226 SCIENCE AND FRUIT GROWING 



the percentage mortality must be taken as merely comparable 

 inter se under the particular conditions of the experiment. 



Substance. Mortality. 



Alcohol ". . . . .96 



Tobacco . . . . -85 



Lead arsenate . . . -75 



Ivy extract . . . .28 



Quassia . . . . -27 



Hellebore . . . . .20 



Hyoscyamus vir. . . .19 



Nux vomica . . . . 17 



Aconite . . . . . 15 



Quassia (Bentley) ... 8 



Hyoscyamus fol. . . .6 



Thus, with the exception of alcohol, the only substances 

 giving satisfactory results were tobacco and lead arsenate, and 

 it is noticeable that the ineffective substances included many 

 which act as virulent poisons towards animals. It must, of 

 course, be remembered that the sensitiveness of a caterpillar to 

 poisons must depend very much on the age of the insect, and 

 in these experiments the caterpillars were in the adult stage. 



In the following season a further examination was made of 

 those substances in the above list which had proved most effec- 

 tive, and other experiments were added with pyrethrum, Paris 

 green and paraffin emulsion (containing i per cent, of solar 

 distillate). The general results indicated that an equal degree 

 of effectiveness might be obtained with tobacco, Paris green, 

 lead arsenate, pyrethrum or decoction of ivy leaves, provided 

 the strengths were suitably adjusted ; hellebore gave less satisfac- 

 tory results. The paraffin emulsion caused a higher mortality 

 than that obtained in any other case, but this result was looked 

 upon with suspicion, as it was higher than that caused by the 

 emulsion when used in conjunction with other insecticides: 

 in eight cases various* insecticides had been used alone, and 

 with the emulsion added to them, and this addition had raised 

 their efficiency from an average of 42 to 60 per cent., whereas 

 with the emulsion alone an efficiency of 85 per cent, had been 

 indicated. 



To set this question at rest, a comparison was made in 

 the following season between emulsions and lead arsenate, 

 the experiments in this case being carried out under ordinary 

 spraying conditions. About 300 dwarf apple trees of a large 

 number of different varieties were divided into similar sections, 



