156 I. HARPAZ 



ture as a diapause-inducing factor is the last quarter of the 

 duration of larval development, in other words the last instar. 



Undoubtedly diapause in the codling moth in general is 

 governed by a variety of factors, i.e. photoperiod, temperature, 

 diet and perhaps others yet unknown, each one of which may 

 become predominant according to the prevailing circumstances. 



Perhaps a more striking example is the phenology of the 

 onion fly (Hylemyia antiqiia Meigen) in Israel as recently 

 reported by Yathom^. The same species is a well-known pest 

 in Europe and North America where according to the phenology 

 of its liliaceous host plants of the genus Allium, it enters the 

 pupal diapause during August and September, and development 

 is only resumed after winter, in May^. In Israel, in contrast, 

 Allium plants under natural conditions germinate or sprout 

 only after the first winter rains in November-December, and 

 grow till the end of the rainy winter season, at the end of which 

 they become domiant for the dry summer season. In close 

 synchronization with this cycle the onion fly emerges in 

 November and breeds one generation up to January. The 

 majority of the offspring of the following generation enters the 

 pupal diapause in March. This takes them through the dry 

 period of absence of fresh Allium growth till November or 

 December, depending on the first rain. A similar instance is 

 that of the aphid-eating syrphid fly Epistrophe balteata de Geer, 

 which in Switzerland, for example, diapauses as an adult during 

 winter, when aphids are very rare^^, whereas the same species in 

 Israel raises several generations during winter and spring, when 

 aphids are abundant, and is dormant during summer when its 

 prey becomes extremely scarce^^. 



One cannot assume that the factors which induce winter 

 diapause in the onion or syrphid fly in the temperate zone are 

 precisely the same ones that cause the same species to aestivate 

 in a subtropical region, though the reason for arresting develop- 

 ment is the same, namely, to survive a period of absence or 

 shortage of food. This must lead one to suspect that the changes 

 which take place within the host plant in anticipation of an 



