27 



plants are not concerned, as in the control of mosquito larvae, house- 

 hold pests, etc., but where plants are involved, dilution with water 

 into the form of an emulsion is necessary. The simplest form of an 

 emulsion is produced by the mechanical agitation of oil and water 

 until a momentary emulsion is formed ; this must be applied to the 

 plant before it breaks up again into its constituents. Most of the 

 eumlsions used for spraying are rendered more or less permanent in 

 character by the addition of a third substance, known as the emulsifier, 

 which must be colloidally soluble in the dispersing liquid. Soap, 

 which is colloidally soluble in water, is the usual emulsifier in petroleum 

 insecticides, though other substances, such as iron hydroxide, soluble 

 silica, zinc sulphide and gelatine all give excellent emulsions. The 

 function of the emulsifier is to form a layer or pellicle around the oil 

 drops to keep them from coalescing, though how this is effected is 

 still a matter of conjecture. The author has found that heat is 

 absorbed in the formation of an emulsion which would indicate a 

 molecular rearrangement, and this is further demonstrated by the 

 fact that in a true emulsion, where there is a maximum of oil emulsified 

 in a minimum of water, the viscosity of the resulting emulsion is always 

 greater than that of its component liquids. 



In making a spray emulsion, the emulsifier is added to the water 

 and the oil then added gradually. If the emulsion is to be very much 

 diluted, the oil can be put in all at once. It is necessary to follow 

 this general scheme in order that the emulsion shall contain the oil 

 in the form of drops. For spraying purposes, an emulsion approaches 

 perfection as the drops become smaller and more uniform in size, 

 the emulsion being much more stable with, small even drops, while 

 the application is more uniform, thus increasing the insecticidal 

 power of the spray. There is also less fear of scorching the plant 

 when no drops of free oil can gather. Up to a certain point the addition 

 of more emulsifier ^^^ll decrease the size of the drops. The minimum 

 size of the drops seems to be a function of the oil. The method of 

 determining the size of the drops, by means of a compound microscope 

 equipped \nth an oil immersion lens and an eye-piece micrometer, is 

 described. The so-called miscible oils are a commercial preparation in 

 which the emulsifier is held in the oil either by suspension or by colloidal 

 solution. These miscible oils when mixed with water make excellent 

 emulsions, but it is difficult to make a uniform product and the price at 

 which the oil is sold makes it uneconomical for orchard use. 



Davidson (W. M.). The Pear Woolly Aphis.— ilM?t/. Biill. Cal. State 

 Commiss. Hortic, Sacramento, vi, no. 10, October 1917, pp. 390- 

 396, 2 figs. 



The biology of Eriosoma ]}i/ricola, Baker and Davidson (pear woolly 

 aphis), which until recently has been confused with E. lanigerum, 

 Hausm. (woolly apple aphis), has previously been dealt with [see this 

 Revietv, Ser. A, iv, p. 369]. The root Aphids suffer very little from 

 natural enemies, a single Scymnus larva being the only predator 

 observed by the author, while no parasitic enemies have been 

 encountered. Against this Aphid, fumigation of the trees in air- 

 tight boxes is recommended, 1 oz. sodium or potassium cyanide being 

 used to each 100 cubic ft. of space, the process lasting 45 minutes. For 



