INSECTS 223 



case it is in the nature of a spring, and not a winter treatment. 

 Further experiments with lime washes under such conditions were 

 not made, but other forms of spring treatment were examined, 

 branches of trees being treated in situ on April 24, May 6 and 

 May 20 the trees were in full bloom on this last date and 

 the results observed, both on branches left on the trees, and on 

 other branches which had been removed and kept in jars. The 

 results were, that of the spray-fluids tried, a 2 per cent, decoction 

 of tobacco proved the most effective, causing a mortality of 

 nearly 100 per cent, in the applications on the first two above- 

 mentioned dates : a I per cent, solution, with 0*33 per cent, 

 of lead arsenate added to it, was considerably less effective, 

 especially in the case of the twigs left on the trees, and the 

 addition of I per cent, of solar distillate to this mixture, did not 

 much improve its action (VIII, 58). 



This is not the only case in which tobacco was found to be 

 highly satisfactory as an insecticide ; but, unfortunately, tobaccos 

 vary enormously in the percentage of nicotine which they con- 

 tain, and consequently in their effectiveness. The same applies 

 to what is known as tobacco powder, which is in use for the 

 treatment of hops, and which contains 68 per cent, of tobacco 

 mixed with sulphur, asafcetida and sago flower (the powders 

 used for fumigation and sheep dipping have an entirely different 

 composition) ; it is preferable, therefore, to use a solution of 

 nicotine of known strength, and such solutions are now prepared 

 by dealers in insecticides. 



In 1908 further experiments were made at Woburn (X, 26) 

 to determine the strength of nicotine solutions which should 

 be used for psylla and other pests. The strengths examined 

 varied from 0-113 down to 0-038 per cent., and three series 

 were undertaken, the trees being sprayed, in one case on 

 May 7, before the blossom opened, in another on May 17, 

 when the blossoms were just open, and in a third on May 26, 

 when they had nearly fallen. The effect in the case of the 

 latest spraying was not quite so good as that in the earlier 

 ones, but all three series indicated that a mortality of from 90 

 to 99 per cent, would be obtained by using a solution of 0*075 

 per cent, strength. The addition of lead arsenate to the nicotine 

 produced no enhancement of the effect; and treatment with 

 i per cent, carbolic acid, with paraffin emulsion, or with salt 

 solutions on May 7 were quite ineffective. 



In the case of nicotine, it was found that moderate rain falling, 

 even within a few hours of the application, did not reduce the 



