192 JOHN D. TOTHILL. 



it is generally considered to-day. Writing in 1856 (First and Second Keports) Fitch 

 says " The bark-louse is on the whole the most pernicious and destructive to the apple 

 tree at the present time of any insect in our country. Everywhere throughout the 

 northern States it is infesting the orchards to a grievous extent," and again (I.e.) 

 " In those districts bordering upon Lake Michigan in particular it is at the present 

 time making the most appalling havoc, surpassing anything which has hitherto 

 been reported of this species." This seeming difference may be due to an absence 

 of Hemisarcoptes on this continent for many years after the introduction of the scale. 

 Harris (" Insects Injurious to Vegetation ") says " The first account that we have 

 of the occurrence of bark-lice on apple trees in this country is a communication by 

 Enoch Perley of Bridgetown, Maine, written in 1794." As the mite was not discovered 

 here until seventy-four years later it seems possible that the mussel scale at first 

 enjoyed a mite-less regime on this continent. 



Control in Postembryonic or Summer Stages. 



Weather. 

 After the eggs begin to hatch inclement weather is probably the greatest enemy 

 until the insects come to be covered with a protecting roof. Regarding this period 

 Franklin Sherman, Jr. (N.C. Dept. of Agr. Bull. no. 185) says " We believe that 

 heavy showers at the time when the tiny young are crawling wash off hundreds of 

 them and leave them to perish on the ground. Sudden gusts of wind must blow 

 away many more, and though by this means a few may find lodgment on other trees 

 . . . still the majority which happen to be blown off the trees must inevitably perish. 

 But . . . the extent to which the weather aids us is uncertain." 



Overcrowding. 

 It sometimes happens that the scales become so numerous on a tree that over- 

 crowding and starvation take place ; this seems to act as a very important check in 



Fig. 5. Showing over-crowding of oyster-shell 

 scales on an apple twig ; the full-sized £ is 

 surrounded by a number of dwarfed $ $, some of 

 which did not live to lay eggs and none of which 

 laid more than a fraction of the normal quota. 



Canada. Many cases have been found in which practically all the 1916 females on a 

 twig died from this cause before egg-laying had begun. In one case in which there was- 



