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.