Cabbage Pests and Diseases: Caterpillars, Aphids, Root Fly and Clubroot

Insect mesh does more than anything you can spray

The three most damaging cabbage pests, cabbage white caterpillars, cabbage moth caterpillars and cabbage root fly, all arrive as an adult insect that has to land on the plant to lay. Fine insect mesh over hoops from the day you plant excludes all three, and it works for the whole season without you doing anything else. Nothing you apply afterwards comes close, because by the time you notice damage the caterpillars are already inside the head where no spray reaches them.

The mesh has to be sealed. Bury the edges or weight them down continuously, keep the fabric off the leaves, and lift it only to weed and water, closing it again the same day. Check the inside of the mesh occasionally too: a single moth that got in while you were working lays several hundred eggs.

Caterpillars: find the eggs, not the damage

Cabbage white butterflies lay clusters of yellow eggs on the undersides of leaves, and cabbage moths lay smaller, scattered eggs. Both hatch in about a week. Turning over the outer leaves once a week and rubbing off any egg cluster you find takes two minutes per plant and it is far more effective than dealing with caterpillars that have already tunneled into the head.

Cabbage white caterpillars feed openly on the outer leaves and are easy to pick off. The cabbage moth caterpillar is the serious one: it bores into the center of the head, and the only outward sign is a small entry hole and dark crumbly frass around it. Once it is inside, hand picking is impossible and the head is usually spoiled. If you find frass, cut that head and use whatever is undamaged.

Where numbers get away from you, a bacterial spray based on Bacillus thuringiensis is effective on young caterpillars, harmless to bees and approved for food crops in most places. It only works while the caterpillars are small and it has to hit them, so it goes on the undersides of the leaves and it goes on early.

1. Turn ten outer leaves per plant, weekly Look on the undersides for egg clusters and rub them off with your thumb. Do this on the same day each week and you will rarely see damage.
2. Look for frass at the head Dark crumbly grains where the leaves meet the head mean a caterpillar has bored inside. Peel back the leaves at that point and remove it, or cut the head.
3. Spray young caterpillars only A Bacillus thuringiensis product works on small caterpillars and does nothing to large ones. Apply to leaf undersides in the evening and repeat after rain.

Aphids in the growing point

Cabbage aphids form dense gray, waxy colonies tucked into the growing point and between the folded leaves of a developing head. Their waxy coating sheds water and most sprays, which is why they are so hard to shift once established, and once they are inside a heading cabbage you cannot wash them out at the sink either.

Check the center of each plant weekly from the moment growth speeds up. A colony the size of a coin can be squashed between finger and thumb or blasted out with a jet of water. Left another fortnight it covers the whole growing point, growth distorts and the head that forms is unusable. Yellow sticky traps at plant height catch the winged adults arriving, and ladybirds and hoverflies will clear light infestations if you have not sprayed everything in sight.

Do not put aphid-covered leaves on the compost pile next to the bed. Take them off the plot. Winged forms move back onto the crop within days from a heap sitting in the sun.

Cabbage root fly: wilting on wet soil

A cabbage that wilts in the afternoon despite moist soil, then recovers overnight, then wilts a little more each day, has lost its roots. Cabbage root fly lays at the base of the stem and the maggots eat the fine roots and tunnel into the taproot. Pull a suspect plant up and you will see white maggots and brown rotted root tissue.

There is no cure once the maggots are in. Lift the plant, take it off the plot and do not replant a brassica in that exact spot this season. Prevention is a stem collar or mesh, and both have to be in place from the day you plant, because the flies are drawn to the smell of freshly disturbed soil and the newly planted crop is what they are looking for.

1. Fit a collar at planting Cut a disc of card or roofing felt about five inches (13 cm) across, slit it to the center and fit it snugly around the stem, flat on the soil. It stops the fly reaching the soil at the base.
2. Confirm before you pull Scrape the soil away from the base of a wilting plant and look for white maggots. Wilting can also be dry soil or clubroot, and the treatments are different.
3. Remove affected plants whole Lift the plant with the soil around the root and bin it. Leaving it in place lets the maggots pupate in your bed and emerge as next season's flies.

Clubroot: the one that means moving the crop

Clubroot is a soil-borne organism that infects brassica roots and turns them into swollen, knobbly galls. Above ground the plant is stunted, wilts easily in sun and never heads properly. It thrives in acid, poorly drained soil, and the resting spores survive in the ground for up to twenty years, so a single infected bed is a long-term problem rather than a season's setback.

There is no treatment. What you can do is limit it: lift infected plants complete with the root and dispose of them off the plot, never compost them, lime the bed to bring pH up to 7.0 or above, improve drainage, and clean soil off boots and tools before you walk to another bed. Raising your own transplants in clean compost matters as well, because bought-in plants are one of the commonest ways clubroot arrives in a garden.

Where a bed is known to be infected, grow cabbage elsewhere or in large containers of fresh compost. Raising plants in bigger pots so they go out with a substantial root system also helps them outgrow a light infection, though it will not save them from a heavy one.

Keep a note of what appeared and where

Write down which pest or disease appeared, the date you first saw it and which plant position it was on. Pests arrive at strikingly consistent points in the season, and after one year of records you know to have the mesh on and to start checking leaf undersides a week before the trouble usually starts rather than a fortnight after.

Position matters most for clubroot and root fly, because both live in the soil. A sketch of the bed with the affected plants marked tells you which ground to avoid and which to lime hard, and that information is worth far more three seasons later than any memory of it.