210 SCIENCE AND FRUIT GROWING 



the changes which they themselves undergo when they are 

 sprayed on to trees ? 



As to the changes undergone by the insecticides, but little 

 work has been done, except in the case of lime-sulphur washes 

 in America, 1 and in that of alkaline washes at Woburn (p. 166) : 

 as to the action of the insecticide on the eggs, there are three 

 main possibilities. 



Firstly, the action may be a mechanical one, the insecticide 

 coating the eggs so as to deprive them of such access of air and 

 moisture as may be necessary for their development. This is 

 not a very acceptable explanation, as the eggs are in a dormant 

 condition, and even the drying up of the infested twigs produces 

 no visible effect on them. The glueing of the scales to the 

 branches is another form of mechanical action which certainly 

 is operative in one case. 



Secondly, the action may be of a physical character, the 

 watery contents of the egg-cells being drawn out by the insec- 

 ticide, through plasmolytic action. The mussel scale eggs when 

 immersed in brine, become shrivelled up and killed in this 

 way. But most of the insecticides used are too weak to effect 

 appreciable plasmolysis, and some of them, such as paraffin, 

 would not produce plasmolysis at all. 



Thirdly, the action may be purely chemical, the insecticide 

 destroying the cell- walls, and gaining an entry into the cells. 

 This is, perhaps, the best explanation, but one which is not free 

 from difficulties. It harmonises with the alteration in colour 

 observed in the eggs when they have been attacked (see p. 201), 

 and it is an action which substances such as the caustic alkalis, 

 paraffin or sulphuric acid are capable of causing. On the other 

 hand, it does not remove the difficulty of explaining why 

 the action of caustic alkalis in aqueous solution is so feeble in 

 comparison with their action when a certain amount of soap, 

 or other substances, is present with them ; nor does it explain 

 altogether how the action becomes so much enhanced by sub- 

 sequent wetting. It can only be suggested that moisture, 

 being necessary for the chemical action, the presence of soap, 

 glycerin, etc., helps to attract or retain this moisture, and so 

 prolongs the action till the cells are effectively destroyed ; whilst 

 in the case of both those substances, as well as of paraffin, the 

 penetrative power of the wash is enhanced by their presence 

 (see p. 208). 



1 H?ywood, Jonrn. Amer. Chem. Soc., 27, 3, 224; Smith, New Jersey 

 State Bull., 178, 8. 



