Look for something that uses hemp, which is tough and lightweight. Patagonia makes something that might fit the bill (even has a gusseted crotch): https://www.patagonia.com/product/mens-all-seasons-hemp-canvas-double-knee-work-pants-regular/55785.html

>These lightweight yet durable double-knee work pants are made from our innovative 9.6-oz All Seasons Hemp canvas, which needs no break-in and offers the toughness, comfort and breathability needed for warmer weather work.