THE CODLING MOTH IN MICHIGAN. 55 



It is of interest to note the extent of variation in the length of the 

 life cycle of the first generation. The figures of Table LIV show a 

 maximum length of time for the entire life cycle of 87 days and a 

 minimum of 29 days, or a range of variation of 58 days. From the 

 above results it becomes evident that reliable conclusions can not be 

 made from a limited number of observations no matter how accurate 

 the records may be; they only represent the results under limited 

 conditions. Such conclusions may readily become extremely mis- 

 leading when used as a basis for timing spray applications. By using 

 the average length of the life cycle of 51 days it will be found that 

 three broods of the codling moth could have existed in the Michigan 

 fruit belt in 1911. Or should we, on the other hand, choose to use 

 the records for the minimum length of the life cycle we could on that 

 basis account for the existence of a fourth brood of the codling moth. 

 Our observations, however, only show evidence of two broods, since 

 out of several thousand larvae of the second brood not a single insect 

 pupated in 1911. 



THE SECOND GENERATION. 



SECOND BROOD OF EGGS. 



Time of incubation. — Eggs of the second brood occurred in the 

 field for about three months. During this long period the different 

 eggs were often subjected to strikingly different climatic conditions, 

 which resulted in an unusual degree of variation in the time of incu- 

 bation. In Table LVII are included the records for 110 observa- 

 tions. The time of incubation here varied from 6 to 16 days and 

 averaged 9.35 days for the whole period. During the latter part of 

 July and first half of August, when the greatest abundance of eggs 

 was found, the time of incubation varied from 6 to^8 days. As for 

 the first brood of eggs, observations were also made on the embryo- 

 logical development of the second brood of eggs, namely, the time of 

 appearance of the "red ring" and the " black spot." The summa- 

 rized results in Table LIX show that the red ring appeared on an 

 average within 3 days after egg deposition and the. black spot 2 days 

 previous to hatching. A number of eggs deposited during the middle 

 part of September failed to hatch, mostly due to the prevailing low 

 temperature. The fact that the red ring had already appeared in 

 these eggs proved them to be fertile. 



All of the eggs used in these tests were laid in the rearing cages, 

 and there were 4,643 eggs under observation as listed in Table LVII. 



