PROGRESS REPORT ON THE EUROPEAN CORN BORER 93 
tender tips of the husks and upon the silk, or work their way 
down between the silks into the ear and feed upon the grain and 
cob. 
After the larva has entered the stalk it tunnels upward or down- 
ward. The character of the tunnel is subject to great variation, 
but typically the larva follows a nearly straight course through 
the pith and generally lengthwise of the plant. In some instances 
the tunnel is more or less winding and occasionally small cells 
are excavated along its course. Sometimes the larva also excavates 
a large horizontal chamber either just above or just below the 
entrance hole and starts its tunnel from this chamber. Stalks bear- 
ing this type of tunnel are greatly weakened and soon break over. 
All parts of the stalk may be tunneled down to and including 
the base or stubble. There is a tendency for the larvae to work 
in the internodes of the stalk, but many of the nodes are also perfor- 
ated, especially where several larvae are present in the same stalk. 
Even during the period of active growth of the larva it is ap- 
parent that not all of the plant substance removed by the larva 
during the process of tunneling within the stalk is actually de- 
voured. The larva appears to prefer as food the portions of the 
interior of the stalk which are most succulent and rich in sugar. 
The harder and less nutritious portions of the stalk are merely 
bored out and cast aside. This discarded material, together with 
the excrement of the larva, is pushed out of the entrance hole in 
the form of yellowish-white frass, which later becomes darker in 
color. Much of this frass is held together in masses by silken 
threads spun by the larva and usually hangs suspended below the 
entrance hole or collects below in the axils of the leaf blades and 
upon the ground. Some of the frass, instead of being ejected from 
the tunnel, is packed by the larva into the cells or chambers of 
the tunnel. 
When the larva bores an ear of corn it may enter directly at the 
tip, base, or side of the ear, or indirectly through the short stem by 
which the ear is attached to the stalk. Once inside the ear the larva 
tunnels through all parts of the grain and cob. The actual feeding 
areas on the grain may consist of long irregular surface furrows 
between the rows of kernels, or tunnels just underneath the upper 
surface of the kernels; or large irregular areas may be fed upon 
with no apparent regularity of procedure. The larvae tunnel the 
cob in a similar manner to that described, for the stalk, the tunnels 
extending either longitudinally or transversely through the cob. 
The feeding habits of the larvae when attacking plants other than 
corn are essentially the same as described for corn. 
LARVAL ESTABLISHMENT AND SURVIVAL 
In view of the known fecundity of the P. nubilalis female, as 
exhibited in confinement, and the number of eggs present in fields 
under close observation compared to the relatively small number of 
fully grown larvae which develop subsequently in such fields, it has 
been apparent that there must be a very high egg and larval mor- 
tality from natural causes, since the wide discrepancy between the 
number of eggs present and the number of fully grown larvae devel- 
oping from such eggs could not, in most instances under observation, 
be accounted for by nonfertility, nonhatch, parasitism, disease, pre- 
dators, or other assignable causes. 
