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Nearly everything that attacks Brussels sprouts arrives by flying in and laying eggs. Cabbage white butterflies, cabbage moth, cabbage root fly, whitefly and aphids all have to land on the plant first. A fine insect mesh, put on the day the plants go out and kept on until you harvest, removes most of the problem before it starts.
Mesh only works if it is a complete barrier. It has to be supported on hoops so the leaves are not touching it, since butterflies lay through mesh pressed against foliage, and the edges must be buried or weighted all round. A mesh with a gap is worse than none, because pests that get in are then protected from birds and predatory insects.
| Problem | First sign | What to do |
|---|---|---|
| Cabbage white caterpillars | Ragged holes, green or yellow-black caterpillars underneath leaves | Rub off egg clusters, pick caterpillars, keep mesh sealed |
| Mealy cabbage aphid | Gray-white waxy colonies, puckered and curled leaves | Squash colonies early, strip infested leaves, check inside buds |
| Whitefly | Clouds of tiny white insects when you brush the plant | Tolerable in small numbers; remove lower leaves, wash sprouts |
| Cabbage root fly | Young plant wilts in sun and never recovers | Collar at the base, or mesh from planting day |
| Pigeons | Leaves shredded to the ribs, usually in winter | Netting; mesh alone is not strong enough |
| Slugs | Holes in lower leaves, slime on the stem | Clear fallen leaves, trap around the base |
| Clubroot | Plants wilt in heat, roots swollen and distorted | Lime, raise plants in large pots, rotate for seven years |
Mealy cabbage aphid, the one that ruins the crop
This is the pest that specifically spoils Brussels sprouts. It is a gray-white aphid covered in a waxy powder, and it forms dense colonies that hide inside the growing point and, crucially, inside the sprouts themselves. You can have a plant that looks reasonable from the outside and find every bud packed with aphids when you cut them open in the kitchen.
It builds up through late summer and autumn, exactly when the sprouts are forming, and once the colony is inside a closed bud nothing you spray will reach it. The whole approach is to catch it early and to keep looking.
- Check the growing point weekly from midsummer The colony starts at the top of the plant in the tightly folded young leaves. Pull the top leaves apart and look. A gray dusty patch the size of a coin is the start of an infestation.
- Squash the first colonies by hand Rub out small colonies between finger and thumb, or cut off the affected leaf entirely. Dealt with in the first week, that is the end of it.
- Strip badly infested leaves Take heavily colonised leaves right off and remove them from the garden. Do not shake them out over the bed.
- Open a sprout and look inside Once a fortnight in autumn, cut one sprout in half. If aphids are inside the buds, pick that plant over promptly and use the sprouts rather than leaving them to fill further.
- Clear old stems in winter The aphid overwinters on brassica debris and on old stems left standing. Pulling and destroying finished plants breaks the cycle for next year.
Encourage the predators. Ladybird and hoverfly larvae clear mealy cabbage aphid effectively, but only if you are not spraying broad insecticides that kill them too. Mesh plus hand removal plus predators is the workable combination.
Caterpillars and what actually eats the plant
Cabbage white caterpillars are the most visible damage but rarely the most serious. A well grown sprout plant can lose a good deal of leaf and still crop. The exception is a young plant in its first month, which has no reserves, and a plant where caterpillars have got into the growing point.
Look on the undersides of leaves, which is where the eggs are laid in yellow clusters and where the caterpillars sit. Cabbage moth caterpillars are worse than the whites because they bore into the heart of the plant and into the sprouts, where you cannot see or reach them. Any caterpillar found near the growing point should be removed the same day.
Clubroot, and why it changes your whole plan
Clubroot is a soil-borne disease that swells and distorts brassica roots into useless lumps. Above ground the plant is stunted, wilts in warm weather, and never fills its sprouts. There is no cure and no treatment. Once it is in your soil the spores survive for up to twenty years.
It is far more active in acid, wet soil, which is why liming and drainage are the standard defenses. If you find it, the response is not chemical but structural: change where and how you grow brassicas.
- Lime to raise the pH Test the soil and lime to bring it to 7.0 or slightly above. Apply well before planting, and never at the same time as manure. Clubroot is markedly less severe in alkaline soil.
- Improve the drainage Clubroot thrives in wet ground. Raised beds and added organic matter reduce infection appreciably on heavy soils.
- Raise big plants before planting out Grow seedlings on into large pots so they go out with a substantial root system. A big plant infected late still produces a usable crop, where a small one collapses.
- Rotate for seven years or more Do not grow any brassica, including radish, turnip, swede and mustard, on infected ground. Three years is enough for most brassica problems; clubroot needs far longer.
- Do not spread it Spores travel on boots, tools and soil on transplant roots. Work infected ground last, clean your boots, and never compost infected roots.
Keeping a record that is worth having
Pest problems on a nine month crop are about timing, and timing is the thing you forget. Write down the date you first saw each problem, which plants in the row it started on, and what you did about it.
The first sighting date is the most valuable entry. If mealy cabbage aphid appeared in the middle of one summer, you know to start checking two weeks before that next year, which is the difference between rubbing out one colony and losing the crop. Also note which end of the bed was worst: the sheltered, still corner is usually where aphids and whitefly build up first, and that tells you where to put the mesh hoops next season.