40 Roosevelt Wild Life Bulletin 



Every strength of copper sulphate used proved lethal in time and 

 every leech exposed to its full action died, except in tvv^o cases in 

 which a 3,000,000 and a 5,000,000 dilution in which leeches had 

 previously died, were used a second time. These had evidently lost 

 their toxic properties through their previous reaction to organic sub- 

 stances. In the stronger solutions death (determined by the com- 

 plete relaxation of the leech and its failure to respond at all when 

 pinched with forceps) occurred in from five to twelve and one-half 

 hours, in the one-five-millionth solution in from sixteen to eighty- 

 six hours and in the intermediate strengths in corresponding periods 

 of time. The variation in the time required by any particular strength 

 to kill showed a rough correlation with temperature and amount of 

 organic matter in the form of minute plant life present in the water, 

 being negative to the former and positive to the latter; that is, the 

 toxicity for the leeches increased with increase in temperature and 

 diminished with increase in the quantity of minute algae present. The 

 exact cause of death, whether due to the direct action of the metallic 

 poison or to its indirect action by absorption of the dissolved oxygen 

 in the water with consequent suffication, was not determined. Inas- 

 much as leeches are highly resistant to oxygen starvation it is prob- 

 ably not the latter. 



In the half-filled jars the leeches continued to live through the 

 period of the experiments. None seemed to be harmed in the least 

 by the solution from contact with which they early escaped. In the 

 check jars they lived normally, not a single one having died during 

 the course of the experiments. 



Concerning the effect of copper sulphate upon leeches in the lake 

 itself I was fortunate to be present during two applications for 

 water bloom. Though a very careful inspection of the shore was 

 made not a single dead leech was found, although in one case the 

 mortality among fishes and particularly among catfishes was con- 

 siderable. Noticing that the water bloom in sheltered places close to 

 shore, especially in shallow water, was often little harmed by the 

 application of copper sulphate, it was thought that the poison might 

 not have reached the leeches. To remove this doubt some of these 

 places were sprayed with a one-five-thousandth solution. No effect 

 on the leeches was noticed except that several were stimulated to 

 activity, which might have been due to the fact that the operator in 

 applying the solution often waded in the water. 



It becomes clear, therefore, that copper sulphate cannot be used 

 successfully to kill the leeches. If its use can be recommended at 

 all, it is only as a possible repellent. If applied in considerable 

 strength almost daily at the swimming docks it might drive the 

 leeches away but its value for even this purpose is doubtful. 



Because of its large use as a disinfectant for camp latrines, drains, 

 etc., chloride of lime was selected for a similar series of experiments. 

 It proved to be much less toxic than copper sulphate, the weakest 

 solution that was lethal within four days being one-one-millionth and 

 a few leeches survived this. A one-five-hundred-thousandth solution 

 proved invariably fatal in from eighteen to forty hours. Chloride 



