269 



There is more than one kiud of iujury produced by arseuites. One 

 form generally shows itself as a spotting of the leaf. It is this form 

 that is usually observed and written about, but on certain plants it is 

 doubtful if the most injury be not produced in another way which I have 

 denominated chronic poisoning. This does not show itself immediately 

 according to my experience, but unless the leaf has also acute local 

 poisoning there is for some time no effect observable. Later the leaf 

 passes through the same process as is normal later in the year, becoming 

 dryer, generally coloring red or yellow and dropping. In the summer 

 or fall, after the spring growth has ceased, the limbs to which an appli- 

 cation is made will sometimes be left entirely bare from this cause, while 

 the other parts of the tree retain their foliage. Chronic poisoning is 

 most severe on peach of any plant with which I have experimented, 

 apple perhaps being the next. A similar thing occurs in the use of the 

 alkalies. 



Cross-section paper is a great requisite when this kind of experimenta- 

 tion is conducted. Each reading should be plotted so as to give the 

 investigator a clear idea of his results. Any deviation from a regular 

 curve, any exceptional maximum or minimum reading should be inves- 

 tigated, for it is only by attention to such details that we can be able 

 to find the cause of variation and to eliminate in subsequent experi- 

 ments that source of error. In reporting the experiments, the detailed 

 figures will be of little value, but the averages, the number of experi- 

 ments, and the maximum and minimum readings will give one an idea 

 not only of the results, but also of the trustworthiness of the work. 



Such experimentation as I have outlined presents a promising field 

 for cooperative work, and, as before stated, it is this kind of experi- 

 mentation that is at i)resent most needed. We must have a theoretical 

 knowledge of the subject, a rational for our practice ; for with eco- 

 nomic entomology, as with any other science, all investigation is hin- 

 dered and of a more or less transient value until the fundamental prin- 

 ciples are well established. 



Mr. Beckwith then read the following paper : 



- • 



PRACTICAL NOTES ON THE USE OF INSECTICIDES. 



By M. H, Beckwith. 



So much has already been written concerning this subject that it 

 would seem to be scarcely worth while to attemjitto add anything that 

 will be of value. Although I may not be able to give you any new 

 method of using insecticides, the work in this line that I have been 

 able to accomplish during the past season may prove interesting. 



It is not always those experiments that are the most successful that 

 prove most valuable. Frequently it is our failures that finally produce 

 the best results. Thus it is with the use of insecticides ; in attempting 

 to prevent the ravages of certain insects by the method that appears 



