Scolytus scolytus likes elms and long hot summers. The elm bark beetle, as it is also called, lays its eggs beneath the elm bark, and no one would object particularly were it not for the fact that the beetle carries with it spores of a fungus called Ophiostoma nuvo-ulmi. As the fungus thrives and propagates, the tree becomes a victim of Dutch elm disease.
No one knows exactly where Dutch elm disease began. It is assumed to have existed unobtrusively in some oriental habitat, the spread of Ophiostoma nuvo-ulmi controlled by natural enemies indigenous to that region. But when it was introduced inadvertently to Europe in the second decade of this century, becoming established and acquiring its name in the Netherlands around 1917, Dutch elm disease began a lethal march that has plagued palatial avenues and rural groves in the intervening years.
The disease reached Britain in 1927, carried by infected logs, and migrated to the US a few years later. Then, however, it more or less died out, until a new and more virulent strain appeared in the 1960s, causing a destructive epidemic that many readers will remember. Within a few years, all over southern England, elm trees that had stood for centuries were wilting, losing their leaves, and showing signs of great distress. Here in Ireland, the damage was more patchy, but serious nonetheless. That particular episode peaked in England after the long hot summer drought of 1976. During any long drought, many trees become weakened and vulnerable to enemies. But such conditions also favour the lifestyle Scolytus scolytus, which, like all cold-blooded organisms, is very sensitive to warmth, and is capable of flight only when the temperature is above fifteen or sixteen degrees Celsius. Warm weather therefore, particularly in May and June when the beetle lays its eggs, allows it to venture far afield and deposit its infection in may different places.
Some twenty-five million elms, out of a total population of thirty million, succumbed to Dutch elm disease in Britain in that first major epidemic. Less tropical summers and other factors in the following years slowed the progress of the plague and it faded from public consciousness - but it never disappeared completely; reservoirs survived in isolated pockets of elms, both beetle and fungus waiting until the next generation of elm trees grew large enough for them to breed in. First hints of a return to destructive viability came in the early 1990s, and the resurgence was greatly helped by the hot summer of 1995.
Since then, ninety per cent of young elms around London, for example, have fallen victim to the deadly fungus.