THE TOBACCO BEETLE. 15 



varies from 45 to 70 days. Eggs are deposited in the food substance, 

 and under usual conditions the incubation period is from 6 to 10 days, 

 the larval period from 30 to 50 days, and the pupal period from 6 

 to 10 days. Adults live ordinarily from 3 to 6 weeks after emergence. 

 In cold climates the species passes the winter mainly in the larva 

 stage. It thrives best in localities where the temperature and hu- 

 midity are high, and in substances in which the larva? are protected 

 from rapid evaporation. 



THE EGG. 



PERIOD OF INCUBATION. 



The time required for hatching depends upon the prevailing tem- 

 perature and probably upon moisture conditions, although consider- 

 able variation in the incubation period has been observed, even with 

 eggs from the same female which were kept under identically the 

 same conditions. The variation in the time required for hatching is 

 least when the eggs are not subjected to extreme changes of tempera- 

 ture and humidity, and when the temperature is relatively high. At 

 Key West, Fla., where the temperature is remarkably even, the incu- 

 bation period varied from 5 to 10 days in numerous experiments con- 

 ducted in April and May. During the experiments the maximum 

 temperature ranged from 91° to 93° F., and the minimum from 81° 

 to 83° F. The average period of incubation was found to be approxi- 

 mately 7 days. 



The length of the egg stage at room temperature at Clarksville, 

 Tenn., in May, ranged from 7 to 14 days. In 22 records obtained 

 during July and August the average period of 7.6 days was recorded. 



At Appomattox, Va., at room temperature in July, the average 

 period of incubation was found to be 8.2 days. The shortest period 

 observed was 6 days and the longest period 14 days. 



Records obtained at Tampa, Fla., in July and August, show an 

 average incubation period of approximately 8 days. The longest 

 period observed was 10 days and the shortest period 6 days. 



In determining the incubation period at a fixed temperature at 

 Richmond, Va., eggs were kept in an electrically heated and auto- 

 matically regulated incubator. The temperature was fairly constant 

 at 30° C. (86° F.) and the relative humidity at about 80 per cent. 

 During the period of incubation the variation in temperature was 

 less than 1° C. All eggs used in the experiments were obtained from 

 beetles kept in the incubator, the incubation period ranging from 

 6 to 8 days. In all of 20 different experiments hatching commenced 

 during the sixth day. 



THE LARVA. 



Injury to tobacco by feeding is caused mainly by the larva or worm 

 stage of the insect. It is this stage of the tobacco beetle that is often 

 referred to by tobacconists as the "grub," Within the egg the embryo 



