THE POTATO TUBEE MOTH. 19 



will migrate to another leaf or to the petiole before starting a mine. 

 In the case of a tuber the larva generally begms 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 fii'st 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 aroimd 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 skui 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 hot 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 larvao 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. 



