168 MASS. EXPERIMENT STATION BULLETIN 179. 



(c) Linseed Oil Emulsion. — Thus, out of nine mixtures, only those- 

 containing linseed oil proved at all promising. Mixtures 7 and 8 were 

 rather poorly saponified (chemically), while 9a and 96 were completely 

 saponified; but 7 and 8 proved efficient, while 9a and 96 were not. This 

 could only be explained by the fact that the free linseed oil was really 

 the toxic agent, and when it was only partly saponified there remained 

 some free linseed oil which established the efficiency of the spray. Upon 

 this supposition were based other preparations containing linseed oil 

 mechanically emulsified in a solution of soap in water. These emulsions 

 proved to be efficient when a 1 per cent, oil spraj^ was used. 



Two types of linseed oil emulsion may be made, depending upon the 

 length of time these emulsions are to be retained before use. 



Experimentally it was found that the most stable stock emulsion could 

 be made as follows : one-eighth of a pound of Ivory soap (one-half a 5-cent 

 cake) dissolved in a pint of very hot water. After the soap is completely 

 in solution add 1 pint of cold water followed by the addition of 1 pint of 

 raw linseed oil. The oil should be completely emulsified by the use of a 

 bucket pump. This solution is stable, pro\dded the water contained in 

 it is not allowed to evaporate. In using this stock emulsion, especially 

 after it has been kept for some time, it is best to mix one part of stock 

 with an equal volume of water before diluting to desired strength. One 

 part of stock emulsion in 20 parts of water proved to be efficient in killing 

 mites, both in the quiescent and feeding stages. 



If spraying is to be done soon after mixing thd emulsion it is best to 

 increase the amount of water and soap, and make the emulsion as follows: 

 shave 6 ounces of Ivory soap (1| 5-cent bars) into 1 gallon of hot water. 

 Add 2 quarts of cold water to cool the solution, then add 1 quart of raw 

 linseed oil and emulsify with a bucket pump. This emulsion, used at the 

 rate of 1 part in 9 parts of water, is very efficient, kilhng quiescent and 

 feeding mites without injury to leaf tissue. 



Soy bean oil substituted for linseed oil proves to be efficient, and in 

 some localities could be used to advantage. 



Action of Linseed Oil Einulsion upon Mites. — The majority of oils used 

 as insecticides are regarded as contact poisons. These poisonous oils 

 are supposed to enter the body of the insect, either directly through the 

 thin membraneous chitin of the body segments or by entering the spiracles, 

 where they immediately pass through the tracheal lining and produce 

 an active effect upon the internal structures essential to the life of the 

 insect. 



In a previous part of this paper it has been showii that the spiracles 

 are very few, — probably two in number, — and that the body of a red 

 spider is covered by a rather thick and continuous coating of chitin. For 

 these reasons sprays which prove effective in killing aphids are of little 

 value when applied to mite-infested plants. 



Many of the spray materials which have given partial success in con- 

 trolling mites have a marked adhesive action, and from this property 



