INSECTS 227 



one section being sprayed with arsenate (0-4 per cent, of the 

 paste), and two others with emulsion (i per cent, of paraffin), 

 made, in the one case, with copper sulphate, and in the other 

 with iron sulphate. The difficulties in forming any trustworthy 

 estimate of the results in such experiments is considerable : the 

 method adopted was to examine each of the trees, and give it 

 "marks" according to its freedom from caterpillar attack; 

 this " marking " was done by an observer ignorant of the treat- 

 ment applied to the trees ; as a general result of two inspections 

 at different times, a balance of 12 per cent, in favour of the 

 arsenate was obtained. Where copper, instead of iron, had been 

 used in making the emulsion, no advantage had accrued, nor was 

 there any appreciable difference between the trees sprayed before 

 or after the blossoms had opened. 



As a general conclusion, therefore, lead arsenate would seem 

 to be the most serviceable insecticide for dealing with cater- 

 pillars. Though tobacco, Paris green or pyre thrum might yield 

 equally good, or even better results, there are objections to all 

 three of them ; tobacco is a very expensive remedy, and the 

 results obtained with it in practical spraying for caterpillars have 

 not borne out the anticipations based on laboratory experi- 

 ments (see p. 226), Paris green is very poisonous and liable to 

 cause serious scorching, and pyrethrum varies greatly in strength, 

 and deteriorates on keeping. 



Lead arsenate, however, has its drawbacks, for it also is poison- 

 ous, and should not be used on trees within five or six weeks of 

 the fruit being gathered. This generally renders it impracticable 

 for the treatment of gooseberries or currants for sawfly, and 

 a similar objection applies to treatment with hellebore. In 

 that case paraffin emulsion (made with iron or soap, and not 

 with copper) , is a valuable substitute for arsenate ; indeed 

 when the percentage of solar distillate in it is increased to 1-5 

 per cent. as it was found possible to do without any risk of damage 

 to the foliage it proved to be even more efficacious, or, at any 

 rate, more prompt in its action, than lead arsenate. An attack 

 of gooseberry sawfly was thus dealt with, and on the second day 

 after the spraying no live caterpillars could be found; whereas 

 in the case of similar bushes treated with arsenate, there were 

 still a few which had not succumbed. The leaves of the bushes 

 sprayed with the iron emulsion were somewhat stained, but the 

 fruit was unaffected, and exhibited no flavour of paraffin ; some 

 of it, indeed, was marketed within two or three days of the 

 spraying (X, 41). 



