﻿6 BULLETIN 1371^ U. S. DEPAETMENT OF AGRICULTURE 



record trees.* These were placed under a binocular, the scale cov- 

 ering was lifted with a needle, and the scale examined to determine 

 whether it was living or dead. Two thousand scales were counted 

 from each plat, except from those used in the 1921 experiments in 

 Indiana, where only 1,000 were counted. In this examination only 

 the hibernating females were recorded, the last season's dead scales 

 and the young scales, which would not in any case survive the 

 winter, being disregarded. These counts were made approximately 

 one month after treatment.® 



In addition to the examination of the hibernating scale to deter- 

 mine the percentage actually killed by the spray, a careful count of 

 the newly settled scale on 80 linear inches oi new wood was made. 

 This was done for the first and second generations on peach trees and 

 for the first generation on apple. These records show the continued 

 effect of the dormant spray as well as the value of the treatment in 

 preventing reinfestation of the treated trees. 



For this purpose a number of twigs with a total length of 80 linear 

 inches were selected from each plat, care being taken to secure repre- 

 sentative twigs from branches with approximately the same original 

 infestation. The number of newly settled scales on these 80 inches of 

 wood was counted and the average number per inch taken as a meas- 

 ure of the reinfestation resulting from the scale not killed by the 

 dormant spray. 



PERCENTAGE OF CONTROL 



In computing the actual value of a given treatment against an 

 insect where it is possible to make an accurate count of the living and 

 dead individuals and no satisfactory data can be obtained to show the 

 effect of the infestation on the host plant or the crop produced, it is 

 necessary to take into account the number of dead insects in the 

 untreated check. In the following experiments the figure designated 

 ''percentage of control' ' is obtained as follows: 



Let X = per cent living in the untreated check. 

 Let Y = per cent living in the treated plat. 



ThenX— Y = per cent actuall}^ killed by the treatment. 



The ratio of the percentage actually killed by the treatment 

 {X —Y) to the percentage living in the check {X) will give the actual 



X —Y 

 efficiency of the spray, or — ^ — ^ 100 = per cent control. 



DRY CALCIUM-SULPHURS 



These so-called dry lime-sulphurs, which are sold as substitutes for 

 liquid lime-sulphur solutigns, are of comparatively recent develop- 

 ment, and their sale has reached large proportions. The great 

 quantity of water in lime-sulphur solutions makes this product 

 objectionable as a commercial preparation from the standpoint of 

 packing and shipping. Many efforts have been made to eliminate this 

 feature and to obtain a lime-sulphur product in dry form that is at the 

 same time susceptible to ready solution. Several ''dry" methods of 

 preparation have been tried, but in general the method of manufacture 



8 A. L. Melander (19) has shown that an examination made at this time gives the most rehable index of 

 the efficiency of a dormant spray against the San Jose scale. 



