Do your brassicas have small circular holes in the leaves, giving them a sieve-like appearance? Do they appear like they’ve been peppered with fine shot? If you don’t control the beetles they can eventually kill the plant.
The culprit is the flea beetle which like its namesake jumps away when disturbed. The small, shiny, black beetles emerge in April and May. After winter hibernation they’re ready to start feeding. Eggs laid near host plants hatch in late summer. The beetles feed for a few weeks before over wintering.
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. Flea beetles are attracted to the colours of white and yellow. Coat one side of a card or plastic about four to six inches square with something sticky, grease, petroleum jelly or lard have all been successful.
In short term brush the sticky side of the card onto your plants and then lay the card or plastic in the vicinity of the plant. When the beetles hop into the air they’ll stick to the grease. Check the card regularly and wash the captured critters off then re-use.
Flea beetles like hot, dry soil, so misting or fine watering to keep the top soil moist helps as do mulches.
Natural repellents consist of nicotiana, catnip and wormwood. Make a tea and spray he affected crop. Another natural repellent is a garlic and hot pepper spray. Beetles hate the combination but re-apply after watering or rain.
If all else fails insecticides based on pyrethin or carbaryl can be applied (if still available??)