23. _Tobacco._ A good remedy against larvæ and adults; _doubtful against the eggs_. Fumigation has no effect on scale-insects, except sometimes on Dactylopidæ, or "mealy bugs" (Hubbard; Comstock; Personal experiment). The tobacco should be applied in a pretty strong solution; but the expense in this country would be probably too great for general use. 24. _Whale-oil and whale-oil soap_ have been already alluded to under the head "Kerosene." They are both useful ingredients in mixture with that substance, if procurable cheaply. From the foregoing list it will be gathered that, if experiment, combined with knowledge of the habits and life-history of scale-insects, can be relied on, there is no substance better adapted for their destruction than kerosene, mixed with oil, or milk, or soap solution, and _carefully applied_. It has been already observed that the killing of the eggs is absolutely necessary for thorough clearing-away of the insects; and, to quote again the words of Mr. Hubbard, kerosene is "almost the only substance which will with certainty kill the eggs without at the same time destroying the plant." But precautions must not be neglected. Persons who recklessly use any remedy, or who apply it too thickly or in too strong proportions, must expect their trees to suffer. Nor must the weather and the time of the year be overlooked. _Winter is the best season for all remedies_; and, preferably, cool and cloudy days. Again, if substances soluble in water, such as potash or soda lye, soap solutions, &c., be employed, it must be expected that a day's rain will wash a good deal of them off, and greatly reduce their efficacy. These are things which many people forget; they fancy that because somebody has cleared his trees with, say, castor-oil in winter they can do the same thing in full heat of summer; or, because a lye solution has done well in the dry climate of California, that it will be equally good in the rains of New Zealand. Still more is it a fallacy to imagine that rule-of-thumb methods, not founded upon any knowledge of the nature, habits, and life-history of the insects, are likely to be really efficacious. Little need be said here of a remedy which has had, to some extent, the authority of Professor Riley, and which is recommended by Mr. Howard (Report U.S. Dep. of Agric. 1880-81, p. 351): viz., the transportation or acclimatization of parasites on scale-insects. Doubtless the thing could be done, as experiments in America have shown. But there are plenty of parasitic insects in New Zealand already, and, although they seem to have hitherto confined their work to the native and mostly to the innoxious Coccids, they may at any time begin to attack the others, and it is only a question of time when they will act usefully as efficient checks (see Chap. IV.).