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The culprit is the flea beetle, which, like its namesake, jumps away when disturbed. The small, shiny, black, brown or blue beetles emerge in April and May, after winter hibernation, ready to start feeding. Eggs laid near host plants hatch in late-summer. The beetles feed for a few weeks before overwintering.
Solution
Organic
Seedlings are especially vulnerable to attack, so protect them by waiting until they're a good size before planting out. Exploit the beetles' habit of jumping and catch them with a sticky trap. Coat a piece of card with grease, such as insect barrier glue, leaving a clean strip along one edge. Brush the clean edge of the card over the top of your plants - when the beetles hop into the air they'll stick to the grease. Repeat as necessary.
Chemical
Use rotenone, derris or pyrethrum chemical controls, or spray seedling brassicas with a thiacloprid insecticide. Take care not to disturb the beetles before an application, or they'll jump out of harm's way.
