INSECTICIDES, FUNGICIDES, ETC. 155 



it appeared almost superfluous to examine the effect of such a 

 spraying on them while still in leaf : yet observations were made 

 on the subject, and proved to be of some interest. As in the 

 previous case, the object was to ascertain whether under any 

 circumstances such a treatment might be applied for the destruc- 

 tion of woolly aphis. 



The oils were applied to the trees (apple trees) by means of a 

 pneumatic sprayer giving a very fine mist. The general results 

 were, that the heaviest and lightest oils used both did much dam- 

 age, but that those of intermediate volatility the lighting oils 

 were comparatively -innocuous. With the heavier oils, such as 

 solar distillate, the damage was not noticeable at first, and 

 did not attain a maximum till five days later, when the leaves 

 and some of the shoots were found to be killed, the other shoots 

 being flabby, though these subsequently recovered, and gave 

 rise to fresh leaves. With the lightest oils (petrol, etc.) the 

 maximum damage over 90 per cent, of the leaf surface being 

 affected was noticeable within a few hours of the spraying, and 

 remained at this maximum for a month, after which the shoots 

 recovered, and new growth started. It seemed probable that in 

 this case much of the damage was attributable to the low tempera- 

 ture resulting from the evaporation of the volatile oil, for, with 

 oils of intermediate volatility (lighting oils) no damage was 

 visible till from five to ten days after the spraying, and the 

 maximum reached only after one to two months did not 

 entail injury to more than one-quarter or one-third of the whole 

 leaf -surface (X, 13). 



It is, therefore, quite possible to spray trees in leaf with a 

 lighting oil without any serious consequences ; but, at the same 

 time, such treatment should be regarded in the light of an heroic 

 remedy, which it would be well to apply only to trees which are 

 of no great value. 



One further point of interest was, that it is better to apply the 

 oil undiluted than diluted in the form of a strong emulsion. 

 To be effective in killing woolly aphis, the emulsion would have 

 to contain 50 per cent, of oil, but such an emulsion was found 

 to do more damage to the trees than undiluted paraffin. A 

 similar conclusion was deduced from experiments on the black 

 currant gall mite (p. 215). Doubtless the soap and water in the 

 emulsion retarded the evaporation of the oil, so that it had a 

 longer time during which to act on the foliage (VIII, 104). 



In the treatment of nursery stock with paraffin, the injury was 

 much more noticeable on the scions than on the stocks, indicating 



