190 JOHN D. TOTHILL. 



conditions at Havelock are typical for the four places ; in this case, of the hundred 

 new scales turned over, nine still had a full complement of healthy eggs, and of ninety- 

 one that were infested with Hemisarcoptes only four contained a few healthy eggs. 



In these two districts the scale has been plentiful and the mites have had an 

 abundant food supply. With these favourable conditions the mites have evidently 

 increased until the scale has been almost completely destroyed. The conditions 

 at many other places, however, were found to be quite different. In the Arboretum 

 at Ottawa the scale is abundant and the mite is rather less so than at Moncton 

 and the group of places in Huntingdon County (vide table). On Mount Royal, 

 Quebec, the scale is also abundant and the mite still less abundant than at Ottawa. 

 At the end of this series comes Chipman with the scale exceedingly scarce and difficult 

 to find and with apparently no mites whatever. 



Such a series of conditions seems capable of interpretation by supposing that when 

 the scale is thick the optimum conditions for the mite prevail and that the mite 

 then increases until finally, as at Moncton, it practically exhausts its food supply. 

 The conditions in the Ottawa Arboretum and at Mount Royal would be transition 

 stages, in which the mite is gradually increasing in proportion to its host. Chipman 



Fig. 4. Au apple twig showing extermination of oyster-shell scale by the mite 



(Hemisarcoptes) ; the old (1915) wood is still covered with dead scales, which 



were killed between September 1915 and April 1916, none being left to infest the 



1916 or 1917 wood. (Original.) 



has probably been all but freed of scale in this way, as my assistant, Mr. A. B. Baird, 

 reports that it was abundant there only a few years ago and that the elimination has 

 come about without spraying. 



Such an interpretation would explain also the almost complete absence of the scale 

 at Campbellton, N.B., and along the St. John River in Madawaska and Victoria 

 Counties, where its food-plants are somewhat abundant and where very little spraying 

 has been done. It would also explain the present scarcity of scale on some derelict 

 apple trees close to this laboratory that four years ago were heavily infested. The 

 curious eliminations of oyster-shell noted by L. Caesar may have been due to an attack 

 by Hemisarcoptes ; he says (Ont. Agric. Coll. Bull. no. 209, 1914) " I have on several 

 occasions seen trees that were badly infested throw off the scales in some inexplicable 

 manner and take, as it were, a new lease of life." It at least seems certain that when 

 the scale is abundant this mite is the most important single factor operating towards 

 control in eastern Canada. In places where the host is less abundant the mite becomes 

 proportionately less efficient. 



Before concluding this part of the discussion it may be pointed out that Fitch 

 and his contemporaries seemed to regard this scale as a much more serious pest than 



