278 CONTROLLING INSECTS AND DISEASES 



Tobacco preparations in dust form are also available. 

 They are used extensively on vegetable crops and to some* 

 extent for fruit. They have been developed recently and their 

 use is likely to expand as they are further perfected. 



Tobacco dusts are obtained by impregnating some suitable 

 carrier, as hydrated lime, with free nicotine or nicotine sul- 

 fate, or by grinding tobacco refuse into a fine powder. Tobacco 

 refuse gives dust of an uncertain strength and effectiveness. 

 Some growers make their own dust from hydrated lime and 

 nicotine sulfate. 



Dusts made with free nicotine are more volatile and 

 quicker in their action than those made from nicotine sulfate 

 containing 40 percent nicotine. This is an advantage since 

 insects succumb more readily to the dusts of higher volatility. 

 Such dusts, however, are more expensive than those of slower 

 action. The strength of nicotine in these preparations is ex- 

 pressed either as the percentage of actual nicotine in them, or 

 the percentage of nicotine sulfate, which is but 40 percent 

 nicotine. A 2 percent nicotine dust is thus nearly 2% times 

 as strong as 2 percent nicotine-sulfate dust. (Nicotine is about 

 95 per cent pure.) This difference should be carefully noted 

 in making calculations. 



Nicotine dusts should be purchased in tight containers. The 

 containers should be kept closed and as nearly full as possible, 

 because the dusts lose strength rapidly on exposure to air. 



a : 8. Oil sprays possess considerable insecticidal value. 

 They spread well, permeating every crack and crevice. Their 

 use is limited, however, because of their caustic effect upon the 

 foliage and tender bark and because they cannot be used in 

 combination with any sulfur sprays. They do not corrode 

 metal parts of a sprayer but cause the rubber parts to de- 

 teriorate rapidly. Oils may not be used in freezing weather, 

 jor upon stone fruits at any season. 



Emulsification. So that the oil may mix freely with water 

 for dilution in a spray tank, a third material, an emulsifier, 

 is added. Several emulsifying agents are on the market under 



