Flea Beetle

These tiny jumping beetles chew little holes in the leaves of the Brassica crops and their relatives. Generally you can ignore them because their damage is generally minimal and plants will usually grow out of it (a foliar feed may help them to recover). Young plants with little leaf area sometimes suffer badly and seedlings occasionally die.

Control

One suggested remedy is to move a grease covered board along the row of plants, the idea being to catch the beetles as they jump.  If they are really bad you could try a botanical insecticide.

Norman E. Rees, USDA Agricultural Research Service - Retired, Bugwood.org