Leaves perforated with small holes are a sure indicator of flea beetles, who especially like the leaves of cabbage, arugula, tomatoes and eggplants. The beetle itself is tiny, shiny and black, and will hop away as you approach it.
I read of a gardener who capitalized on this habit by building a contraption that looked like a high-riding skateboard, with a handle, which is pushed over a row of plants. A horizontal metal wire down across the front of the board disturbs each leaf — and the flea beetles — with each pass. Flypaper tacked on the underside of the board catches the beetles as they hop away. I’ve never tried this method.
I have thwarted flea beetles by covering plants with a lightweight “row cover,” a sheer material through which light, air and water can pass, but not pests.
Plants tolerate a certain amount of leaf damage from the likes of slugs and flea beetles, with remaining leaf areas becoming more efficient to make up for lost ones. Keep your plants healthy and they’ll usually grow vigorously enough to keep ahead of or outgrow such damage.