The object of the milk is not only to lessen the injurious qualities of the kerosene, but also to induce it to mix more freely with the water; but it is the oil alone which destroys the insects _and their eggs_.
(_c._) _Kerosene and soap._ When milk is not obtainable, or too dear, nothing is so excellent as this mixture. Soap itself (see below) is a useful insecticide, and in combination with kerosene includes the good qualities of both substances. The cheapest possible qualities of soap will do. The mixture, which is, even more than the last, purely _mechanical_, must be made first of all an "emulsion." The American experiments result in the following recipe and method of using:--
Common soap 1/2lb. Kerosene 2 galls. _Soft_ water 1 gall.
Dissolve the soap in the water heated to boiling, then add the kerosene, and churn the mixture until a creamy fluid results which thickens on cooling. Dilute with nine or ten times the quantity of water: the quantities given above will make about thirty gallons of liquid. Whale-oil soap, soft-soap, or any other kind will do. As with the milk emulsion, apply in the form of the finest spray for evergreens (Riley; Hubbard; Personal experiment).
(_d._) _Kerosene and oil._ Castor-oil, linseed-oil, whale-oil, may be used. A mixture of this kind, in the proportion of 1 part kerosene to 3 or 4 of oil, has been found very efficacious for apple- and other fruit-trees attacked by the common apple-scale (_Myt. pomorum_). But, as observed above, the mixture must not be laid on too thick. Thinly brushed all over trunk and branches, at dead of winter, it has been found quite successful in destroying both insects _and eggs_, without injury to the trees (Personal experiment). It would probably not answer for evergreens, on account of expense.
On the whole, it may be said that, as far as certainty can be attained in the matter, there is no substance better for destroying Coccids _and their eggs_ than kerosene in the form of milk or soap emulsion, diluted with water for evergreens or for trees with insects on the leaves as well as on the bark. Probably, for deciduous fruit-trees the kerosene-and-oil mixture is the best.
The great point in favour of this substance is that _it destroys the eggs_; this, few if any of the others will accomplish.