INSECTS 201 



air during treatment, though protected from the rain. One 

 series of experiments was conducted in February and March 1905, 

 whilst eleven other series extended from November 1905 to 

 May 1906. 



The effect of the application was measured by examining at 

 intervals under the microscope 20 to 80 scales from each branch, 

 and ascertaining the percentage of eggs killed. The change 

 in the appearance of the eggs when affected is not difficult to 

 recognise : from being white and opaque, they become trans- 

 parent, and, as the action proceeds, turn brown, and become 

 shrivelled. So far as could be ascertained, the change showed 

 the same characteristics in all cases where any action occurred. 

 The effect of an insecticide on the eggs is easily distinguishable 

 from that of the parasitic chalcid when it attacks them, for this 

 insect confines its attention to one egg at a time, and sucks all 

 the contents out of it, till nothing but the empty egg-sack is 

 left. 



The efficacy of an insecticide for scale must depend conjointly 

 on its virulence in destroying the eggs, and on its power of getting 

 at them, by soaking under the scales, for the substance of the 

 scale itself is most resistent, not being destroyed by even a 10 per 

 cent, solution of caustic soda. Various experiments were made 

 to ascertain whether, in consequence of the branches having been 

 severed from the trees, any drying up of them occurred, sufficient 

 to. affect either the adherence of the scales to the bark, or the 

 vitality of the eggs : but the results were satisfactorily negative 



(VI, 34). 



As a rule, though with sundry exceptions, the effect of the 

 insecticide was complete within 14 to 21 days. In the spraying 

 of trees in the open the action might be more prolonged, owing 

 to it recrudescing after a fall of rain (see p. 209), but, as the 

 application of the insecticide by means of a spra}dng machine 

 would be less perfect than that by means of a paint-brush in 

 the laboratory experiments, the sum total of the effect could 

 not be assumed to be the greater with the growing trees. 



In the first place, it was ascertained that the destructive action 

 of caustic alkalis when these were used alone, and in solutions 

 of moderate strength, was very small. Out of 37 cases where 

 they were tried, at strengths ranging up to 2 per cent, of caustic 

 soda or its equivalent of caustic potash, no mortality was 

 observed, except in three instances, and then it amounted to 

 only 2 per cent. Even with three or four dressings, and with the 



