22 PROTECTION OF PLANTS, 1915-16 



not dead, but no metabolic changes occur. Moreover, it is not in a lethar- 

 gic condition, for in a state of lethargy metabolic changes occur. Lower, 

 it enters the stage of permanent cold torpidity, which ends in death. The 

 temperature of permanent cold torpidity varies considerably. With larvae 

 it lies between -4° and-42°C., with pupae between -4° and -25°C., and with 

 adults between -1.5° and -35°C. 



III.— HOW INSECTICIDES KILL 



Contact Insecticides. 



Until recently our information regarding the action of insecticides 

 was vague and often incorrect. It was commonly stated that contact 

 substances kill insects by stopping the breathing pores or plugging the 

 tracheae, producing death by suffocation. The investigations of G. D. 

 Shafer of the Michigan Agricultural College (Technical Bulletins 11 and 21), 

 however, have shown that the old idea is mainly wrong inasmuch as insects 

 are not readily suffocated. The death-producing action is mainly due to 

 the absorption of the chemicals into the tissues. It was shown, for example, 

 that the volatile portions of kerosene, carbon bi-sulphide, gasoline, creolin, 

 pyrethrum, etc., were effective long before the liquids had time to penetrate 

 the chitin or the spiracles into the tissues. 



With the penetration of the volatile substances the nervous system 

 is seriously affected, and results resembling narcosis produced, with a dis- 

 turbance of the respiration activity. Insect tissues soon become satur- 

 ated when exposed to the vapors of the substances mentioned, and death 

 ensues through the inability of the tissues to absorb oxygen in the presence 

 of these vapors. 



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. 



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, it was shown by Mr. Shafer that gasoline, carbon bisulphide, 

 hydrocyanic acid gas, sodium fluoride, etc., when applied to insects, acted 



