Studies on Eggs of Apple Plant Lice 27 



one species are not alike ; some are less resistant than others, conse- 

 quently some are killed by weaker strengths of insecticides. This 

 is well illustrated in all experiments, particularly in the experi- 

 ments where the percentage of hatch runs between 10 and 25 per 

 cent among treated eggs. 



The percentages of hatch or kill in all the spraying experiments 

 in the greenhouse or out-of-doors at the laboratory for the two sea- 

 sons are figured on the basis of considering the number of eggs 

 present at the time the final observations were made. In a former 

 paper (19) the percentage of hatch was figured on the basis of 

 considering the percentage of hatch in respective checks for each 

 set of experiments as 100 per cent, and then the number of eggs 

 hatched in the various experiments compared with the check. Ex- 

 perience has shown that this method is .subject to considerable 

 variation due to tlie variation in the percentage of hatched eggs 

 in the checks; consequently, this method was abandoned for the 

 above mentioned form. In all the tables the percentage of hatched 

 eggs is given for each experiment and one may obtain the percentage 

 of killed or dead eggs by subtracting the percentage of hatched eggs 

 from 100 per cent. The results given in table 4 will be considered 

 in the various discussions on the ditferent insecticides and chemicals. 



Upon completing the investigations on the aphid eggs for the 

 season of 1916-17, it was observed that the susceptibility of the 

 eggs to various insecticides and differe)it percentages of humidity 

 seemed to be greatest just prior to the emergence of the nymph. 

 With this observation in mind, a large series of spraying experi- 

 ments, starting with November 30, 1917 and continuing until April 

 1, 1918, were conducted out-of-doors at the laboratory for tne 

 purpose of determining the relative susceptibility of the eggs 

 throughout the dormant season. 



Diagram 1, by an intersection of plotted lines with perpen- 

 dicular lines (representing dates of application) shows the results 

 of the more important experiments conducted with the eggs of 

 A. avence. The dates at the top of the chart indicate the time of 

 application of the various sub.stances used. Other applications 

 were made throughout the month of December and January, but 

 the results of the spraying experiments for December 15 are typical 

 of all these. The points of intersection of the various types of 

 plotted lines with the perpendicular date lines indicate the per- 

 centage of hatch if one examines the column of figures to the left, 

 and the percentage of dead eggs if one examines the column of 

 figures to the right. Each treatment or. substance used as a spray 

 is given a definite letter, the explanation of which may be found 

 at the bottom of the chart. The chart also shows at the top the 

 source or orchard where the eggs were collected and on the bottom 



