Brassica Pests

Want to know how to manage brassica pests? When it comes to pests that prey on brassicas, there is good news and bad news. The good news is that the colder the weather, the fewer the pests, if any. The bad news is that when it starts warming up from August onwards, the pests come out to breed and feed. Of course, it all depends on the temperatures, which means that the two critical periods are in autumn with its warm days, when the seedlings are planted out, and in spring when there is active growth and heads are developing.

Five Main Brassica Pests

Snails and Slugs

These pesky critters are still active in autumn and are a threat to newly planted seedlings.

What to use: Biogrow Ferramol is pet-friendly and not poisonous to birds. Natural remedies include salt (but this can affect the soil pH), beer traps, crushed eggshells and other snail traps.


Aphids not only weaken plants by sucking out the sap, but they can also spread viruses. They are the greatest threat to cabbages because once an aphid infestation gets into the head, the whole cabbage is ruined. The same can be said for cauliflower. Nip any aphid activity in the bud by spraying seedlings in autumn with an organic or biological insecticide. Monitor throughout winter but be extra vigilant from August and spray as soon as an infestation is evident.

What to use: Organic insecticides like Margaret Roberts Organic Insecticide smothers and repels. Also, Ludwig’s Insect Spray + contains canola oil, garlic and pyrethrum that kills on contact.

Biological pesticides like EcoBuz PestPro includes beneficial fungi, Beauvaria bassiana, that penetrates the pest cuticle, growing and feeding until the target pest dies.

Make your own spray by finely chopping one onion and two cloves of garlic. Put in a blender with two cups of water and blend at high speed. Strain and pour the liquid into a garden sprayer. Apply as a full cover spray every five to seven days.

Bagrada bugs

Bagrada bug (a stinkbug species) attacks young seedlings of cabbage, Brussels sprouts, cauliflower and broccoli. By sucking the plant sap, they deform or weaken the plants and if the growing shoots are damaged, the heads will not form. Adult bugs are shield-shaped with yellow and orange spots and an orange band across the back.

For the best protection, spray when seedlings are transplanted in early autumn and re-spray regularly. Bagrada bugs are less active in winter but adult bugs can overwinter in old plants or cabbage stumps.

What to use: Organic insecticides containing pyrethrum that kills bugs on contact.

For a natural method, wash bugs off the plants with a strong jet of water or, if there are only a few, hand pick off the plants and kill. Don’t leave cabbage stumps in the soil.

READ MORE: Find out how to use herbs to manage Garden Pests!

Diamond-back moth

During its larvae (caterpillar) stage, the diamond-back moth feed on cabbages, broccoli, cauliflower and Brussels sprouts. The moth lays the eggs next to the main leaf veins and the hatched larvae are initially brown but turn green as they mature. They feed on the underside of leaves and new shoots, which affect the formation of a head. A sign of their activity is transparent patches on the leaves which later become holes. Take preventative action by spraying newly planted crops with an organic insecticide or biological caterpillar insecticide.

What to use: Organic insecticides that also control African bollworm such as Ludwig’s Insect Spray+. Biological controls such as Margaret Roberts Biological Caterpillar Insecticide or EcoBuz LarvaePro contain the beneficial bacteria, Bacillus thuringiensis subsp. kurstaki (Btk). It is applied as a full cover spray in the late afternoon when the caterpillars are feeding. It is not harmful to natural predators.


Other caterpillars such as the cabbage semi-looper, which is a light green caterpillar that has a looper action, and African bollworm larvae (green, brown or black), are more active from spring through to summer. They damage the leaves and can work their way into the heads. Use the same controls as listed for diamond-back moth larvae.

Source: The Garden Guardian’s Guide to environmentally-responsible garden care by Johan Gerber (Aardvark Press).

The Gardener