VAL BOURNE on stopping brassicas falling prey to Cabbage Whites

The year 2008 was designated the Year of Reading and the Year of the Potato. But I have renamed it the Year of the Cabbage White Caterpillar. It seems others feel the same because whenever I see other gardeners they mention their lace-tattered brassicas in hushed, rather anguished, tones.

As you probably know, I am an organic gardener. It’s partly Yorkshire tightness, partly self-preservation and very largely a wish to preserve the natural order.

So my classic treatment has always been to net my brassica plants as soon as they go outside in May using a double layer of fine green netting — the sort for supporting peas. A lot of black square netting, although it looks good, allows these Houdinis to squeeze through. Fleece and enviromesh stop the rain getting to the crop and the material of both holds moisture, causing problems.

Netting is my way of excluding two white butterflies. The Large white (Pieris brassicae), which lays clusters of eggs on the back of leaves throughout summer and into autumn, and the Small white (Pieris rapae) which lays single eggs on the backs of leaves. The latter produces a green caterpillar which is hard to see. But they generally munch alone or in twos and threes. But the Large white caterpillars, pictured, appears in hundreds and shreds a crop in days.

The problem is nothing will eat a Large white caterpillar because as it munches the leaves it builds up mustard oil in its body. This makes it taste very bitter and unpleasant. However, lots of predators pick off young ones in their early stage (the first instar) and I have seen wasps and birds eat them. You can apparently water on a nematode (Steinernema carpocapsae) at this early stage too.

But a couple of years ago I noticed a predatory insect with very long, curved antennae laying eggs inside tiny cabbage white caterpillars. I managed to photograph it. It was Cotesia glomerata and the caterpillars came to a very nasty end. Their bodies ruptured after 15 to 20 days to expose a mass of bright, sulphur-yellow cocoons. Some emerge after 30 days, if the weather’s warm, but others will overwinter somewhere sheltered.

I have seen the eggs — fluffy masses that look like yellow loft insulation — sticking to greenhouse shelves and ledges and I have cleaned them off without realising how valuable they could have been for next year. If you see them be aware that those yellow clusters are beneficial and leave them alone.

Levels of parasitism can rise to 75 per cent by summer and, if that were not enough, the aptly named C. glomerata can also spread a debilitating granulovirus as well. It goes without saying that pesticides, which cannot discriminate, knock C. glomerata out along with spiders and other predators. In South Africa, Large whites appear, but nearly every chrysalis is predated so they are not a problem.

Rainy, dull weather probably kept predation low this year and even my netted caterpillars were savaged.Some butterflies penetrated the barrier I know. Then, of course, the netting works against you because you are excluding most of your predators too. Once I noticed small caterpillars perhaps I should have removed my net to allow them in. But I had another problem. Caterpillars can walk and I think many were feeding on un-netted crops nearby. Once they were stripped, they took a short hike to mine. As a result, I won’t be growing brassicas on my allotment again. They will be grown in the garden — a long way from other cabbage crops. If I spot young caterpillars next time I will take the net off and let battle commence!