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(2) Soft soap and quassia for hop aphis ; soft soap alone or with 

 quassia extract for aphids generally. (3) Sulphur in some form for 

 red spider. (4) Washes containing free caustic alkali for cleaning tree 

 trunks in winter. (5) Paraffin in very weak emulsion with soft soap 

 or paraffin stirred up in water for garden pests. (6) Lime, salt and 

 sulphur in vague proportions for apple-sucker, etc. (7) Lime washing 

 for winter use. (8) Nicotine as a general panacea for all pests, as a 

 stomach poison for caterpillars, as a contact poison for apple-sucker, 

 aphis, etc. 



These are probably all used as the result of tradition and purely 

 empirical practice, with experience on a large scale as the deciding 

 factor. It is pointed out that even the so-called experience is of little 

 value in the case of many-brooded or migratiiig insects, such as aphids ; 

 plums are sprayed in June against the leaf-curling aphis and when, 

 a week later, none are found, the spray has the credit, whereas the 

 aphids have really only migrated. It is not known how paraffin, soft 

 soap or quassia act on the insects against which they are used, and 

 probably much of the good done is more or less accidental and a better 

 and correct knowledge of how and why these substances kill would 

 assist the discovery of better and more certain insecticides. The 

 spreading over and wetting of the plants by the insecticide is generally 

 necessary, and this is practically a question of surface tension and the 

 problem may be stated thus : — When a solid body (the leaf) is in 

 contact with two fluids (air, wash), if the tension of the surface separa- 

 ting the solid from the second fluid (wash) exceed the sum of the 

 tensions of the other two surfaces, the second fluid will gather itself 

 into a drop and the first fluid spread over the surface. If the reverse, 

 the wash spreads over the whole surface and will wet the solid. As to 

 the action on the insect, there are three points to be considered — 

 mere spreading over mechanically, wetting with spreading, toxic 

 action after wetting. In the first case, of w^hich lime-wash is an 

 example, the cause of death is probably suffocation. In the second 

 case, the question is whether the whole insect is wetted or only parts 

 of it, or there may be a film of liquid lying over the whole insect 

 but not actually in contact w^ith it. True wetting should mean that 

 the liquid is at least in contact with the spiracles. In the third case, 

 there is further a toxic action, but when an insecticide spreads, wets 

 and penetrates into the insect, it is not really known what happens. 

 Experiments were made with meal-worms with the following results : — 

 No result is obtained by applynig any liquid for a short time to the 

 skin, but many fluids act at once if applied to a single spiracle ; inter- 

 ference with the mechanical functions of at least five pairs of spiracles 

 is necessary to produce any symptoms due solely to mechanical 

 interference ; closing all spiracles produces a condition that may be 

 called " rigor," in which a liquid will enter the spiracles ; it is impossible 

 to cause a liquid to enter one spiracle unless this condition is produced ; 

 dipping a meal-worm into a liquid produces the condition in which 

 the liquid enters. We can therefore compare the eftects of liquids by 

 this means. Of liquids tested, the following results were obtained : — 

 Killed all : clove oil, xylol, turpentine, nitro-benzene, chloroform, 

 amyl acetate, cymene, pseudocumine. Killed some : quinoline, 

 carbolic acid, formol 4 per cent., pyridine, acetic acid, eucalyptus oil, 

 methyl sahcylate, aniline, acetic ester. Killed none : picric acid in 



