May 12.1923 Injury to Foliage by Arsenical Spray Mixtures 
505 
the market is not either of the above forms, but is a double salt of a com¬ 
plex nature. For the purpose of this paper the lead arsenate will be desig¬ 
nated as one of the above forms, or a mixture containing the two forms. 
USE AS AN insecticide 
Arsenate of lead was first used as an insecticide during the summer of 
1893. In the early work of eradication of the gipsy moth it became 
evident that the known arsenicals could not be used at the requisite 
strength for killing gipsy moth caterpillars without serious injury to the 
foliage. Arsenate of soda was suggested as a substitute, but when used it 
burned the foliage to a greater extent than the other arsenicals. Mr. F. C. 
Moulton, who was experimenting upon these insecticides during the 
winter of 1892-93, proposed acetate of lead to precipitate arsenate of 
soda and in this way obviate the burning of the foliage caused by the 
latter; and by this method the first arsenate of lead was produced for 
insecticidal purposes. Arsenate of lead has the following advantages: 
(i) On hardy foliage it can be used at almost any desired strength without 
serious injury, (2) it is visible whenever used, and (3) it a^eres well. 
The principal objection is its rather slow killing properties, probably due 
to its comparatively low arsenic content and slow solubility. 
INJURY TO EOUAGE 
When lead arsenate was first used it was thought to possess all the 
necessary qualifications for an ideal insecticide. It has proved of great 
value and is used very extensively on apple and other more hardy 
foliage, but reports from its use on peach are variable, sometimes no 
injury being reported and in other cases the injury being so severe as to 
defoliate the trees. Femald {8y 9) states that— 
it [arsenate of lead] can be used in large proportions, if necessary, even up to 25 pounds 
to 150 gallons of water, without injury to the foliage. ... It does not injure the 
foliage of the most delicate plants, even when used in as large a proportion as 25 
pounds, or even more, to 150 gallons of water. * 
Smith {26, p. 4S7, 2j, p. 8) states: 
Its great advantage is its harmlessness to plant life of all kinds ... it is absolutely 
harmless to foliage at any strength . . . It is the only effective poison of this character 
that can be safely applied to peach foliage and on conifers. 
Many other statements similar to the foregoing are available to show 
that many considered lead arsenate as almost an ideal insecticide. 
However, other investigators conducting careful experiments occasionally 
reported serious injury. Haywood and McDonnell {12) say: 
Rather severe injury was caused to the foliage and fruit of the peach by pure lead 
arsenate, made either from lead acetate or lead nitrate. 
Woodworth {36) states, in speaking of spraying in the Pajaro Valley 
with lead arsenate, that: 
While the codling moth was well controlled, the amount of burning was so large 
that the progress of spraying was entirely checked. . . . The most significant dis¬ 
covery of the year 1906 was that where a lead arsenate was so compounded that all 
the arsenic acid present was combined with lead no injury was produced on the most 
delicate foliage. Such a compound is known as the neutral or ortho-arsenate of lead. 
At that time no manufacturer was able or willing to produce an arsenate of lead of 
this description, and to this day, excepting the product manufactured here at Watson¬ 
ville, there is no strictly neutral lead arsenate on the market. 
These statements serve as illustrations that foliage injury is not due 
directly and entirely to the arsenical but is greatly influenced by other 
