Cabbage loopers (Trichoplusia ni) are caterpillars that will destroy plants in the cabbage family, including lettuce, broccoli, Brussels sprouts, cauliflower, collards, kale, etc. They move like inchworms do (hence the name looper), and after their metamorphosis, they become common-looking brown moths that are often found at outdoor safety lights.
At first you might not be able to spot these small 1.5-inch, pale green caterpillars in your balcony garden. Instead you will see the holes they chew in your container plants and the black pellets of waste they leave all over your plant leaves. If you see this type of damage, search thoroughly for the cabbage looper immediately to cut down on damage. Also look for pale green eggs on leaves and cocoons.
Once you have found the caterpillars in your balcony garden, kill them immediately by cutting them in half, squishing them, or taking them away from your container plants and using an insecticide. Use a vegetable-safe insecticide on infected plants until the problem disappears completely. If you do nothing, your crop will be gone in a matter of days. Use an insecticidal spray that includes B.t. (Bacillus thuringiensis), a soil-dwelling bacteria. B.t. is a common organic way of controlling cabbage loopers in the garden. This environmentally friendly insecticide is marketed under trade names such as Dipel and Thuricide. Once a cabbage looper caterpillar ingests B.t., it will stop feeding and will die within several days. Cabbage loopers can also be controlled with predators, such as spiders, ladybugs, wasps, birds, etc., but if you find these caterpillars in your balcony garden, do not leave the problem to predators. It is best to kill the caterpillars as soon as possible.
If you are having caterpillar problems on a plant outside of the cabbage family, it most likely is not a cabbage looper. Another common caterpillar pest for vegetable gardeners is the tomato hornworm caterpillar, which affects tomato and potato plants.