What’s wrecking my brassicas?
QCaterpillars are making a mess of my brassicas (cabbages, Brussels sprouts and sprouting broccoli). How can I stop them defoliating my plants? Margaret Wheeler, Chelmsford
ALarge and small white butterflies lay their eggs on garden brassicas and between them, have the plant covered.
The large white lays its yellow eggs in clusters of around 60, hatching to speckled caterpillars, feeding mainly on the outer leaves. The small white lays eggs singly and the soft green caterpillars target the heart and then eat outwards.
Both make copious holes and foul plants with frass (caterpillar poo). In most years, there are two generations of these butterflies. It is the second, active from July to September, that causes trouble.
I often boast about the efficient bird population of our garden but these caterpillars accumulate foul-tasting essences from the brassicas which put birds off, and even our chickens used to baulk at them. Some caterpillars become infested by eggs and larvae of a parasitic wasp but the caterpillar still feeds to maturity when, instead of pupating, it dies, surrounded by cocoons of the wasp. At least heavy wasp infestations mean a few less pupae hatching the next year. My preferred solution is to prevent egg-laying from the start, by covering brassicas with fine netting that also protects from cabbage root fly and pigeons. This is best held above the foliage, on a frame. Some gardeners make their own garlic solution to spray on and repel egg-laying butterflies and Grazers make a caterpillar repellent spray based on calcium chloride, but both will need regular application.
Biological control involves sending off for a packet of nematodes which, made into a solution, are sprayed on to damp leaves to control the caterpillars. If you opt for an insecticide, make sure it is suitable for use on edibles and check the interval between application and harvest.