No. 6. DEPARTMENT OF AGRICULTURE. 579 



on file in the office of the State Zoologist, that the public fully 

 appreciates the value of this very important work, and all interested 

 persons appear anxious to avail themselves of the needed assistance 

 thus offered them by the State. 



TESTS OF COMMERCIAL INSECTICIDES. 



By T. C. LeFEVRE, Field Assistant to Prof. Surface. 



In testing an insecticide, commercial or noncommercial, it is neces- 

 sary to consider it from at least three points of view, viz.: 



(1) Does it kill a sufficient number of the insects for which it is 

 applied to warrant its use? 



(2) Does it injure the tree or plant to which it is applied? 



(3) Is the cost so great as to render it too expensive for practical 

 use? 



In considering the efficiency of an insecticide we frequently find 

 mistakes which greatly alter the final results and conclusions. 



Among these mistakes it might be well to mention that many 

 users of insecticides, fail to consider the scales which are already 

 dead at the time of spraying. On close examination in the early win- 

 ter or late fall, we found on peach and plum trees, moderately in- 

 fested, from 10 to 20 per cent, of the scale dead, and on apple and pear 

 from 5 per cent, to 10 per cent. dead. It is our opinion that the 

 number of San Jose" Scale killed by agencies other than insecticides, 

 at the end of an ordinary winter, may reach from 50 to 75 per cent, 

 of all these pests on the tree. This being the case, it can easily 

 be seen what a great difference this would make in our final con- 

 clusions, based only upon the percentage found dead when inspecting 

 after spraying. It was our plan to calculate accurately the percent- 

 age of scale insects killed by each insecticide. To do this we found 

 the percentage dead previous to spraying on one tree of every lot 

 treated with each insecticide. By taking the differences of percent- 

 ages before and after the treatment it would seem to give the 

 desired accurate results. Although in theory this method should 

 be a proper means of calculating the percentage of dead scale insects, 

 it will not work out in practice. 



After second thought, the obvious impossibility of making any 

 accurate percentage of either dead or living scales within one or 

 two per cent, will be seen. At the best, such a calculation is 

 only an approximation. We must bear in mind that no two un- 

 treated trees, nor any two untreated branches of the same tree are 

 likely to have the same percentage of dead or living scales, and the 

 same may be said of any two parts of any one branch. We have fre- 

 quently examined trees on one branch of which not over 50 per 

 cent, dead scales could be found, on an average, while on another 

 over 50 per cent, were found. In the same way, we have often 

 examined sprayed twigs on which there were spots of several 

 square inches area with not over 1 per cent, of the scales dead, 

 while within 6 inches, 90 per cent, were dead. These facts should 

 enter into the final calculations of percentages of living and dead 



