32 BULLETIN 427, U. S. DEPAETMENT OF AGRICULTUEE. 



is SO high that an increase of the insect from this source is highly 

 improbable. Rains and sudden changes in climatic conditions kill 

 many of the larvae, and the large number of predacious and parasitic 

 enemies further reduce the numbers of the insect. The figures will 

 be considered later with the discussion of natural enemies. 



When the insect attacks stored tubers the percentage of insects 

 developing safely is very high. Figures show that practically all the 

 eggs deposited hatch. In storage there is always an abundance of 

 food and all stages are protected from most of their enemies, so most 

 of the larvEe develop successfully. 



POSSIBLE RATE OF INCREASE. 



The theoretical rate of increase for the tuber moth is very rapid. 

 Taking 150 as the average number of eggs deposited and comiting 

 half the adults as females, the progeny of one pair would give about 

 60,000,000 adults at the end of the fourth generation. 



While this theoretical rate is seldom even approached, it serves to 

 show that under favorable conditions for reproduction the insect may 

 increase to damaging numbers in a short time. 



NATURAL ENEMIES AND CHECKS. 



Where the tuber moth works as a leaf-miner on the potato tops, its 

 numbers are kept down very well by its enemies and chmatic changes. 

 Its numerous parasitic enemies play the most important part, rains 

 and cold weather probably come second in point of importance, and 

 the predacious enemies last. 



In southern California the parasitic enemies of the tuber moth 

 form a fine series and work on every stage. The egg and pupa each 

 has its parasite, while several attack the partially grown larvae and 

 at least two the mature larvae. 



Only three of these work on the tuber worm infesting potatoes, 

 and here they are only partially effective. The burrowing habit of 

 the larva protects it from parasites except while spinning its cocoon 

 and pupating. Parasites are also hampered by the storage of pota- 

 toes. Altogether it is doubtful if parasites could be of practical 

 importance when the insect infests stored tubers. Certainly the 

 stored potatoes examined have discouraged such a belief. 



Experiments to ascertain the percentage of parasitism in the potato 

 tops show that the parasites, taken altogether, are valuable in the 

 control of the tuber moth. The impracticability of direct methods 

 oi control necessitates the use of all possible measures to limit the 

 number of moths before harvest. This is well accomplished by the 

 parasites, resulting in lessened injury to the leaf surface and dimin- 

 ishing the number present to infest the potatoes just before and during 

 harvest. 



