How do you protect your cabbages from the prolific cabbage white butterfly? There are actually two types, the Large White Pieris brassicae and the Small White Pieris rapae. Both overwinter as pupae with the Small White appearing in spring, slightly earlier than the Large White with a second generation emerging in July. The Small White lays a single egg whilst the Large White lays in batches of ten to 20; eggs hatch about a fortnight later. The eggs and caterpillar will often be underneath the leaves only becoming obvious as tell-tale holes appear. The Small White caterpillar tends to eat its way to the heart of the cabbage. Both species feed on the cabbage for a month or so before leaving the plant to pupate. They overwinter in secluded areas and the life cycle repeats itself.
So how can we stop them? Getting rid of the pupae is tricky as they will often be outside your own garden. We can try and stop the butterflies from landing on the cabbages but to do so requires a frame and netting with the netting holes needing to be no larger than 7mm square. It needs to be large and soft enough to drape over your framework and the framework has to be tall enough that as the cabbage grows it does not touch the netting. If your cabbages touch the netting, butterflies will land on the net and lay eggs through the holes. The net needs to have a good 150mm overhang and must be well pinned down. Laying down the odd brick is insufficient. I have seen cabbage whites walk along the soil under poorly pegged down netting to reach a plant and lay eggs.
Having done all this you can you can still expect on a hot August day to see cabbage whites under your netting. You’ll need to remove the netting to get rid of the butterfly and search for eggs. In the hope of not having to remove netting, I have seen gardeners lose their sense of proportion and get into all sorts of contortions with garden canes to try and kill the trapped butterfly. Poorly applied netting can also trap birds and kill them.
Caterpillar eggs take two weeks to hatch and removing them by rubbing them out with your fingers is very satisfying You can do the same with small caterpillars, wearing gloves if you’re squeamish. Removing large caterpillars this way gets a little messy and I prefer to collect them in a jar sometimes by removing a leaf, and then put them on the bird table. The cabbage white also lays eggs on nasturtiums so a good bed of these can be used as a sacrificial crop. You’ll need to check your cabbages regularly and for most of us with busy lives, a few holes in our cabbages will be likely. But there’s nothing to beat a cabbage freshly harvested from your plot so go with nature and give brassicas a go.
Mike I’Anson – July 2016