CONTACT INSECTICIDES 51 



Tobacco dust is also used around plants as an insect repellent as well as a 

 fertilizer. 



Nicotine obtained as above indicated, is varia})le in its strength and 

 the amount it should l)e diluted for use is uncertain. It is also quite 

 volatile and this is a disadvantage when it is used as a spray. Com- 

 mercial nicotine compounds on the market avoid these difficulties by- 

 supplying a material containing a fixed percentage of nicotine combined 

 with sulfuric acid, known as nicotine sulfate 40 per cent. This can be 

 diluted to the proper strength with accuracy, and does not pass off into 

 the air rapidly. Nicotine uncombined, of the same strength, can also 

 be obtained, but should be used for fumigation and not as a spray. 



Nicotine sulfate is an excellent material to use for plant lice and other 

 delicate insects. It is generally diluted at the rate of 1 gal. to 800 or 

 1,000 gal. of water, and in some cases a greater dilution even, than this is 

 possil:)le. Sometimes dilution at the rate of 1 to 500 is desirable. 



Standard formula for nicotine sulfate 40 per cent, 1 part to 800 of 

 water: 



Per Barrel Per Gallon 



Nicotine sulfate, 40 per cent J-2 pint 1 '-4 tcaspoonfuls 



Soap 2 to 3 lb. 1 oz. 



Water 50 gal. 1 gal. 



Three-eighths of a pint in 50 gal. of water, or 1 teaspoonful in a 

 gallon, gives nearly a dilution of 1 to 1,000. The addition of soap 

 causes the material to spread more and adhere better. 



Among the various sulfur compounds, those with lime have thus far 

 been found to be the most successful. 



Lime-sulfur Wash. — This is prepared by boiling lime and sulfur 

 together in water. Several substances are produced by this boiling, 

 but apparently its insecticidal value is determined by the quantity of 

 calcium polysulfids (CaSi and CaSs) and possibly the calcium thiosulfate 

 (CaSoOs) which are formed in the mixture. The wash can be made at 

 home but it is generally easier to buy it in concentrated form and dilute 

 as needed. It will vary in specific gravity in different cases, and its 

 reading must be taken (a Beaume hydrometer is generally used for this 

 purpose) in order to dilute it properly. The range in readings of differ- 

 ent lots may vary as much as 5° or more, but is usually about 33°Be. 

 Thus, a sample of this density should have 634 gal. mixed with 43^ gal. 

 of water to be of the proper strength for use as a winter spray for the 

 San Jose Scale, when this insect is dormant; while if its density is 30°Be., 

 7 gal. should be mixed with 43 gal. of water. Tables of density, Beaum6 

 readings, and the amount of concentrate to add to water to make a total 

 of 50 gal. of spray, both for winter use and as a foliage spray in summer, 

 can be obtained by applying to any state experiment station or to the 

 U. S, Bureau of Entomology. 



