Potato SPRAYING IN MINNESOTA 31 
would keep the arsenical in suspension for a longer time than would 
otherwise be the case, would help to distribute the material over the 
leaves,’ and would possibly improve its sticking power. It is doubt- 
ful, however, if soap could be used with calcium arsenate, owing to the 
excess of lime usually present in that material. 
In the field experiments this season, the spray for leaf hoppers was 
applied when the nymphs were most abundant. It was thought that 
this would be the best time since the insects would then be less able to 
jump and fly ahead of the machine. This spraying would probably 
have proved effective if the under side of the leaves could have been 
thoroly wet with the spray. But, as was pointed out before, this 
was impossible with the machinery used. Delaying spraying until the 
nymphs are most abundant has the decided disadvantage of withhold- 
ing the treatment until a large part of the leaf hopper injury has been 
done. These two facts coupled with observations of leaf hopper 
adults flying into the spray and falling to the ground in a stupified 
condition suggests the possibility of spraying when the adults first 
appear on the vines in the spring and driving them through the spray. 
In summing up the possibilities for satisfactory leaf hopper con- 
trol, it appears that the tobacco extracts when used with soap are 
effective in destroying these insects. The present mechanical methods 
of applying these materials, however, are faulty and future efforts 
should be directed toward improvement along this line. 
4 Stokes law—V=2R2+(S—S’) ¢ 
ON 
V=Velocity of settling. S=Specifie gravity of solid. S’=Speceifie gravity of 
the liquid. R=Radius of particles. g=Gravity. N=Viscosity of liquid. 
