267 



Two methods of experimentation may be distinguished. One we 

 may call the field method and the other the laboratory method. The 

 first method is exemplified in the splendid sets of experiments con- 

 ducted by the United States Division of Entomology. They are to 

 test practical questions by practical methods ; that is, they take the 

 conditions as they occur in nature and apply remedies wit-h methods 

 by which they could be applied in practical work. The laboratory 

 method may produce conditions that seldom or never occur in nature, 

 and may use remedies by methods entirely impractical in the field. It 

 is essential to the field method that we take things as they are; that 

 we do everything in a rough way. The laboratory method, on the other 

 hand, is especially characterized by the greatest possible accuracy in its 

 detail and by the elimination as far as possible of all sources of error. 

 The results obtained by the former method are practical, but almost 

 wholly empirical, while the latter gives theoretical information. Both 

 methods are valuable, both are essential to economic entomology. The 

 field method heretofore has been chiefly followed, so that the practice, 

 though often of a very doubtful character, is far in advance of the 

 theoretical knowledge of this science. It is to call your attention to 

 the value and importance of the laboratory method that this paper is 

 prepared. 



That part of the therapeutics of economic entomology that deals 

 with the effects the insecticides have on plants is deservedly receiving 

 considerable attention. The arsenites are the most important insecti- 

 cides from this point of view, both on account of their extensive use aud 

 because of the great injury to the plants for which they are sometimes 

 accountable. 



The injury they do to vegetable tissue seems to depend to a great ex- 

 tent upon their absorption by the plant. The critical points are the 

 time of ai^plication [when the poison is api^lied wet] and each subse- 

 quent rain or dew. A knowledge therefore of the conditions favoring 

 rapid absorption is of great importance. Some experiments made by 

 us on oak leaves prove that the leaf in drying increases its power 

 to absorb water up to a certain point and then decreases it. Is this the 

 case with other plants ? If so, the best time to spray on a clear day is 

 in the afternoon, but on a less clear one, in the morning. There is need 

 of a large amount of experimentation along this line. 



In making applications of the insecticides to the leaf to determine the 

 injury produced it is essential that we be enabled to make a uniform 

 application, that is to apply the same amount of poison each time. The 

 use of water as a dilutant seems more accurate than if the poison is 

 used in the dry way, and dipping is the mode of application which in 

 my hands has been most satisfactory. 



To compare different insecticides it is essential that we make our solu- 

 tions of some standard strength. Unquestionably the standard for 

 comparison should be their effectiveness in killing insects. This will 



