Defense of harlequin bug:
Boundary Crop
Surround brassicas with strong-scented plants, such as chamomile, celery, basil, garlic, mint, rosemary and sage, to help mask their strong odors. You can even plant chrysanthemum, a natural pyrethrin producer.

Trap Crops
Grow an early crop of mustard, broccoli or kale to attract and trap pests. Harvest and discard this crop before your main crop is planted.

Instead of growing them as a boundary, inter-mix main crop with the plants listed above.
Flea beetles are repelled by catnip and basil. They are attracted by nasturtium and radishes. You can plant these as traps for the flea beetles so that they do not attack other more valuable plants.

Cabbage worms are repelled by thyme, so it would be a good idea to plant thyme near your susceptible plants. Cabbage worms are attracted to mustard plants, so planting mustard near more valuable plants can be a good trap for cabbage worms. Once they take over the mustard, you can destroy the plant.
Arugula