THE POTATO TUBER MOTH. 19 
will migrate to another leaf or to the petiole before starting a mine. 
In the case of a tuber the larva generally begins where the egg was 
located, since the irregularity chosen by the moth for oviposition 
affords a favorable location for starting a tunnel. For this reason 
the damage to potatoes first becomes evident in the eyes. The fact 
that the entrance hole is very small and webbed over makes it difficult 
to detect infestation in potatoes shortly after the eggs have hatched. 
After a few days a pink coloration may be detected around the 
injured eye, and closer examination evidences the presence of excre- 
ment held in the web at the entrance to the burrow. The larvae first 
burrow straight through the skin and into the substance of the tuber. 
Some then turn their mines so that they follow close under the skin of 
the potato. A fungus grows in the burrow and discolors it so that the 
course of the work may be easily followed. Later the skin of the 
potato partially dries and sinks so that the scar becomes very promi- 
nent; this is commonly called subepidermal injury. This type is the 
most noticeable, but is not so injurious to the tuber as the deeper 
channel; and as it dries out more easily, it is not so apt to be the cause 
of rot. 
The channel of the tuber-moth larva is as a general rule deeper and 
may even go through the center of a large potato. This form of 
injury is more difficult to detect from the outside than the sub- 
epidermal form, but from its greater injury to the potato is the more 
important. The surface injury may be cut out without much loss 
to the tuber, but to remove the burrow through the center, the tuber 
must be cut to pieces and much of it lost. 
There seems to be no definite course followed by the larva which 
might determine the character of injury or the direction taken in the 
tuber. Some channels are partly subepidermal and partly deep, while 
other larvae construct subepidermal channels and still others deep 
ones. 
The channels or galleries generally measure 1 to 3 inches in length 
and are quite tortuous. The portion occupied by the larva is fresh 
and white, but the older parts are covered with a matlike brown 
fungus and often partially filled with excrement. In older injury the 
mycelium of the fungus may entirely fill the channel. The growth is 
very compact, and if the tuber is cut in such a way that the injury 
is exposed the fungus may be lifted out in one piece. 
It should be added at this point that, in its occurrence in Cali- 
fornia, the larva does not prefer to feed upon the tubers in the ground 
as long as the potato tops are green and succulent. However, as 
soon as the tops become dry and hard the larvae do not hesitate to 
attack the exposed part of a potato, and will even dig through a thin 
layer of soil in order to reach the buried tubers. 
