214 MAINE AGRICULTURAL EXPERIMENT STATION. 1916. 



Hardly a colony of aphids can be found that is not being 

 preyed upon by from one to half a dozen species and from one 

 to dozens of individuals. They kill the individual aphids out- 

 right but on account of the rapid rate of reproduction of the 

 latter the colony usually exists for some time at least. 



I have been interested in getting some definite data as to the 

 capacity of these larvae for devouring their prey. 



Specimens of Aphis brassicae proffered to a four-day old 

 un fasted larva of Syrphus americanus were devoured at the 

 following rate: Four minutes and 30 seconds, 2 minutes, i 

 minute, I minute, 30 seconds, 3 minutes and 1 5 seconds, 2 

 minutes, I minute and 45 seconds, and I minute and 30 seconds ; 

 a total of nine aphids in seventeen and a half minutes (32, p. 

 481). In this test the aphids were touched to the mouth-parts 

 of the larva as rapidly as the previous skin was discarded. It 

 was thought that this represented an abnormal rate of feeding. 

 Last summer the larva of Syrphus nit ens was observed while 

 feeding normally on the large aphids Pterocomma flocculosa. 

 During a period of twenty minutes, twenty-one aphids were 

 caught and destroyed. The larva had not been kept away from 

 food nor were the aphids supplied for it or the process in any 

 way hurried or artificial. 



How much longer feeding at this rate may continue or how 

 frequently such periods of rapid feeding may succeed each 

 other, observations have not yet shown. However, it seems 

 certain that the number of aphids destroyed by a single speci- 

 men during its larval existence of one or two weeks or longer 

 must be very considerable. 



Particularly when we consider the enormous numbers of 

 progeny which may be prevented from developing /by the 

 destruction of even a single agamic female, the value of these 

 agricultural benefactors may be more correctly judged. A 

 point of great importance in this respect is that the Flower- 

 flies infest the aphid colonies very early in the establishment of 

 the latter; indeed, sometimes the Syrphid eggs are deposited 

 on a plant in advance of its infestation by the aphids; so that 

 numerous voracious larvae may be awaiting the first appearance 

 of the stem-mother and her progeny. 



Another important point which enhances the destructive 

 powers of the Syrphid larvae, is that they eat not the entire 

 aphid but only the soft and rapidly digestible body-contents. 



