Appendix. 171 



ADDITIONAL INSECTICIDE EXPERIMENTS FOR THE SAN JOSE 



SCALE. 

 By Prof. S. A. Forbes, State Entomologist, Urbana, Ills. 



The last observations reported in bulletin 71 of the Illinois Agricultural 

 Experiment Station were made March 25, at which time it now appears 

 that the insecticide effect of the experimental applications made to trees 

 infested by the San Jose scale was not yet complete, or, at least not yet 

 fully manifest. At the above mentioned date there were found on trees 

 which had been treated with the Oregon and California washes twenty and 

 twenty-two days previously, living scales in numbers varying from six 

 to thirty-one per cent of those alive in the beginning for trees treated with 

 the California wash and from one to seventeen per cent for those treated 

 with the Oregon wash (see tables in bulletin 71). 



A careful examination of these experimental trees, made by Mr. E. S. 

 G. Titus May 12, and a systematic count of dead and living scales showed 

 that by that time extremely few scales remained alive on any of these 

 trees. Five hundred young scales of the preceding year were critically 

 examined on each of eighteen hundred trees — nine thousand scales in all 

 — care being taken to choose lots from all parts of the tree up to the 

 terminal twigs. Of these nine thousand scales, only thirty-five were 

 living, the ratio of living to dead varying from none at all to*a maximum 

 of one per cent — as near complete destruction as any field operation is 

 likely to accomplish. 



One half the trees on which these counts were made had been sprayed 

 with the California wash and the other half with the Oregon wash; and 

 sixteen of them were chosen in pairs such that the only difference between 

 the treatment of the trees of each pair was the difference in the insecticide 

 applied. Comparison shows that at the time these counts were made all 

 the difference of effect between the two insecticides had disappeared, one 

 proving finally as efficient as the other. 



These eighteen trees were so selected as to represent ten variations of 

 treatment with water, ranging from daily spraying for seven days in 

 succession beginning with the next day after the insecticide treat- 

 ment, to simple applications of water after an interval so long as to 

 have practically no effect. On a comparison of the reports concerning the 

 different lots of these trees, I find no evidence that variations with the 

 water treatment made any difference with the final effect of the insecticide. 

 The destruction of the insects was retarded in some cases by frequent and 

 early wettings, as shown in bulletin 71, but was practically complete in 

 every instance before May 12. 



By those who have read bulletin 71 in will be remembered that 

 large percentages of the young scales of the preceding year were 

 dead at the beginning of the experiment in consequence apparently of the 

 character of the weather of the preceding summer. If allowance is made 

 for this fact and the ratio of living to dead scales is reckoned with refer- 



