A 1.25 oz pump spray of 30% DEET insect repellent.

The size to buy in multiples. One in the day pack, one in the glovebox, one in the camp box — repellent is only useful if it is with you when the sun goes down.

Higher concentration does not repel harder, it repels longer. 30% covers an evening comfortably; 100% is for all-day exposure or genuinely bad conditions. For most evenings in the Valley or the foothills, 30% is the right answer.

Pump rather than aerosol, so it travels anywhere and does not spray itself empty in a hot vehicle.

One warning worth knowing: DEET attacks plastics and synthetics. It will haze a watch crystal, cloud sunglasses, soften a rain jacket's coating and weaken fishing line. Spray it on your hands away from your gear, and wash your hands before you handle anything you care about.