Plant marigolds or tagetes with Brussels sprouts. Companion planting controls pests and works by attracting beneficial insects that feed on the aphids, such as ladybirds and certain types of flies. Turnip, kale or mustard plants attract beneficial insects that feed on harlequin bugs.
Apply insecticides early in the season when the risk of killing beneficial insects is lower. Wash the leaves weekly with insecticidal, fatty-acid soaps according to recommended dosages, and then rinse the plants with a strong stream of water, and spray with organic pesticide containing neem oil. Repeat this weekly as the effects of the soap and the neem oil only last for a day or two. (Refs 4)
Place a layer of aluminum foil underneath the plants. This reflects light against the leaves’ undersides, heating them and making it uncomfortable for the bugs.
Introduce parasitic wasps. These insects lay eggs inside the aphids, killing the aphids, forming brown, crusty case called a mummy. Once mummies start appearing on the plants, the aphids remaining time is short-lived. The wasp also parasitizes the diamond back moths, killing a large percentage of them.
Spreading garden fabric or floating row covers over the Brussels sprouts deters flea beetles, which are tiny beetles that jump like fleas. Regular hoeing keeps the soil around the plants clear of eggs and larvae. This also offers protection for the plants against cabbageworms.
Kill cabbageworms and loopers with a biological insecticide called Bacillus thuringiensis, also called Bt, which is sold under different brand names. Spray the plants with Bt from the time you transplant them until ready for harvest.