Question: I am growing Arugula and a couple of other mustard greens, and all the plants have these little round holes eaten in the leaves. The plants still grow fine, but what causes that?
Answer: The holes in the leaves are feeding damage caused by adult flea beetles, which include the cabbage flea beetle (Phyllotreta cruciferae).
Flea beetle adults are metallic greenish in color and up to 0.12 inch long. They derive their name from their habit of jumping like fleas when disturbed.
Larvae dwell in the soil and feed on underground parts of the plant. Although young plants can be harmed by flea beetle feeding, older plants are more resistant. Adult flea beetles overwinter in trash around field margins. They become active in late March, and eggs are laid on or near host plants. The larvae feed on the foliage, mine the leaves or attack the roots, depending on the species. Depending on the species, there may be one or two generations per year.
If damage has been a persistent problem, you may wish to treat plants when small holes show on transplants.