DIAPAUSE AND INSECT PHENOLOGY 155 



factorily explained on the basis of Dickson's studies on the 

 photoperiodicity of this moth, which conclusively demonstrated 

 that the codUng moth responds to a critical day-length of 15 h, 

 while with 12 h of daily illumination almost all larvae enter 

 diapause^. It should be pointed out that in all the countries 

 mentioned previously the codling moth caterpillars enter dia- 

 pause as from mid-July at the earliest, i.e. at a period of 

 shortening day-light hours. 



However, in the vicinity of Baghdad, where conditions would 

 be expected to be favourable for an even greater number of 

 generations than in Israel, it was observed by the late Prof. 

 Bodenheimer that the larvae entered diapause as early as in the 

 middle of May, and thus gave rise to no more than one annual 

 generation. As diapause occurs there during a period of in- 

 creasing day-length (the average number of day-light hours 

 during May in Baghdad is 13 h 35 min), the diapause-inducing 

 factor in Iraq should be sought elsewhere than in the photo- 

 period effect. A simple experiment carried out in our laboratory 

 at Rehovot^ revealed that larvae hatched during the first half 

 of April, which after penetrating fruits were transferred to an 

 incubator at 27°, all went into diapause, regardless of host fruit 

 variety. The same thing happened when the larvae were exposed 

 to 27° following some 17 days of natural outdoor development, 

 which constitutes about three quarters of the total duration of 

 larval development. On the other hand, when fully grown 

 larvae were collected outdoors in May and exposed to the 

 same 27° constant temperature, only 42% went into diapause 

 and the rest pupated and emerged the same summer. Simul- 

 taneously with this experiment, when another group of larvae, 

 hatched in May, were exposed to a temperature of 23°, all 

 pupated at the expected time. The average monthly tempera- 

 tures in Baghdad^ are 22° in April, 28° in May and 32° in June, 

 whereas in Rehovot the monthly mean temperature reaches 

 26.5^ only in August, which is the time our codling moth begins 

 its diapause. Another conclusion to be drawn from this experi- 

 ment is that the stage of development sensitive to high tempera- 



References p. 157 



