INSECTS 209 



preferably painting it on with a hard brush, when the insects 

 are emerging from the scales (VI, 123). 



The effect of rain after the application of an insecticide was 

 examined by comparing the results obtained with two sets of 

 branches infested with mussel scale, both of which had been 

 painted with various insecticides, but in one case without, and 

 in the other case with a wetting of water, this being applied 

 by means of a scent sprayer, a few days later. The results 

 showed that the wetting had no effect in all those cases where 

 the wash was weak or ineffectual, and had left no appreciable 

 amount of substance on the bark, whereas with stronger washes, 

 which, had already much damaged the eggs, that damage was 

 materially increased by the subsequent wetting, the extra dose 

 of insecticide thus reaching the eggs, though probably very 

 small, being sufficient to kill many eggs which had already 

 been injured by the original application. This was very notice- 

 able in cases where the washes contained much soap or non- 

 volatile paraffins (VI, 106). The manner in which rain, falling 

 after the spraying, enhances the action of an alkali, has already 

 been explained (p. 166). 



These experiments on scale brought into prominence some 

 of the conditions on which the successful action of insecticides 

 depends : namely, a sufficient prolongation of the action, and 

 the cumulation of independent actions which, by themselves, 

 might each be inoperative. The effect of rain after spraying is, 

 as just stated, due to insecticidal action being prolonged, and 

 other instances leading to the same conclusion may be quoted: 

 thus, three applications of the same weak insecticide caused far 

 more than three times the mortality that one application did, and 

 the effectiveness or non-effectiveness of various paraffins was, 

 also, entirely a question of the duration of the action, as deter- 

 mined by the volatility of the particular paraffin (p. 153). 



The cumulative effect of insecticides which, separately, have 

 but little action, was demonstrated fully by the action of soap- 

 soda solutions, and emulsion-soda washes, as compared with soap 

 alone, soda alone, and emulsions alone. Even an emulsion-soda 

 wash may be thoroughly effective when made with a paraffin 

 which by itself is comparatively inoperative, though the emulsion 

 contains only 6 per cent, of that paraffin, together with a per- 

 centage of soap and soda, neither of which alone are capable of 

 occasioning an appreciable mortality. 



These facts led to some questions of fundamental importance. 

 Why, and how, do these insecticides destroy the eggs ; what are 

 p 



