268 



S. MAUI^lK. 



fctreiijS^li.s on the tre»^s with the usual checks, etc. None of them entirely prevented 

 the insects fi'om hatching out. Besides, the application of caustic alkali is beset 

 with mechanical difficulties and causes much inconvenience to the operators. 



The treatment in vogue at the present day consists in using either of the three 

 washes, viz. :- (1) caustic alkali wash, (2) paraffin enudsion, or (3) the Woburn wash. 

 The ingredients comjiosing these spraying fluids are caustic soda, lime, paraffin, 

 iron sulphate and copper sulphate. Tieating the scale in the test-tube with the 

 above reagents separately, as well as in their combinations forming the washes, it is 

 found that they have no solvent action, except caustic soda. It is on record that 

 these washes have been found useful to a certain extent. This may be attributed to 

 the purely physical action of the paraffin owing to its low surface tension. This 

 propertv enables 'the oil to penetrate minute cracks and crevices, thus wetting the 

 surface well. Insects hatchins out and coming into contact with the oil are killed. 



Fiff. 1. 



Low.'r surface of Lepldosu plies ulnti, L , 

 111, moutli ; .s. sniraoles. 



The paraffin may also get access to the eggs by penetrating under the scale where 

 its contact with the bark has been loosened by weather condition.-^. If in this way 

 the eggs under the scale become thoroughlv soaked with the oil it is very improbal)le 

 that they will hatch. On the other hand, the chances of the oil thoroughly wetting a 

 whole batch of eggs under the scale are verv remote. Thus we find that even after 

 a good spray a large percentage of insects hatch out. 



