(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.
The treatment of a deciduous plant infested by Coccids is simple as to its method. For two reasons the dead winter-time must be chosen for it—first, because, the leaves being off, the whole plant can be easily got at; secondly, because the eggs of the insect have not yet been hatched, and the whole brood can be destroyed at once. The first operation should be the pruning of the tree, so as to reduce the labour required to a minimum. A brushing over all the trunk and branches with a good hard stiff brush and one of the liquid remedies given below is then often successful. Brushing with a dry brush is adopted by some persons; but, although this doubtless clears away a good many insects and scales, and may do the tree itself some good by cleaning off fungus-growths and incrustations, yet it necessarily fails to destroy all the eggs, and in consequence the work is only half done. Any one who wishes to extirpate Coccids must make certain that he has destroyed the eggs—a fact which is quite ignored by numbers of those who glibly talk of their own success, and blame the advice of others. The object being, therefore, twofold, the operation should be performed with a hard, stiff brush dipped in one of the fluids recommended below; and care should be taken that there is no part of the trunk or branches escaping untouched. In fact, what should be aimed at is a kind of painting of the tree, but with a thin coating of the fluid, so as to close the pores as little as may be; while at the same time the brush clears away as many as possible of the "scales" and their enclosed broods of insects and eggs.