Ask Us About Insecticides/Miticides By Jim Bethke

Q What is the recommended mesh size for anti-thrips screening?

A Years ago, we measured some morphological features of various pest species in ornamentals and correlated them with the hole sizes of various types of exclusion screens. We also tested the insects' ability to pass through the various commercially available screens. We found that thrips were thwarted with a screen mesh size of 180 microns or less.

Do insect exclusion screens work? Absolutely, but there are a few things to consider. First, before selecting materials for screening greenhouses, you need to consider the price of the material (including installation), type and economic value of the crop being grown, pests to be excluded and effect the screening will have on greenhouse conditions. Whichever screen is considered, costs should be balanced against savings from reduced pesticide applications and losses from tospoviruses. Secondly, the impact of screens on the greenhouse environment can be significant. Screens reduce airflow into and out of greenhouses and may, depending on position, reduce light levels. Screens must be kept clean to minimize these effects. If greenhouses are retrofitted with screens, modifications may be needed, such as enlarging or adjusting the vents and adding more vents or additional fans or cooling systems.

Before installing screens, you are strongly urged to consult your horticultural farm advisor, an environmental engineer or your greenhouse manufacturer for information on how screens may affect the greenhouse environment.

I think in the most serious cases where tospoviruses devastate a crop, exclusion screening is essential. A good example is the commercial production of New Guinea impatiens where thrips exclusion screening is crucial. These Web sites have published extensively about the subject and will be helpful if you are considering the installation of exclusion screens: and

Q What is the optimum time interval between applications for thrips control?

A If you are using an effective insecticide, weekly applications should work just fine. If you are working with a pesticide-tolerant population, more frequent applications may be necessary.

However, I must stress that pesticides should not be used unless they are necessary. Preventative sprays may be required if tospoviruses cause crop loss, but if the crop is not susceptible resist regular applications because it will promote resistance development.

Chemical class rotation is very important. Chemical classes should be rotated every new thrips generation, and for thrips that is about every three weeks to a month depending on temperatures. Consider changing chemical class more often during the height of the thrips season.

Another complication to a treatment regimen is the constant influx of thrips from drying and dying plants in the environment outside the greenhouse. Sometimes this influx can be due to native plants and sometimes due to commercial agriculture. If the influx is from native plants, then pesticides should be efficacious against them and applications should be made as needed to avoid population development within the greenhouse crop. If the influx is from a commercial source, the population is most likely pesticide tolerant and a more diligent method will be needed to keep it under control. You may consider getting to know your neighbors and determine the pesticide regimen they use. Then, use a different chemical class and consider excluding the use of chemicals they commonly use for thrips control.

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Jim Bethke

Jim Bethke is staff research associate in the Department of Entomology, University of California, Riverside and floriculture farm advisor with the University of California Cooperative Extension in San Diego County. He can be reached at [email protected]

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