THE CONTROL OF INJURIOUS INSECTS 4OI 



In the case of lime-sulphur, its effectiveness is due to its reducing 

 power, and with scale insects to its softening action on the wax about 

 the margin of the scales and to its effect on the waxen covering making 

 it less permeable to oxygen. 



Alkaline washes, corrosive sublimate solution and other liquids, 

 which are able to dissolve or precipitate certain constituents of the 

 tissues, pass through the chitin slowly. 



Moreover, gasoline, carbon bisulphide, hydrocyanic acid gas, sodium 

 fluoride, etc. act strongly on the oxidases and other enzymes in the 

 tissues of insects causing serious disturbances. 



Fat or fat-Hke membranes absorb the vapors of gasoline and 

 chloroform, but in doing so become less permeable to oxygen; and waxen 

 membranes when wet with lime-sulphur also become less permeable 

 to oxygen. 



Non-volatile finely powdered solids, such as borax, hellebore, sodium 

 fluoride, etc., in addition to being stomach poisons, are effective also 

 as contact insecticides because they adhere to exudations on the body 

 wall, and later become dissolved and absorbed through the integument 

 into the tissues. 



The Utilization of Parasitic Insects 



The economic use of parasitic insects shows three phases: (i) the 

 utilization of the native parasites of the district; (2) the transportation 

 of the parasites from one district to another; and (3) the importation 

 of parasites from one country to another. 



1. In the great majority of cases of insect outbreak the native para- 

 sites are able to control it in time. In fact injurious forms are mainly 

 held in check by their parasites. Occasionally, however, through the 

 operation of some obscure factor, the multiplication of parasites is 

 prevented; then the injurious forms are permitted to reproduce with 

 much less check and much loss occurs before parasites are able to 

 "catch-up" again. 



2. Some successes have been reported where parasites were trans- 

 ported from one locality to another. LeBaron of Illinois in 1872 

 introduced Aphelinus mali, a parasite of the Oyster Shell Scale. 

 Webster in 1907 transported Polygnotus hiemalis from Marion, Pa., 

 to a field of wheat infested with Hessian fly at Sharpsburg, Md., and 



