Answer: Without knowing exactly what insect you are having trouble with, it's difficult to recommend a specific treatment. At this time of year (the cool season), two insects that often invade members of the cabbage family are aphids and cabbage loopers.
Aphids are tiny, soft bodied insects (they squish easily) that suck the juices out of leaves, often clustering on the undersides. They can be green, greyish, black or other colors. If your aphid problem is not too severe, a strong blast of water from the hose should work. Spray underneath leaves, in between, etc. Do this daily. If that doesn't work, try an insecticidal soap spray.
Ladybugs and their larvae are voracious predators of aphids. They often "arrive" a week or two after the aphids, so not spraying with chemicals is a good idea if you'd like to attract them to your garden to consume aphids for you.
Cabbageworm or cabbage loopers are small green caterpillars that eat holes in leaves. If you catch them early, handpicking them is an easy and effective control. Look under leaves and all around the plant. You can often spot their dark, pellet-shaped droppings which can help you locate them. If they are out of control, you can use Bacillus thurengiensis, often called Bt. It is a biological control--a bacteria that kills only caterpillars and is safe to other insects, humans and animals. It will kill all caterpillars however, including butterflies.
You can also put a barrier of garden fabric over plants to prevent pests from finding them. There are "summer-weight" garden fabrics that act as a barrier without causing the air under the cover to heat up too much. Put the cover over plants as soon as you plant them, and that should keep the pests off. I hope this info helps.
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