June, 1922 bionomics of alphelinus semiflavus 



219 



sary to bring about a parasitized condition in the adult after 

 she had started to produce young. Out of many trials to bring 

 this about, but three were successful; notwithstanding the fact 

 that several parasites were confined for days with the adult 

 aphids time and again. 



It is believed that several factors operate here to determine 

 this preference for the younger stages of the host; namely 

 (1) size, it being easier to ovipost in a small aphid than a large 

 one, due to position alone; (2) irritability, the large aphids are 



Table II. 



Showing the Percent of Parasitism in the Various Instars, Dissected Immediately 

 After the Parasites Were Removed. 



more irritable than the small nymphs, which often renders ovi- 

 position impossible; and (3) toughness of the integument which 

 makes it difficult for the ovipositor to penetrate. 



Number of Eggs per Host. — In more than 3,000 separate dis- 

 sections for egg counts, etc., there were only two or three cases 

 where more than one egg was deposited in a single host. These 

 few exceptions occurred only where a number of parasites were 

 confined with a few aphids for several days, and even then no 

 more than two eggs were ever found in a single host. In some 

 of these trials a parasitism of 100 percent was obtained for 20 

 first to third instar nymphs confined with one parasite for a 

 single day of twelve hours. Instances were observed several 

 times where parasites apparently oviposited in hosts already 

 parasitized, even to the blackened condition; but in no case 

 did subsequent dissection show that an egg was deposited. 

 Aphelinus has also been observed to return several times to a 

 single host, but here, likewise, only a single egg is left, or ia 

 many cases none at all. 



