screens.
The holes of No-Thrips and Econet S are small and regular. In contrast, of two other reputable screens, one is poly-spun material and the other a woven material covered in acrylic, giving both screens high variability in hole size and shape. Only one low resistance screen - BugBed 110UV - was capable of excluding both thrips and whiteflies. High airflow resistance, often costly in terms of requiring greater screening area to maintain adequate airflow in a greenhouse, neither ensures nor is necessary for significant exclusion of whiteflies and thrips. The bottom line Insect screens should be included in a good integrated pest management (IPM) system. However, the material to be used should be selected only after careful consideration of a number of factors, including air-flow reduction, ability to exclude pests and, lastly, cost. Screening in Australia The use of insect screens is not widespread in Australia at this time, with most production nurseries and cut flower growers in both thrips and whitefly susceptible areas utilising other, effective IPM strategies to eliminate or exclude these pests in their crops. Australian greenhouses generally utilise minimal artificial ventilation (predominantly exhaust fans rather than blow-through) and the installation of screens reduces air-flow markedly. However, some new greenhouses are using insect screening for the main wall and roof coverings in place of traditional materials, thus ensuring both good natural ventilation and insect exclusion. At present, screening materials are imported, mostly from the United States, Israel and Europe - there are no local manufacturers of this product. Few of those tested by Dr Bell are available in this country under their US trade names. However, Antivirus is advertised occasionally. Other well known manufacturers include Ludvig Svensson, supplied through Living Shade Pty Ltd (NSW) and Powerplants Australia (Vic), and Cravo Equipment (also available through Powerplants Australia). The majority of commercial greenhouse construction and/or manufacturing companies will supply and install insect screens when requested. Fitting at the time of construction is desirable in terms of cost but screens can be fitted to existing structures (insect screening costs around $10 per metre). Further reading Handbook of the Major Pests and Diseases of Nursery Plants. Second edition 1998. From NIAA. Keeping pests out with screening. The Nursery Papers issue 1997 # 007. Web sites www.ngma.com - National Greenhouse Manufacturers Association (USA) (More specifically, the pages covering Articles and Insect Screens ) www.dpi.qld.gov.au/iobc - An international bio-control site hosted by Queensland DPI
Acknowledgments This article was researched and written by Dr Michelle L Bell, currently Assistant Professor and extension floriculture specialist, University of Florida, Gulf Coast Research and Education Centre, Bradenton, FL, USA (e-mail mbell@nersp.nerdc.ufl.edu). Dr Bell was previously employed at the North Carolina State University where the research for this article was undertaken. The article has been modified by Noelle Weatherley, Melbourne, for Australasian readers. Table 1. Grouping of screening materials for relative exclusion of whiteflies and thrips, characterisation of relative airflow resistance and manufacturer or distributor. Most sources listed are manufacturers. Screening material Exclusion rating (1) Static pressure (2) (inches of water) Airflow resistance category Source Silverleaf whitefly Thrip BugBed 123 *** *** 0.054 Moderate Green Thumb Grou No-Thrips ** *** 0.263 Very high Green-Tek Inc Econet S *** ** 0.095 High LS Americas BugBed 110UV *** ** 0.028 Low Green Thumb Grou BugBed 85 *** ** 0.039 Moderate Green Thumb Grou Econet T *** * 0.074 High LS Americas Pak 52x52 *** * 0.049 Moderate Pak Unlimited Inc Pak 44x44 ** ** 0.051 Moderate Pak Unlimited Inc Kontrol 45604 ** * 0.044 Moderate Baycor Products G Lumite 52x52 ** * 0.040 Moderate Synthetic Industries Lumite 42x42 ** * 0.042 Moderate Synthetic Industries Antivirus ** * 0.042 Moderate Green-Tek Inc Reemay ** * 0.058 Moderate Reemay Inc Typar ** * 0.271 Very high Reemay Inc Kontrol 45504 ** * 0.056 Moderate Baycor Products G
FlyBarr * ** 0.064 Moderate Hydro-Gardens Inc Pak WP87 * * 0.152 High n/a Kontrol 45404 * * 0.022 Low Baycor Products G Kontrol 45304 * * 0.010 Very low Baycor Products G Kontrol 45204 * * 0.008 Very low Baycor Products G Durascreen * * 0.024 Low DuraGreen Marketi Insecta 500 * * 0.024 Low Green-Tek Inc Econet L * * 0.020 Low LS Americas Econet M * * 0.022 Low LS Americas Lumite 32x32 * * 0.022 Low Synthetic Industries Pak 32x32 * * 0.022 Low Pak Unlimited Inc (1) *** - Screens that excluded more insects than the fibreglass fly wire screen and about the same number of insects as the top performing screen for that pest. ** - Screens that excluded more insects than the fibreglass fly wire screen but not as many insects as the top performing screen for that pest. * - Screens that excluded no more pests that the fly wire screen. (2) Static pressures at an approach velocity of 300 ft/minute on which the designated relative airflow resistance category is based. Return to Nursery Papers index page Maintained by NIAA Webmaster Last updated on: Thursday, March 29, 2001 14:28:11. URL: http://www.ngia.com.au/np/991/np99_1.htm.