22 The European Elm Scale 



plete extermination of any scale insect is something not to be hoped 

 for will agree that on the whole these methods were thoroughly suc- 

 cessful ; for every elm treated made a vigorous growth all through the 

 following summer and showed no further signs of injury due to the 

 insects; while elms on the same street which had not been treated 

 were still badly infested, sickly and unsightly. Out of all the treated 

 elms very few were sufficiently infested with winter larvae in the 

 winter following to require further treatment. The few which did re- 

 quire further treatment at that time were those which were sprayed 

 with lime-sulphur in February, 1907, but which were not washed down 

 with the garden hose early in the summer following. 



In 1907 spraying alone proved highly beneficial to the elms in Lots 

 V and VI. Spraying followed by washing proved much more bene- 

 ficial to those in Lots I, II, III and IV. Thus, in general, the result 

 of the methods employed was good; still, a closer examination of the 

 details give rise to many questions. "We naturally ask: 



(1). How does the lime-sulphur cause the death of those larvae 

 which it kills? 



1,2). "Why is its action so slow and how long is it active? 



(3). How is the lime-sulphur affected by rain and snow after it 

 is applied to the tree and how does this affect its power as an insecti- 

 cide? 



In answer to these questions we will say that the lime-sulphur is a 

 very complex combination of sulphur salts of calcium when applied to 

 the tree, that after its application in contact with air and moisture it 

 undergoes a long, slow process of chemical change lasting for weeks, 

 perhaps for months, during which time it is injurious to some forms 

 of insect life in ways not yet fully understood. The efficiency of the 

 wash is reduced by rains following soon after its application to the 

 -tree. For a further discussion of the chemical nature of the lime- 

 sulphur wash and the changes which take place after its application, 

 see Bulletin No. 101, Bureau of Chemistry, U. S. Department of Agri- 

 culture, Haywood. 



These considerations lead to a final series of questions which are 

 of the utmost significance: 



(1). The good result attained in 1907 was due to two things, 

 spraying and washing. "Which was the more effective ? 



(2). Which would have killed more if the elms had been washed 

 first with the hard stream of water and then sprayed ? 



(3). "What is the use of spraying the elms at all? "Why not let 

 them go until spring and then simply trim them and wash down the 

 scale from the bark ? 



