8 CIRCULAR 4 2 2, U. S. DEPARTMENT OF AGRICULTURE 



number of eggs laid, and this problem should receive further investi- 

 gation. 



THE EGG 



The eggs have soft shells when freshly laid, and are covered with 

 a glue which in most cases fastens them securely to the surface on 

 which they were laid for the length of the incubation period. When 

 eggs are pressed together in clusters they often dry into distorted 

 shapes, as the shells harden in a few minutes after they are laid. 



The eggs of the tobacco moth are elliptical and grayish white, and, 

 when viewed through a binocular microscope, the shell appears to 

 have a granular texture. They measure from one-fortieth to one- 

 fiftieth of an inch in length and can be seen on tobacco leaves with 

 the unaided eye. Figure 4 shows a characteristic arrangement of 

 eggs laid on a leaf of flue-cured tobacco. During the incubation 

 period the grayish white of the newly deposited egg changes to a 

 light brown, and then to a darker brown. 



HATCHING 



The newly developed larva of the tobacco moth can be seen through 

 the eggshell a short time before hatching. When the egg is ready to 

 hatch the larva makes a circular opening in the end of the eggshell 

 and crawls out on the surface to which the egg is attached during the 

 incubation period. The empty eggshells often remain attached to 

 the tobacco leaves or other surfaces, retaining their original shape un- 

 less eaten by the young larvae. In spring and fall, most of the 

 hatching occurs during the warmer parts of the day whereas in hot, 

 summer weather it occurs to a large extent at night. Incubation rec- 

 ords made in 1932 of 3.186 eggs showed that 69.2 percent hatched. 

 Similar records were obtained during 1933 for 2,916 eggs, and 71.3 

 percent hatched. (For conditions of temperature and humiditv see 

 fig. 10.) 



THE LARVA 

 HABITS 



In the warehouses the larvae are active after emerging from the 

 eggshells and crawl away without delay in search of food. The ease 

 and speed with which they crawl indicate that young larvae are 

 capable of migrating considerable distances in search of food. Feed- 

 ing on tobacco leaves begins at once, provided the moisture content 

 of the leaf is satisfactory. It became apparent early in the life- 

 history studies that moisture in the tobacco leaves was a very impor- 

 tant factor in the survival and growth of larvae. The ideal moisture 

 content for the best growth on flue-cured tobacco is approximately 

 13 percent ; detailed observations, however, demonstrated that larvae 

 could survive in tobacco with as little as 10 percent moisture in the 

 leaf. In small rearing containers in which portions of tobacco leaves 

 were used as food, the moisture rapidly evaporated during exposure 

 in the laboratory, and a heavy mortality of young larvae resulted. 

 Feeding is rather voracious from the start ; the young larvae eat the 

 upper or lower cuticle of the leaf until about the first molt, after 

 which the entire leaf between the larger veins and the midrib is con- 





