28 SEVENTEENTH Report STATE ENTOMOLOGIST OF MINNESOTA—1918 
COMPARATIVE Cost 
The cost of an arsenical cannot always be accurately judged by 
its price per pound since the different materials used as insecticides 
do not contain the same proportion of arsenic. Table II shows the 
approximate proportion of arsenic contained in the four poisons treated 
in this article. 
PA BEE LT 
Arsenic content in 
terms of AS, OF 
Per cent 
(Cailontiine ranehioe a petomniaes Ae, alo GEOR Bee ob boob er muon 42-52 
Keacieatseiaceye es hehe cr eo eee ne Sine ct eos horeka tc Jake 30 
ZAC MANS EMIT ROE, HER Mote be Pe SO ne eee one 40 
PaAriSe SHEET swans oor oo J onion a Ln 58 
From this table it is seen that paris green contains almost twice 
as much arsenic as lead arsenate. Thus it follows that to do the same 
work, lead arsenate must be used at almost double the strength of paris 
green. If paris green sells at 70 cents a pound and lead arsenate at 
35 cents, the cost of the arsenic would be about the same in each case, 
altho the arsenic in paris green would not be worth as much as that 
in lead arsenate, owing to the difference in power of adhesion. The 
same comparison may be made between lead arsenate and calcium 
arsenate or zinc arsenite. 
Since calcium arsenate usually costs the farmer about 5 cents 
less per pound than lead arsenate, it is by far the cheapest arsenical on 
the market. When its high power of adherence is also considered 
it stands far above the other materials for potato spraying. 
There is, however, one very serious objection to the use of calcium 
arsenate for general spraying. Calcium arsenate as sold for insecticidal 
use has a tendency to burn tender foliage, due largely to soluble arsenic 
contained in it. This difficulty will undoubtedly be overcome even- 
tually. It is perfectly safe, however, to use calcium arsenate on potato 
vines, since there is not so much danger of burning as with paris 
green. 
CoNTACT INSECTICIDES 
The only contact insecticides used in this series of experiments 
were nicotine compounds applied both with and without soap. These 
were applied July 9, when the leaf hopper nymphs were most abun- 
dant, since it was thought that at this time they would be more easily 
hit than when in the adult stage. The results of the field experiments 
were not by any means conclusive in themselves, but when considered 
