14 CIBCULAK 3 6 5, U. S. DEPARTMENT OF AGRICULTURE 



The volatility is usually expressed as a percentage showing the 

 proportion of oil evaporating in a given time under standard con- 

 ditions. As given by the oil companies this has little meaning in 

 insecticidal practice, because the volatility is measured at much 

 higher temperatures than those prevailing when oils are used for 

 sprays (9). Oils of the kerosene type evaporate too quickly to make 

 efficient insecticides, but if lubricating oils, which evaporate com- 

 paratively slowly, are used it is probable that volatility can be 

 disregarded. 



The viscosity of an oil is a measure of the rate of flow under 

 specified conditions. With soap emulsions, it has been found that 

 the more viscous oils give a greater oil deposit and better insect kill 

 than oils of lower viscosity. Against the camphor scale, oils within 

 the range of approximately 100 to 220 seconds Saybolt have given 

 satisfactory results, oils in the upper part of this range showing a 

 somewhat better kill than those in the lower part. 



The unsulphonatable-residue value expresses the percentage of oil 

 by volume that is not affected by treatment with concentrated sul- 

 phuric acid, and is an index of the degree of refinement of the oil. 

 A high unsulphonatable residue indicates that the oil has been highly 

 refined and that the more chemically active constituents have been 

 removed. Oils ranging from 67 to 95 percent unsulphonatable resi- 

 due have shown no difference in their effect upon the camphor scale, 

 but the refined oils are less toxic to plants than oils showing a low 

 unsulphonatable residue. In spite of the higher cost, therefore, their 

 greater safety to the plant may justify the use of the refined oils for 

 the more susceptible host plants. 



TIME OF APPLICATION' AND OIL CONCENTRATIONS 



The best time to spray the infested camphor-trees and most other 

 trees and ornamental plants (except citrus) is during the dormant 

 season, from about November 15 until the buds begin to burst in the 

 spring. All the scales are in the adult stage at this time, but the 

 plants will stand heavier concentrations of oil than during the 

 growing season. 



The oil concentration to be used will depend upon the type of 

 emulsion. For a boiled emulsion prepared according to formula 

 1, 3 percent of oil should be used in the dilute spray. Since the 

 concentrated emulsion contains approximately 60 percent of oil, 1 

 part of emulsion should be mixed with 19 parts of water to give 

 a 3-percent spray. The cold-pumped emulsion made according to 

 formula 2 will deposit more oil than the boiled emulsion and should 

 be used at the rate of 2.5 percent of oil, or approximately 1 part of 

 oil in 23 parts of water. The oil-depositing qualities of the casein- 

 lime spreader and soap emulsions have never been compared, but 

 some investigators have found that the casein-lime gives a kill 

 equal to that obtained with the boiled emulsions. 



When only a few gallons of emulsion are needed, the grower will 

 probably find it more satisfactory to purchase one of the commercial 

 emulsions. These are ready for use when diluted with water, and 

 should be applied in the strength given in the manufacturer's di- 

 rections. 



