32 BULLETIN 427, U. S. DEPARTMENT OF AGRICULTURE. 
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 larva?, 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 larvae 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 counting 
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 climatic 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 il 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 
of 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. 
