Broccoli Pests and Problems: Caterpillars, Aphids, Clubroot and Buttoning

Netting is not optional

Cabbage white caterpillars, pigeons, cabbage root fly and flea beetle between them account for the great majority of broccoli failures, and a single physical barrier stops all four. Fine insect mesh, 1 mm or smaller, on hoops with the edges buried or weighted, is the most effective thing you can do for this crop and it needs doing on the day you plant.

The mesh has to be held clear of the foliage. Butterflies lay through netting that rests on the leaves, and pigeons peck through it. Allow enough headroom for a three foot (90 cm) plant from the start rather than raising the hoops later, because the week you spend meaning to do it is the week the eggs get laid.

Caterpillars: find the eggs, not the damage

By the time you can see the damage the caterpillars are well grown and half the leaf area is gone. Cabbage white eggs are laid in yellow clusters or as single skittle-shaped eggs on the undersides of the leaves, and they hatch in about a week. Rubbing off egg clusters on a weekly round is quicker and more reliable than dealing with the caterpillars later.

For caterpillars already feeding, picking them off by hand works on a home-scale row, and a bacterial spray based on Bacillus thuringiensis is effective on small larvae without harming pollinators. Both work far better on the undersides of the leaves and down in the crown, which is where they hide during the day.

1. Turn ten leaves per plant, weekly Look at the undersides for yellow egg clusters and rub them off with your thumb. This one habit prevents most caterpillar damage.
2. Search the crown, not just the leaves Larger caterpillars sit deep in the center of the plant during the day. Part the leaves and look right into the growing point.
3. Repair holes in the mesh immediately One tear is enough. Check the edges after windy weather, since mesh lifted at the corner is the usual way butterflies get in.

Cabbage aphid: the one that ruins the crop you meant to eat

Gray, mealy cabbage aphid colonies settle in the tightest parts of the plant: the developing head, the crown, and the side shoots. Unlike leaf damage, this makes the crop itself unusable, and once a colony is inside a head there is no way to wash it out. Look for a distorted, cupped leaf and check underneath it, because that puckering is usually the first visible sign.

Catch it while a colony is the size of a coin and you can squash it between finger and thumb or blast it off with water. Left three weeks it covers the plant. Ladybird and hoverfly larvae will clear moderate numbers if you have not sprayed the bed, so it is worth tolerating a small population on the outer leaves rather than reaching for an insecticide immediately.

1. Check the growing point weekly Part the central leaves and look at the base of the developing head. That is where the colony starts and where it does the damage that matters.
2. Squash or wash off small colonies A jet of water directed at the underside of the leaf removes most of them. Repeat after two days, because the ones that fall off can climb back.
3. Cut out a badly infested shoot A shoot packed with aphids will not clean up. Cut it off and bag it rather than leaving it as a reservoir for the rest of the plant.

Clubroot and root fly: read the roots

A broccoli plant that wilts on warm days, stays stunted and never fills out has a root problem, and there are two common ones. Lift the plant and look. Swollen, distorted, club-shaped roots mean clubroot, a soil-borne organism. Roots eaten away with small white maggots present mean cabbage root fly. The treatment is completely different, so the diagnosis is worth the lost plant.

Clubroot cannot be cured. It survives in soil for up to twenty years, so the response is management: lime to raise pH towards 7.0, improve drainage, raise plants in large pots so they go out with a big root system, and never move soil or tools from an infected bed to a clean one. Root fly is prevented outright by collars and mesh at planting.

Buttoning, blindness and bolting are timing faults

Three broccoli disappointments have nothing to do with pests. Buttoning is a tiny head on a small plant, caused by a check to growth: root bound transplants, cold at the seedling stage, or a shortage of nitrogen or water early on. Blindness is a plant with plenty of leaf but no head at all, usually because the growing point was damaged by slugs, frost or a careless hoe. Bolting is a head that stretches and opens into yellow flowers, caused by heat or by the plant reaching maturity in weather that is too warm.

None of these can be reversed on the affected plant, which is why the record you keep matters. Note the sowing date, the planting date, the date the head appeared, and what the temperature did while it was forming. Buttoning almost always traces back to something identifiable in the seedling stage, and bolting almost always traces back to a head maturing in a warm spell. Both are fixed next season by moving the sowing date, not by anything you do to the plant.