Leptinotarsa Decemlineata is a little creature somewhat larger than a ladybird, but similar in shape, and it has, as hinted by its name, 10 distinctive black and yellow stripes running the length of each wing. Until little over a century ago, it lived quietly, unobtrusively and quite anonymously in the Midwest of the US, minding its own business and feeding from the leaves of a local plant, the wild sand-bur or Solanum rostratum, of the Rocky mountains.
Then came the Gold Rushes of the 1840s, and the miners on their way west introduced Solanum tuberosum, which we know as the potato. The insect developed an addictive liking for the leaves of this new delicacy, and has eaten little else in the intervening years.
With such an abundance of new food available, successive generations of what had now come to be known as the Colorado beetle, migrated eastwards across the US and reached the east coast by 1875. After its inadvertent importation into the south-west of France, near Bordeaux, in 1922, le doryphore de la pomme de terre spread inexorably, reaching the English Channel by 1940. It continued its march eastwards as the Kartoffelkaefer in Germany, madelinka ramborova in Czechoslovakia, Stonka ziemniaczana on the farms in Poland, and finally reached Russia in the guise of Koloradski zhuk.
Temperature is the key to our deliverance from what I suppose might have been the ciarog Colorado here in Ireland. The beetle migrates by using its wings to become airborne and then allows itself to be carried along by the wind. But its metabolism is such that it seems to be capable of flight only when the temperature is around 25 degrees celsius. Temperatures over the sea are almost invariably less than this critical value, so when the insect is carried away from the land, the temperature falls, and the beetle stops flying and drowns. For this reason, the English Channel - and in our case the Irish Sea - is an effective barrier.
Even when the insect is imported into Ireland inadvertently by some other means, as happens every now and then, there are doubts about its viability in our traditionally cool climate. The reproductive success of the Colorado beetle, in terms of the number of generations per year, is highly temperature dependent, and Irish temperatures are unlikely to be high enough to facilitate its increase to plague proportions. But scientists are far from totally confident on this score; average temperatures have been higher in recent years than heretofore, and in any case troublesome insects have a habit of adapting - within limits - to a local climate.