What mesh size stops thrips, aphids and whiteflies?
If you have ever compared two insect nets both sold as “50 mesh” and wondered why one stops thrips and the other does not, this guide is for you. The short answer: mesh count is not the specification that matters. The opening size in millimetres is. Below is what the aperture actually needs to be for the most common greenhouse and field pests, why finer is not always better, and the research behind the numbers.
In short
To exclude a pest, the mesh opening must be smaller than the pest can pass through. Laboratory work by Bethke and Paine (1991) reported approximate maximum hole sizes of 0.19 mm for western flower thrips, 0.34 mm for cotton aphid, 0.46 mm for the sweetpotato/silverleaf whitefly and 0.64 mm for leafminer. For spotted wing drosophila in berry and stone fruit, US extension guidance works to a different and widely quoted figure: below 0.98 mm. Smaller openings exclude more, but they also cut airflow and raise heat under the cover, so the right choice is the largest aperture that still stops your target pest.
Why “mesh count” misleads buyers
Mesh count is the number of openings per linear inch. Two fabrics with the same count can have different opening sizes because the thread diameter differs: thicker threads leave smaller holes. A supplier who quotes “50 mesh” without the aperture in millimetres has told you almost nothing about pest exclusion. This is the single most common mistake we see in insect-netting requests, and it is why our mesh visualizer works in millimetres, not mesh numbers.
The apertures, pest by pest
The reference values come from Bethke and Paine’s 1991 study on screen barriers for glasshouse crops, cross-checked against university extension guidance.
- Thrips (western flower thrips, Frankliniella occidentalis): about 0.19 mm. The smallest common target. University of Tennessee Extension gives the same figure in imperial units, about 0.0075 inch.
- Aphids (cotton aphid, Aphis gossypii): about 0.34 mm. A useful starting point; other aphid species vary.
- Whiteflies (Bemisia tabaci group): about 0.46 mm.
- Leafminers (Liriomyza trifolii): about 0.64 mm. UT Extension confirms roughly 0.025 inch.
These are starting points, not guarantees. Bethke and Paine themselves cautioned that hole geometry and screen construction affect exclusion, not just the raw opening size.
Spotted wing drosophila: a different figure, from a different source
The four values above come from one glasshouse study. Berry and stone fruit growers face a pest that study does not cover, and it now drives more netting decisions in the United States than any other: spotted wing drosophila (Drosophila suzukii).
The figure US extension programmes work to is an opening below 0.98 mm. It comes from the insect rather than from a net catalogue: an adult female is roughly 2.0 mm long with a body about 1.0 mm across, which is why the same fabric class sold as no-see-um screening is the usual reference. University of Maine Cooperative Extension and University of Vermont Extension both publish exclusion netting guidance built on that threshold, and Cornell has been collecting field data with New York growers since 2013.
Two consequences matter more than the number itself.
Bird netting does not do this job, and believing it does is worse than having no net. Bird netting openings are typically 15 to 20 mm. Against an insect that needs 0.98 mm, that is not a partial barrier, it is an open door. A block that looks protected gets scouted less and sprayed later, so the false sense of protection can cost more than the unprotected block next to it.
The net has to go on at the right moment. Extension guidance for blueberry is to net once pollination is complete. Fine mesh installed while the crop still needs insect pollination excludes the pollinators along with the pest, and it does so exactly as specified. There is no aperture that sorts a pest from a pollinator of the same size, so this is a calendar decision, not a fabric decision.
The trade-off nobody mentions: airflow
Here is what a catalogue will not tell you. Every step finer in mesh increases the thread area and reduces airflow through the fabric. In a greenhouse that means higher temperatures, more humidity, harder work for the ventilation fans and, sometimes, more disease pressure than the pest you were trying to stop. North Carolina State University’s greenhouse-screening bulletin makes this the central point: choose the hole size for your most important pest, then size the ventilation to compensate, rather than defaulting to the finest mesh available.
So the honest rule is counter-intuitive: do not buy the finest net you can find. Buy the largest aperture that still excludes your target pest.
How to turn this into a purchase
- Identify your primary pest. If it is thrips, you are at the fine end (about 0.19 mm); if it is leafminers or larger, you have more airflow to work with (about 0.64 mm).
- Ask the supplier for the aperture in millimetres, not just the mesh count.
- Confirm UV treatment and roll format for your structure. Our buyer’s guide covers those.
- Size the quantity with the coverage calculator.
If you want, send us the pest and the crop and we will confirm the aperture and the specification before quoting. That confirmation step is exactly where a good sourcing quote should start.
Sources
- James A. Bethke and Timothy D. Paine, “Screen Hole Size and Barriers for Exclusion of Insect Pests of Glasshouse Crops”, Journal of Entomological Science, 26(1), 169-177, 1991. DOI 10.18474/0749-8004-26.1.169.
- Michelle L. Bell and James R. Baker, “Greenhouse Screening: Comparison of Materials for Excluding Thrips and Whiteflies”, N.C. Flower Growers’ Bulletin, North Carolina State University, 40(2), 1995.
- University of Tennessee Extension, “Insect and Mite Pest Management in Greenhouses”, PB1594.
- University of Maine Cooperative Extension, Spotted wing drosophila: pest biology and IPM recommendations for wild blueberries.
- University of Vermont Extension Ag Engineering, Exclusion netting for spotted wing drosophila.
- Cornell University spotted wing drosophila programme, Thinking exclusion?
These figures are documented starting points based on laboratory work and published extension guidance. Field performance varies with pest strain, screen construction, installation quality and ventilation. We confirm the specification per project before quoting.