I learned the hard way that mosquitoes and blackflies can bite through my knit polyester and knit merino wool hiking shirts. Another lesson: A rain jacket is effective insect protection, but wearing one in the High Sierra during the middle of the day under a blazing sun is completely unbearable.
A better bug strategy is a dedicated bug shirt. It is Item 3 in my Core 13, a tight collection of backpacking clothing that can be mixed-and-matched to create appropriate systems for all 3-season conditions.
Bug defense options
There are two types of bug defenses:
1. Body armor, via a tightly woven polyester or nylon fabric through which bugs cannot bite. However, because such a fabric has less air-permeability than knits, I discourage using this type of shirt when the bugs are mild or non-existent.
2. Chemistry, specifically permethrin. Sawyer offers a permethrin-based spray treatment, but the performance of factory-treated fabrics such as BugsAway from ExOfficio and Insect Blocker from Columbia is longer-lasting, supposedly “the expected lifetime of the product.” Unlike the body armor approach, it is possible to have a permethrin-treated fabric with high air-permeability.
Fit & features
Regardless of the defense strategy, a bug shirt should have long sleeves, a high collar, and the ability to lock off potential entry points like sleeve cuffs and the neck. A long arm length, perhaps even with thumb loops, would help to protect the wrist and some of the hand, which is otherwise exposed. Venting features like a chest zip and button-front are a must.
To enhance the poor airflow of woven nylon shirts, some manufacturers add mesh vents under the arms and/or along the back. I think the performance benefit is marginal, and it’s entirely offset by the ease with which bugs bite through this type of fabric, especially if it is not permethrin-treated. In fact, I think the only bites I have received while wearing a bug shirt have been along these mesh panels.
These types of shirts are synonymous with “travel” shirts and thus always seem to be styled with large chest pockets. I never use these pockets and should probably cut them off, since the double-layer fabric makes my chest hotter.
My picks and suggestions
Based on personal experience, I can verify the bug-resistance of:
Non-treated woven polyester shirts such as the GoLite Paparoa Long-Sleeve. GoLite is no longer around and I have not seen this fabric used in other shirts, but it serves the point that a tightly woven shirt, even without a permethrin treatment, can offer an effective defense against bugs.
Pemethrin-treated knit polyester shirts such as the Ex Officio BugsAway Impervio Crew, which will have substantially better airflow and dry time than the aforementioned woven nylons and polyesters. Due to limited personal experience, I cannot verify that factory-treated permethrin will “last the expected lifetime of the product,” as is claimed. But I’ve witnessed its effectiveness when new.
If you love a shirt that you already own, you can turn it into a legitimate bug shirt through the aforementioned spray-on permethrin treatment or by sending the shirt directly to Insect Shield, which will give you a much longer-lasting defense.