Scourge of the cabbage white butterfly

LAST Monday I wrote about the birds in my garden and on Tuesday I got a number of emails from readers full of interesting information, particularly about the family life of the house sparrow.

Then disaster struck.

For many years I have been writing my articles on an ancient PC. Younger members of the family decided this was barely a step removed from a quill pen and presented me with a new laptop for my birthday. I was delighted. But the keyboard was rather different and, more crucially, so was much of the software.

On Tuesday night I hit the wrong key and managed to delete the whole day’s emails. They may be retrievable but, in the meantime, please forgive me if you don’t get an acknowledgement of the information you sent me on sparrows.

Today the focus of attention in the garden has shifted from birds to butterflies. Most people love butterflies in the garden, at least most butterflies. The exception is the vegetable gardener and the cabbage white butterfly.

I have a plot, which rotates round the garden on an annual basis, full of cabbages, broccoli, cauliflower, kohlrabi and sprouts. All these vegetables are closely related members of the Brassica family. And today there are white butterflies flitting and floating over the plot.

In a week or two there will be hordes of ravenous caterpillars munching their way through vegetables which are supposed to feed me through the winter.

There are two species of cabbage white butterfly. They are called, accurately if a little unimaginatively, the Large White and the Small White. Both species are normally double-brooded in Ireland. A small hatch of them will be on the wing in May and a much larger brood will appear at around this time of year. The caterpillars of this second brood will over-winter as pupae and the ones that the birds don’t eat will provide the spring hatch next year.

The female Large Whites lay their eggs in clumps on the Brassica leaves, often on the under side of the leaf. These hatch into brightly coloured black, white and yellow caterpillars which crawl all over the plant, making no effort to conceal themselves. They can do this because they contain a substance that is poisonous, or at least very unpleasant, to birds. The only predator they have to fear is a species of parasitic wasp which injects its eggs into the caterpillar’s body where its larvae literally eat their host from the inside out.

The female Small White lays its eggs singly and is a lot less fussy about the plants it chooses. These eggs hatch into well camouflaged green caterpillars with a thin yellow stripe. They lack the protective substance for deterring birds so the caterpillars normally crawl into the heart of the food plant to conceal themselves.

This makes the Small White a more serious garden pest. It can eat a cabbage from the inside and the caterpillars can’t be picked off the leaves as easily as those of the Large White. It’s possible to buy small-mesh plastic netting to protect Brassica from butterflies, but it’s awkward stuff to use. If the caterpillars do get a hold the organic gardener usually has to resort to an organic spray – ones containing pyrethrum are very effective and safe. The cabbage whites are Ireland’s commonest butterflies and there is an inward migration of both species from Britain in August and September.

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