Potato SPRAYING IN MINNESOTA 25 
sprays. One week after this spraying, it was difficult to find any 
plants on which the beetles were working, except a few at the edges 
toward adjoining infested potato fields. The late potatoes adjoining 
the early variety and a small block of a little more than an acre, des- 
ignated as block No. 2, were also sprayed at this time. The beetles on 
these plots of late potatoes were even less abundant than on the early 
plots and the control was almost perfect in each case. 
The remaining block, designated as No. 3, was not sprayed for 
the first time until June 29. At the time of spraying a few larvae 
had hatched but for the most part, the eggs were still unhatched. 
Adults were abundant. On this block three poisons, paris green, lead 
arsenate, and zinc arsenite were used at the same rates as in block 1 
with almost perfect control in each case. This block was sprayed 
again on July 23, altho the beetles were not numerous enough at 
that time to make the treatment commercially profitable. Those pres- 
ent, however, were well grown and so a heavy dose of two pounds of 
lead arsenate and one pound of paris green per acre was used. This 
spraying almost entirely exterminated the beetles in this field. 
From this series of experiments it is evident that lead arsenate, 
paris green, calcium arsenate and zine arsenite are equally effective 
against the colorado potato beetle under favorable conditions of 
spraying. Since this is true, the factors which should determine 
the most desirable of these insecticides to use in potato spraying 
are power of adherence and cost. Since it is a very difficult mat- 
ter to determine the comparative power of adherence of insecticides 
in the field, this was determined by laboratory experiments. 
PoweErR oF ADHERENCE 
In the determination of the comparative power of adherence of 
arsenicals, considerable difficulty was encountered in devising sat- 
isfactory methods of measurement. Two satisfactory methods 
were finally worked out. Both of these methods were used, each 
serving as a check on the other. The first was as follows. The 
materials were mixed with water as for field spraying and a meas- 
ured amount of each was placed on clean microscopic slides and 
allowed to flow evenly over the entire surface. These slides were 
then dried at room temperature in a dust-proof cabinet for 48 
hours, after which they were carefully weighed. They were then 
fastened in a vertical position and sprayed with water delivered at 
a uniform pressure from a spray nozzle. Each slide was placed in 
the same position and the nozzle was clamped in place so that each 
