74 



sulphur prevents a fixation of the young scales more effectually than 

 a treatment with the Iserosene sprays. We also learn from the fore- 

 going table that Scalecide was somewhat less effective than either 

 form of lime-sulphur used ; and that, of the two latter, Rex dip was 

 about 5 percent more effective than the home-made solution. 



THE TRANSITION ZONE IN INSECTICIDE EXPERIMENTS 



In the Twenty-fourth Report of this office (1908) I called par- 

 ticular attention to a generally neglected factor in field experimenta- 

 tion with insecticides and other means of insect control;* namely, 

 the fact that the effects of insecticide treatment of experimental plots 

 are likely to be diminished and disguised by an invasion of these 

 plots by insects migrating from adjacent untreated, parts of the or- 

 chard or field; and in that paper I illustrated this influence by a 

 comparison of the degrees of infestation of check rows near experi- 

 mental plots and experimental rows near check plots with the degrees 

 of infestation of rows in each kind of plot at some distance from the 

 other. The practical outcome of this comparison was a general rule 

 that large enough plots should always be made in field experiments 

 to give interior parts freedom from the influence of one plot upon 

 another, and that the marginal parts of the plots should not be used 

 in assembling data for comparison. 



This principle is amply illustrated in the results of the orchard 

 "experiments now under discussion. The check rows next the experi- 

 mental plots were less infested by San Jose scale at the end of the 

 season, and the experimental rows nearest the check plots were more 

 infested, than were those at some distance from the line of division. 

 This mutual influence of one plot upon the other varied somewhat 

 with the degree of infestation, the neutral or transition zone of un- 

 available trees being five rows wide in the less-infested orchard and 

 six rows wide in that more infested. This transition zone was due, 

 of course, to the movement of the young insects across the boundary 

 between the plots, the marginal rows of the check losing more than 

 they received from the experimental plot adjoining, and the experi- 

 mental plot reversing the process. Even the outside rows of the check 

 plot show, as might be expected, the same loss of infesting insects, so 

 that only three rows were left available for use in a check plot six rows 

 wide. 



The detailed data are given in the accompanying diagrams, one 

 representing the less-infested orchard and the other the more-infested. 

 The vertical lines on each diagram stand for the orchard rows in two 

 adjacent plots, one the check and the other sprayed with home-made 

 lime-sulphur wash. The heavy line between rows 6 and 7 is the 



*' ' Spraying Apples for the Plum-eurculio, ' ' pp. 94-99. 



