Cabbage white seems to fly in the face of evolution

ANOTHER LIFE:ONE OF NATURE'S less pleasing odours is the sulphurous stench that wafts from a stand of Brussels sprouts as the caterpillars of Pieris brassicae get to work in early autumn.

Alerted, belatedly, to their feasting as I went out to measure the rain the other morning, I found scores of the bold, yellow and black larvae reducing the big upper leaves to tattered ribs of upside-down umbrellas. This happens every September, as the butterflies’ second brood start feeding, and every year I forget until reminded by the breeze.

In the long, co-evolutionary liaison between insects and their special food plants, the volatile chemicals exuded from the brassica plant family, notably the mustard-oil glucosides, prompt the large white butterflies to lay a bright mosaic of yellow eggs on the undersides of the leaves. Vigilant gardeners remember to search for these and swipe them off with a calloused thumb. I was left, instead, to pluck the caterpillars with fastidious fingertips and sling each squirming, smelly handful over the ditch.

Chemical scents released as the caterpillars chew, and the dark globules of their smelly “frass” that pile up on the leaves, are also what attract their enemies. Birds are warned off a distasteful feed by the larvae’s bright colours, but tiny, parasitic wasps choose the caterpillars, in turn, as a larder for their young. They inject eggs inside them and give them a dose of virus-like particles that freeze any further development, so that their young can take their own time in growing and eating their way out.

This horrid-seeming manouevre was taken by Charles Darwin as yet another process evolving slowly by natural selection. As he wrote to a colleague: “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Icheumonidae with the express intention of their feeding within the living bodies of caterpillars.”

It is, however, the strange and elaborate process of metamorphosis, from caterpillar into pupa and thence into butterfly, that is used by creationists to challenge the validity of evolution. The change from caterpillar to butterfly – and from larvae to winged adult in a host of other insects, such as bees and dragonflies – seems to them simply too profound and complex to have evolved by gradual means, even over the millions of years that creationists deny.

“Which came first – the caterpillar or the butterfly?” is a question recently entered for discussion on the website hosted by Richard Dawkins (www.richarddawkins.net), the geneticist, Darwinian and atheist. “As the butterfly lays eggs that caterpillars hatch from,” the questioner went on, “does that mean the caterpillar was never a creature in its own right, and the butterfly came first?”

This was to confuse evolution with the development of different stages in the life of a single creature – let’s say the large white butterfly. Each stage, refined by evolution in adapting to different environments and habitats, has a different job: the caterpillar feeds, moulting its skin as it grows; the pupal stage takes time to shape and launch the butterfly; the butterfly disperses the species and lays the eggs. All the successive shapes of the insect are controlled by hereditary genes.

But the evolution of insect metamorphosis remains uncertain. Science is still asking questions about the process, and sometimes coming up with odd-seeming suggestions. Earlier this year, zoologist Donald Williamson, now retired from the University of Liverpool, suggested to the National Academy of Sciences that the radically different forms of caterpillar and butterfly result from an ancient hybridisation between an insect and a genus of velvet worms called Onychophora. “No one knows where caterpillars came from,” he has been quoted as saying, but his proposal of molecular research to back his idea has so far failed to gain much support.

Meanwhile, what becomes of my damaged Brussels sprouts? The sulphurous gases released in their demolition, while attracting the parasitic wasps, clearly have done nothing to prevent this season’s caterpillar assault, so it can scarcely be called a defence. It does, however, exercise some broad control on their global population and thus enters the “balance” of nature. My plants, meanwhile, are sturdy and thigh-high and will grow new leaves to nourish the later, lateral buds that we happen, however eccentrically, to enjoy.

Butterfly food plants are not generally the ones that butterflies themselves take food from. The food for caterpillars of red admiral, lesser-spotted tortoiseshell and peacock butterflies is the stinging nettle, a plant few gardeners will actually choose to cultivate except, perhaps, in a patch behind the compost bin. On the other hand, the nectar-rich garden flowers that butterflies themselves frequent when not laying eggs are probably best kept at a distance from your Brussels sprouts.

EYE ON NATURE

Do we have biting mosquitoes in Ireland? I have heard that a biting variety is now in the Castlegregory area and also in Co Clare. I was bitten twice. I assume they are a recent phenomenon.

Eoghan O Loingsigh, Castlegregory, Co Kerry

We have at least four such varieties who have been here for millennia.

To our delight, we have discovered a family of red squirrels living near our property in Sligo. We are also aware of a pine marten living nearby and that it is the squirrels’ mortal enemy. How can we ensure their safety?

Ann Hearne, Skreen Road, Dublin 7

There is nothing you can do, but while pine martens do prey on squirrels they are a small part of their very wide food source. Pine martens are excellent climbers but they are around four times heavier than red squirrels, and can’t follow them to the outer ends of branches.

When we had the recent high tides, and the Inishes in Ballisodare Bay were flooded, I spied a hare swimming about 30m to safety.

Anne-Marie Ellison, Sligo

A bright green parakeet with a long tail flew over our garden.

Richard Warburton, Churchtown, Dublin 14

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo. Email: viney@anu.ie. Include a postal address.