Nothing need be said here of carelessness or unskilfulness in applying a remedy, beyond the following instance: A person whose apple-trees were being very much damaged by _Mytilaspis pomorum_ was advised to apply, by way of painting the trunks and branches, a mixture of kerosene and some other ingredient. In two or three months he found violent fault with his adviser, for he said every tree was dead or dying. On inquiry it was found that, from over-zeal or want of knowledge, he had applied the mixture as if painting a house, had used it much too strong, and, to make assurance doubly sure, had given his trees two good coats of the oil. An intelligent appreciation of the life-history and habits of scale-insects is necessary to enable any one to select and apply, with a probability of success, a remedy, and the details given in Chapter I. of this work will be found useful for this purpose. It will be apparent from them that, without regarding the generic or specific characters of these insects, we may lay down a few general principles to start with, thus:-- I. _Whatever damage is done is effected by the sucking of the juices of the plant through the rostrum of the insect._ It follows from this that applications of any fluid to the tree externally, with the object of poisoning the insects in their feeding, would be useless, as their food is drawn from beneath the surface. II. Neglecting entomological distinctions, we may divide the Coccididæ, roughly, into-- (_a._) Insects attacking deciduous plants; (_b._) Insects attacking evergreen plants; or, again, (_c._) Insects living usually on the bark; (_d._) Insects living usually on the leaves; (_e._) Insects living on both bark and leaves; or, lastly, (_f._) Insects covered with hard shields or "scales;" (_g._) Insects covered with cotton; (_h._) Insects naked. It will be clear that a different method will be required for destroying these different classes; but any one insect will belong to more than one class. Thus _Mytilaspis pomorum_, the apple-scale, belongs to (_a_), (_c_), and (_f_), and indeed may be placed also in (_b_), as such plants as hawthorns, which it attacks, are as bad as evergreens in the difficulty of reaching the insect on them; or, _Lecanium hesperidum_ is in (_a_), (_d_), and (_h_); Lecanium oleæ in (_a_), (_b_), (_e_), and (_h_). As far as regards the injurious species of Coccids it may usually be taken for granted that those infesting deciduous plants (class _a_) live chiefly on the bark (class _c_), and are either naked (class _h_) or covered with a hard scale (class _f_). If naked they are chiefly _Lecanium_; if covered, either _Mytilaspis_, _Aspidiotus_ or _Diaspis_. _Icerya_ is exceptional, being omnivorous, feeding equally on bark or leaves, deciduous or evergreen plants; it belongs to every class except (_f_). Every method of destruction has therefore to be resorted to against it.