June, 1922 bionomics of alphelinus semiflavus 233 



Observations on colonies of aphids in the open greenhouse 

 among both Aphidius and Aphelinus showed that parasitism of 

 a single host by both parasites was rare at best, if not entierly 

 absent. In fact, it was often difficult to bring this about when 

 they were confined in a small cage. However, when they were 

 put in and taken out separately, the one before the other, results 

 were different. Aphidius would parasitize practically every 

 aphid in a small colony, and all the eggs that Aphelinus would 

 deposit later, which was always considerably less than Aphidius 

 for a given period, would be in the same host with one or more 

 Aphidius eggs or larvae. 



When Aphidius was placed with a colony of aphids a day 

 ahead of Aphelinus and remo^^ed before Aphelinus was intro- 

 duced, a heavy parasitism by both resulted. But in practically 

 every case, the Aphelinus eggs failed to hatch. The conditions 

 were then reversed, and Aphelinus was given a day's start. In 

 this case Aphelinus hatched and began development in the 

 usual way, but they did not interfere in the least with Aphidius 

 which came along later. This very efficient parasite emerged 

 from its membraneous cover in the normal manner and within 

 a day or two the small Aphelinus larva would die. This happened 

 even when the Aphelinus larva was allowed several days the 

 advantage over Aphidius and was quite large when the latter 

 hatched. 



It was, therefore, practically established that for the younger 

 instars, at least, Aphidius would always dominate, regardless of 

 whether it came before or after Aphelinus in the same host; 

 and that Aphelinus, coming after Aphidius for a day, would 

 never hatch. In fact, the egg would never begin development as 

 shown in the characteristic change in appearance (Plate I, 

 Fig. 3, 4 and 5.) 



Possible Ways hy Which Aphidius May Cause the Death of 



Aphelinus. 



Just how the death of Aphelinus is brought about still 

 remains a mystery that will only be solved by more careful 

 investigation. Several theories are suggested to explain it, but 

 all of them require refined methods to prove. They are given 

 here for what they are worth. 



