Broad bean bumble bee beckons

I WAS working in the vegetable garden — actually that’s not strictly correct, I was taking it easy on a garden seat and admiring the work I’d done.

I was also admiring the tireless work of a bumble bee that was moving down a row of broad beans, visiting each flower and taking a quick sip of nectar. In the process, of course, it was pollinating the flowers ensuring that I’d get a heavy crop of beans. It’s good to have helpers.

Then I noticed something that was not so good. A small white butterfly was flitting around over the cabbages. The small white and the large white are sometimes called the cabbage whites because of the damage their caterpillars can do to a crop.

In fact, small whites can damage all the many members of the cabbage and turnip family as well as nasturtiums and a range of related wild plants. The first thing I tried to do is discover whether the butterfly over the cabbage patch was a male or a female.

Males, of course, are quite benign, just flitting around the place sipping nectar from flowers and mating with any unattached female. They seem to have a preference for white or pale-coloured flowers. The sexes are quite easy to tell apart. The female has two black spots on each fore-wing and the male only has one.

Bad news: this was a female. Small whites normally have two broods a season in this country, though three have been recorded in good summers. This female was probably from an over-wintered pupae and she had mated and was looking for somewhere to deposit her eggs. These are deposited singly, unlike those of the large white which are laid in clusters. They are yellow and bottle-shaped and usually placed on the underside of a leaf, where they’re hard to spot.

The first thing the tiny caterpillar does when it hatches is eat its own egg case. Then it starts to eat the leaf it was laid on. As it does so it heads for the heart of the plant, eating as it goes and increasing in size. Large white caterpillars don’t do this. They stay on the outer leaves, making them much easier to remove.

The reason for this is that they contain a substance that tastes unpleasant to birds. Small whites don’t have this defence so they have to hide. The other piece of good news, from the gardener’s perspective, is that small white caterpillars are very prone to attack from a parasitic wasp the larvae of which eventually kills its host.

Small whites are widespread in Ireland and relatively common, though numbers do fluctuate.

This fluctuation is partly caused by massive inward migration across the Irish Sea in some years. Small whites are capable of travelling a couple of hundred kilometres in their life-time, though, of course, most of them never do. Their travelling ability was well demonstrated in Australia in the last century. There was an ill-judged introduction of the species in Melbourne in 1939. Three years later they had reached the west coast, 1,850 miles away. Even given the mild climate which allowed them to breed more frequently than they do here, they had travelled this distance in a maximum of 25 generations. They are now an agricultural and horticultural pest in Australia.

Small whites are prone to what are called ‘aberrations’, occasionally throwing up individuals of non-standard colour or markings. In the early 20th century, at the height of the craze for collecting dead butterflies, these were highly prized. Ireland produced more aberrations than other parts of these islands and one man made a living breeding and selling them.

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