Unfortunately, it became the practice to plant larch and Norway spruce in mixture. No more mischievous combination could have been devised, owing to a peculiarity in the life history of the spruce-gall aphis (_Chermes abietis_), a plant louse which bores into the buds of young spruce and lays eggs therein, causing the tree to throw out a cone-like gall from the site of the puncture. This gall is the nursery whence issues a swarm of sexual and sexless aphides. The sexless form has wings, and, alighting on a larch, speedily lays numerous eggs, which in turn are hatched into minute sexless lice, each with a coat of white down, easily detected as snowy dots on the foliage. In a few weeks these creatures acquire wings, and, despite their sexlessness, lay fertile eggs, successive swarms being produced till the fall of the leaf. Feeding by suction of the juices in the leaves, these creatures seriously, often fatally, reduce the vitality of the tree, the foliage appearing as if blighted by frost.
It must be admitted that this diagnosis of the life-history of the spruce and larch louse is to some extent tentative. It is true that no instance is recorded of the male _Chermes_ being found on the larch, and it is also true that, as stated by Elwes, larches are often infested with _Chermes_ where there are no spruces near.[20] But it is well known that many, if not all, of the _Aphidæ_ multiply by parthenogenesis (that is, without the intervention of the male), and although it has not yet been ascertained that this can be continued for more than four years,[21] that is a period quite long enough to allow of the swarms inflicting deadly injury to any tree not in the most robust health.
Now, whereas larch and spruce may often be found growing together in natural woods on the continent of Europe, it may be asked why the result of planting them together in British woods should be attended with such evil consequences. The explanation is to be found in the climatic conditions to which the larch is exposed in these islands. Naturally a mountain tree, in regions where a high summer temperature, long and strong sunshine, with little rainfall, but with much subterranean moisture from melting snow, promote vigorous growth, to be followed by total rest during severe winter weather, the larch meets in Britain with the reverse of these conditions--namely, a cool, cloudy, generally wet, summer, and an open and still wetter winter. The wonder is that the tree can adapt itself to the change as well as it does; there can be no doubt that its constitution does not remain so well able to resist attack by insect or fungoid parasite. Nature, which is ever as solicitous to provide for the perpetuation of what we consider ignoble vermin as she is for that of more admirable forms of life, has adapted the spruce-gall for a dual existence upon two species of tree growing in company; but she has also endowed these trees with a constitution vigorous enough not to suffer materially from the presence of the parasite. When that constitution becomes impaired by unnatural conditions of climate and environment, the parasite gets the upper hand, just as lice multiply upon a diseased bird or mammal. In the case of the larch, the mischief does not end with the aphides.