4^8 AN ECONOMIC ENTOMOLOGY. 



For winter use as against armored scales, two pounds in one 

 gallon of water are necessary, and this must not be used until after 

 midwinter, as otherwise severe injury may be caused to fruit buds 

 or even twigs and branches. At this strength the application is 

 intensely caustic, and the scales are corroded, lifted, and dried 

 out, while the soap, dissolved by rains, penetrates through the 

 covering to the insect beneath. 



Petroleum or naphtha soaps, or soaps containing a greater or 

 less percentage of mineral oil in solution, are better than ordinary 

 laundry soaps, but not so effective as fish- or " whale-oil " soaps. 

 The percentage of petroleum is usually small, and without a guide 

 as to the actual amount their use is uncertain. Besides, at the 

 prices charged, the ordinary kerosene emulsion is cheaper 



Among the contact poisons none rank higher than the mineral 

 oils, whether in the form of the crude petroleum or the refined 

 kerosene ; but their effect on plant life is often severe and not 

 always identical. 



As against scale insects that hibernate in the partly grown con- 

 dition, there is nothing better than crude petroleum of forty-three 

 degrees specific gravity on Beaume scale, and, when properly 

 applied, it is safe on almost all kinds of trees when they are dor- 

 mant. Given an oil of the proper gravity, with a vaseline base 

 like all those from the Allegheny region, it should be applied 

 slightly warm — about seventy degrees Fahrenheit — -through a 

 fine vermorel nozzle, with a good force behind it, on a dry day 

 to a dry tree, and just enough to wet thoroiighlv. The oil is very 

 penetrating, soaking through the dry scale covering at once, and 

 coming into contact with the insects. The light oils evaporate in 

 a few minutes, and there remains a film of parafifine and vaseline 

 which is either absorbed in time by the surface or by the dust 

 that settles on it. The vaseline coating gives the application its 

 lasting power, and sooner or later it gets through the thickest 

 layer of scales ; but it also constitutes the danger, for when the 

 oil is applied in excess the outer bark becomes soaked, and on 

 the smaller twigs and branches the bast as well as the tissue 

 dies. In the hands of the experienced man there is no more 

 effective material than crude oil ; in the hands of the careless 

 laborer there is nothing more dangerous to plant life. 



Emulsions of crude oil have not proved satisfactory and me- 

 chanical mixtures made by emulsion pumps are equally unreliable. 



