EXPERIMENT STATION BULLETINS. 593 



It was observed that on exposure to the air, the orange color of the 

 poljsulphides disappeared more rapidly from the solution diluted with 

 whitewash than from the water dilution. 



Mr. Winter's estimations showed, further, that when the two dilutions 

 were exposed to air the polysulphide sulphur in the whitewash dilution 

 decreased a little more rapidly, while the thiosulphate sulphur in- 

 creased more than in the water dilution. He concluded, therefore, that 

 the polysulphides oxidized on exposure to air a little more rapidly in 

 the whitewash dilution. 



If, now, this increased rate of oxidation in the whitewash dilution 

 applies to the thiosulphates as well, it may account, in part at least, 

 for the larger amount of '^oxygen used" (Table XIII) in the whitewash 

 dilution ; since the oxidation of 2 c. c. of lime-sulphur used there was 

 not entirely complete in eighteen hours. As has already been stated, the 

 polysulphides disappeared early in the experiments and oxygen was 

 used very slowly at the end of the period. 



Whitewash added to the lime-sulphur seemed to give body to the spray 

 so that more of it would stick on a given surface. This, also, would 

 seem desirable in orchard spraying, since more of the insecticide would 

 be held in contact with the scales. It may be added that hot lime-sul- 

 phur spreads more readily on the bark of a tree than the cold wash, 

 and the heat assists the solution in attacking the wax of the scale. 



Whether the addition of excess of lime (as whitewash) to the lime- 

 sulphur and the practice of applying the wash hot would increase the 

 efficiency of this scalecide enough to prove economical in orchard spray- 

 ing, however, can be decided only by careful practical orchard tests 

 on a large scale. Such tests are out of the province of this project. 



GENERAL CONCLUSIONS. 



Only general conclusions pertaining directly to the project are given 

 in this summary. Many related considerations of importance outlined 

 or discussed in the body of the paper are not mentioned here. 



Usually, contact insecticides do not depend upon one property or 

 means, alone, for their effectiveness, yet as a rule some one property is 

 chiefly concerned. 



Alkaline washes, corrosive sublimate solution, and other fluids, which 

 are capable either of dissolving or of precipitating certain constitu- 

 ents of insect-tissues, are able to penetrate the chitin of insects into the 

 tissues slowly. The weaker the surface tension of the fluid, apparently, 

 and the thinner the chitin with which it is in contact the more rapid 

 the penetration. Gases and vapors may penetrate the chitin of in- 

 sects, especially through the tracheae, into the tissues far more rapidly 

 than liquids. 



It is through absorption into the insect-tissues of the volatile por- 

 tions of kerosene, gasoline, creolin, pyrethrum and such contact in- 

 secticides that they mainly become effective agents against insects. 

 Vapors from these insecticides enter the tissues and become effective 

 long before the liquids as such have time to penetrate the chitin, Kero- 

 75 



