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. 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.
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.
These figures are documented starting points based on laboratory work. Field performance varies with pest strain, screen construction, installation quality and ventilation. We confirm the specification per project before quoting.