76 
them—a fact which indicates that these plants are less suitable 
than corn to the maintenance and multiplication of these in¬ 
sects. We have also considerable reason to believe that many 
winged plant lice flying about in search of feeding and breeding 
grounds must be destroyed by some of the innumerable acci¬ 
dents to which these feeble and helpless insects are necessarily 
exposed. This measure of rotation may consequently have the 
effect to diminish to an important extent the number of corn 
root lice in later generations. Precise proof on these points is, 
however, very difficult to secure. Artificial breeding experiments 
are altogether too variable in result to serve the purpose, as 
our own attempts at a solution of this question show; and evi¬ 
dence must be sought in the field especially by making detailed 
comparative observations of parts of the same previously in¬ 
fested fields, planted here to corn and there to small grain. 
The relative abundance of the lice late in May and early in June 
Avill go far to show the comparative utility of these crops as a 
food resource to the corn root aphis. 
Applications of Fertilizers and Insecticides —Various field ob¬ 
servations have given us reason to conclude that fertilization 
of the soil will serve to support corn under the drain of aphis 
injury, especially by enabling a stunted plant to rally more 
rapidly and completely after the insects have begun to scatter. 
The rapidity and vigor with which, in rich ground and in a 
fairly favorable season, corn will outgrow an apparently fatal 
injury by the root aphis is, in fact, often quite surprising. 
Apart from this general statement I have only to report the 
result of a single series of plot experiments tried in 1891 with 
various fertilizers mingled with petroleum, crude and re¬ 
fined, as an insecticide, and with applications of salt, wood 
ashes, and lime. 
A plot of ground ten hills square, containing ninety-seven hills 
of corn, was selected on the University experimental farm at 
Urbana June 18, 1891, and surrounded by a continuous line of 
six-inch boards sunk about three inches into the ground, with 
close-fitting joints, and with the earth well tramped both out¬ 
side and in. The upper edge of the boards was thickly covered 
with coal-tar, subsequently kept fresh by repeated applications. 
These measures were intended to prevent all interference with 
the experiment, either by escape of the insects within the plot 
or by invasion from without. June 26 this enclosure was en¬ 
larged to contain twenty-three hills more. Fertilizers, and mix¬ 
tures of fertilizers and petroleum, were applied to the hills of 
corn June 18 and 23, being worked into the soil about each 
hill; and on the 26th lime, ashes, and salt were similarly 
applied. Of the ninety-seven hills in the first enclosure, forty- 
eight were found in the beginning to contain root lice and ants, 
and of the twenty-three hills in the second lot sixteen were simi¬ 
larly infested. 
