46 APPLIED ENTOMOLOGY 



this long before it is to be used, nor to mix more than will be used the 

 same day. 



Although the addition of lime to the Paris green reduces the danger 

 of injuring foliage, some plants even then, are liable to be burned some- 

 what. Accordingly, the amount of the poison to use per barrel will vary. 

 Thus, for potatoes the Paris green can usually be increased to % Ib. per 

 barrel, while for the peach it is not safe to use more than ^ Ib. It 

 should not be used on evergreens. 



Applied as a dust it is usually thoroughly mixed with flour, plaster or 

 air-slaked lime, in about the proportion of 1 part of the Paris green to 

 from 6 to 10 parts of the other, by weight. 



Paris green is unsafe to apply on stone fruit foliage, and because of the 

 danger of burning in general, it is now less used than was once the case, 

 arsenate of lead having largely replaced it as a spray. 



Arsenate of Lead. The value of this material as a spray against insects 

 was discovered about 1892 in the course of the work conducted by the 

 State of Massachusetts on the control of the gypsy moth, and it has now 

 been generally adopted as being, under ordinary conditions, the best 

 stomach poison to use. Two forms of it are available: the basic or neu- 

 tral (ortho) arsenate, Pb 3 (AsO 4 )2 and the acid arsenate, PbHAsO 4 . In 

 pure condition the former is about 25 per cent arsenic oxid and the latter 

 about 33 per cent. The latter is the form in most general use, but on the 

 Pacific Coast, because of local conditions, the former appears to be re- 

 garded with more favor. 



Arsenate of lead may be obtained both as a paste and as a powder. 

 By law the paste must not be more than half water, but with about this 

 amount present the percentage of arsenic oxid in it is reduced to from 

 12 or 13 to 19 or 20 per cent. In the powder, water being practically 

 absent, about 32 to 33 per cent is arsenic oxid. As the average in the 

 paste is usually 16 per cent and the price of the powder is about double 

 that of the paste, there is little choice between the two so far as the arsenic 

 is concerned. The Federal law requires that in the paste no more than 

 0.75 per cent of the arsenic oxid shall be soluble in water. 



Either form of arsenate of lead shows well on the foliage, which is 

 useful, enabling the sprayer to see parts he has missed in spraying, and 

 to " touch up" those places. It is also very light, settling slowly in the 

 pump. Under most conditions arsenate of lead does not burn the leaves, 

 being in fact, the safest of the stomach poisons in this regard, and it 

 adheres to the leaves longer than the others (stomach poisons). On 

 the other hand, it acts slowly on insects because of its rather low arsenic 

 content 



As a pound of the paste is approximately one-half water, it is necessary 

 in spraying a given area to use twice as much (by weight) as of the 

 powder, in order to supply equal amounts of the poison. 



