20 BULLETIN 427, V. S. DEPAKTMEISTT OF AGRICULTURE. 



EFFECTS ON THE TUBER. 



The immediate effect of the potato-tuber moth on the potato is in 

 the reduction of its market value. This takes place in two ways: 

 (1) By ruining the substance of the part of the potato actually 

 attacked, (2) by causing the entire tuber to be unsightly and, 

 therefore, undesirable. Losses through the tuber moth may practi- 

 cally all be classed under these two heads. It is true that the larvae 

 cause decay in the tubers, but when this takes place the infestation 

 is generally so heavy and the injury so far advanced that the tubers 

 have ceased to have value, either for food or for seed. 



The growing sprouts of seed potatoes also form a favorite point 

 af attack. When thus attacked they are badly injured or killed, 

 and potatoes which have too long been exposed to the attacks of the 

 moth are likely to have their value as seed materially reduced. 



NUMBER OF LARV^ DEVELOPING IN TUBERS. 



Since several generations of the insect may develop in a single 

 tuber, it is difficult to determine the maximum that a tuber of average 

 size will support. Mature larvae have been noted, apparently of 

 normal development, in tubers which contained no appreciable 

 moisture and which were simply a network of pith holding together 

 the dried burrows of previous generations of tuber worms . In one gen- 

 eration 121 pupae and 3 mature larvae were taken from a tuber 4 cm. 

 by 6 cm. by 9 cm. The substance of the tuber had been so com- 

 pletely destroyed that on removal from the breeding jar it collapsed. 



DEATH OF LARV^ IN TUBERS. 



The normal rate of mortality among the larvae while tunneling in 

 tubers is very low. Whenever too many develop in a tuber and a 

 putrid condition ensues, very few of the partially developed larvae 

 escape. Some may leave the tuber and go in search of other food, 

 but most of them remain in the decaying tuber and die. 



Larvae mining in potato tops are very susceptible to change of 

 weather, and in short cold and rainy periods most of the larvae in the 

 leaves are killed. Those in the stems, being better protected, are safer 

 from weather changes. 



LENGTH OF FEEDING PERIOD OP LARV.^. 



The length of the feeding period varies with the temperature. 

 During July and August the active larval life requires as few as 14 

 days, while during December and January the same period some- 

 times lasts 69 days. This much greater length of the larval stage 

 in winter is a result simply of retarded development and can not be 

 considered hibernation, as the larva is active and feeding at all times. 



