40 BULLETIN 918, U. S. DEPARTMENT OF AGRICULTURE. 



25 per cent, in addition to the 47.1 per cent laid on other parts of 

 the plants and assumed to be lost, would bring the total mortality 

 of young larvae up to 72.9 per cent. This figure (72.9 per cent) is 

 thought to approach more closely what actually takes place in the field 

 than that of 62.1 per cent based on the number of empty shells found. 

 This is borne out by laboratory experience where the mortality was 

 90 per cent or more. 



Once the larvae are inside the bolls there is very little or no mor- 

 tality during the summer months. 



MORTALITY OF HIBERNATING LARVAE IN THE FIELDS. 



There is a very heavy mortality in hibernating larvae left in the 

 fields during the winter, especially when the fields are irrigated. Ex- 

 aminations of old bolls picked up in the fields showed that about 80 to 

 90 per cent of the larvae survived till February, and that the mor- 

 tality rapidly increased from this date onward. The mortality varies 

 greatly in different samples, but by the end of March it is difficult to 

 find any live larvae in the scraps picked up on the ground in the fields. 

 There are still live larvae present, but the old bolls have broken up, 

 scattered, and in cultivated fields are difficult to find. On March 

 13, 1918, only 3 live larvae were found from a bushel of old bolls and 

 trash picked up in a lightly infested field at San Pedro. On April 

 16, 1919, no live larvae were found in the old bolls left on the ground 

 after cattle had grazed over the fields at Tlahualilo, but about 5 per 

 cent were still alive in the bolls left on the stalks. 



To secure more data on this point, two series of experiments were 

 started on November 26, 1918. One experiment was under as nearly 

 normal irrigated field conditions as possible and the other under non- 

 irrigated conditions. The fields in the Laguna are usually irrigated 

 in November, December, or January, when cotton follows cotton, as 

 it does on most plantations. When fields have been lying fallow for 

 a season, the water is applied any time during the year when it is 

 available. About 3 feet of water is placed on the fields and allowed 

 to soak in. This requires from one to several months, depending 

 upon the character of the soil. In the experimental plot irrigation 

 was started on December 2 and continued till December 8, when the 

 motor burned out. Water was again applied on December 23, 1918, 

 and continued daily until January 22, 1919. On account of the 

 small amount of electric current available, the plot could not be filled 

 3 feet deep, but the daily application of a few inches of water kept 

 the ground thoroughly wet and covered with water at least part of 

 each day. 



In each of the series the larvae were exposed under wire cages in 

 bolls in a single layer on the surface of the ground, in bolls buried 6 

 to 7 inches deep, and in double seed buried one-fourth inch deep in 

 flowerpots which were set in the soil flush with the surface. More- 



