For a number of years previous to 1878 we had in Pembroke but little or no severe cold, owing to the prevalence of southeast, south, west, and especially southwest winds. In many places, fuchsias that were left in the ground for the entire year had not been frozen to the root within the memory of man. Some of these plants had grown to be trees five or six yards in height, and with a trunk the size of one's leg. Now, during the same series of years, many insects that are common throughout the rest of Great Britain did not cease to be rare with us, or rather were confined to certain circumscribed limits. Thus, the Noctuellae, with the exception of a few species abundant everywhere, were almost wanting, and I know of no other country where the dearth of common species of nocturnal butterflies was so great. But during the winter of 1878 there supervened a radical change. Persistent winds from the northwest, driving back the currents of warm air from the south, brought on an intense cold that froze everything; or, when some variation occurred in them, clouds formed and dissolved into a rain that immediately froze, so that the large roads remained for weeks covered with a layer of rime from two to four inches thick.
GREEN WOODPECKER SEARCHING FOR INSECTS.
The winters of 1879 and 1880 were equally cold; we may even say that the latter was the severest that had been experienced in fifty years. This year the sea-sand, along with the ice and snow, formed a thick crust all along the tide-line - this being something rarely seen along our coast. The first of these three winters (1878-1879) killed all the arborescent veronicas and a few sumacs. As for the fuchsias and myrtles, they were frozen down to the level of the soil.
I now come to the effects of this severe cold upon the insects.
The Lepidoptera, which before were rare, became more and more common in 1879, 1880, and 1881, and so much so that during the last named year they abounded; and species that had formerly been detected only at certain favored points spread over the entire coast and into the interior of the country. The geometers appeared in numbers that were unheard of. But this change was especially striking as regards the Noctuellae, in view of the previous rarity of the individuals belonging to this family.
We have here an example of the direct relation of cause to effect, although I am not in a position to assert that the effect is always produced in the same way. To me there is no question as to the fact that the constitution of those insects which nature has accorded the faculty of liberating is strengthened, and that their chances of life are increased, if the cold of winter is intense enough to plunge them into an absolute rest, and is not unseasonably affected by warm, spring-like days. It is certain that such cold is capable of contributing largely to the multiplication of the individuals of such species as hibernate in the egg state, and it also has a beneficent influence upon those species which, like the small social larvae, pass this season upon the earth enveloped in a silken envelope, or, like the larvae of the Noctuellae, between dead leaves or upon the ground itself.
On another hand, it cannot be doubted that mild winters greatly contribute to the bringing about of a destruction of larvae and chrysalids in two ways: First, they favor the development of mould, which, as well known, attacks the larvae of insects when these have been enfeebled by an excess of rain or dampness; and second, they permit beasts of prey to continue to exercise their activity. Now, these latter are numerous. Moles, instead of burying themselves deeply, then continue to excavate near the surface, and shrew mice are constantly in search of food. These small mammals, which abound in this district, destroy a large number of chrysalids of Lepidoptera.
It is the same with birds. As soon as severe cold begins to prevail in the north and east, they come in troops to the open fields and the sheltered slope of the hills of our district. But it is scarcely worth while to stop to tell of the skill and perseverance of these destroyer of larvae. We may mention, the woodpecker, however, as a skillful searcher for insects that lie hidden in places where the sun has melted the snow. The carnivorous Coleoptera and the Forficulae are likewise generally in motion during mild winters. Doubtless these last-named do not make very large inroads in the ranks of larvae and chrysalids every day; yet, having no other food, they destroy a goodly number of them. But I believe that the devastations made in the army of insects by all these enemies united do not equal those made by certain crustaceans - the wood lice.
During mild winters these pests multiply, eat, and prosper out of bounds, and to such a point that, in a climate like ours, they become a true scourge that prevails everywhere, out of doors and within. Once in a place, they begin to look for larvae and chrysalids, which they devour. The severe cold seems to have destroyed a certain number of them, since they are now not so numerous by far; and it has at least certainly put a stop to their devastations at an epoch when the larvae are more particularly exposed to the attacks of their enemies. It is to this cause, as well as to the preceding, that I am led to attribute the extraordinary multiplication of so many species during the three last summers, which were separated by severe winters. Last winter was mild, and there is therefore no reason to expect that there will be another multiplication; but I hope that the harm done by such a season will be slight. It is the progressive multiplication of the destroyers, joined to the correlative disappearance of the victims caused by a series of temperate seasons, that is to be feared.
In support of the proposition that I maintain, I may mention still another fact. While this district (Pembroke, Wales) is relatively poor in species whose larvae feed and hibernate in the open air a few species of Noctuellae, whose larvae live buried in the earth, are always abundant. The country is relatively rich in spices of Tortrix, which develop and hibernate in the stalks or roots of plants. It is also worthy of remark that very few of our species seem to be incapable of enduring a severe winter. - C.G. Barret, in Science et Nature.