204 BULLETIN NO. 62. 



but in this connection the habit of the larva of eating from the sur- 

 face of the leaves, in some instances, is of great interest. 



The fundamental aim of spraying for this pest is to have the 

 fruit and leaves evenly and thoroughly covered with a compound 

 of arsenic at the time the young larvae are entering and it has been 

 clearly shown through practical experience that this treatment is 

 very effective in reducing the amount of injury. 



ARSENICAL COMPOUNDS FOR THE CODLING MOTH. 



The essential and active agent of arsenical compounds is the 

 arsenic they contain, yet common white arsenic is not suitable for 

 spraying since it is readily soluble in water and hence injurious to 

 the foliage. So far as the foliage is concerned in order that an 

 arsenical compound may not be injurious it should be practically 

 inert. It is therefore necessary to use a compound of arsenic 

 which has a degree of solubility in water so slight that the leaves 

 do not suffer injury and yet which when taken into the body of the 

 insect will be fatal. 



Paris green has been the most popular insecticide for the codling 

 moth, and most excellent results have been secured with its use. 

 It is not entirely free of soluble arsenic and to take up that which is 

 soluble, freshly slaked lime is used. A safe formula for apple trees 

 is as follows : 



Paris green, i pound 

 Lime, i to 2 pounds 

 Water, 150 gallons 



Arsenate of lead is coming into favor as an insecticide for use 

 against this pest, and has some very strong points of advantage over 

 Paris green. The great weakness of Paris green is that being of a 

 granular form, it is easily washed off by showers. The occur- 

 rence of rains in the spring season is sometimes a great interfer- 

 ence with effective spraying. Arsenate of lead when sprayed ad- 

 heres to the leaves and apples as a thin film which is not easily 

 mashed off. In Missoula where rains often occur when 

 the first application of the poison is being made this compound has 

 become highly approved by the officials of the State Board of Hor- 

 ticulture. Moreover, it is to be recommended for our local condi- 

 tions on account of the fact that since the two broods of the moth 



