120 THE SPRING GRAIN-APHIS OR ' ' GREEN BUG." 



reproduction continually going on during mild weather, Aphidius 

 may also be present either as invisible undeveloped overwintering 

 larvae within the living bodies of its host, or it may be present as 

 mature larvae or pupa? in the dead and dried "cocooned" bodies of 

 the same. Besides this, in the light of recent studies of Aphidius by 

 Mr. Yiereck, the same may be true with reference to its occurrence in 

 a considerable number of other common species of aphidids, inhabit- 

 ing a great variety of vegetation, in the same neighborhood, upon 

 which this same species of Aphidius is parasitic. Thus, it is per- 

 fectly clear why, with Toxoptera swarming in the fields, and the 

 parasite present, about 10 days, with the temperature ranging from 

 40° or 50° to 60° or 70° F., is sufficient to enable the latter summarily 

 to suppress the invasion. The abruptness with which this change is 

 brought about is easily explained by the fact that a parasitized female 

 Toxoptera produces young during only a comparatively few days after 

 being parasitized, although she may survive several days longer, 

 especially if the weather be cool enough to retard the development 

 of the parasite. 



In the North the situation is usually quite different, as parasites 

 can not begin their work here to any extent until after the eggs have 

 hatched, and the stem mothers and their offspring have appeared in 

 the fields, thereby furnishing host insects. The overwintering of 

 immature Aphidius larvae in the bodies of the host is in the North 

 ordinarily precluded by the absence of living host individuals during 

 severe winters, although mature larvae may winter in the dead bodies 

 of the host as in the South. Stem mothers are probably never 

 present in great numbers and considerable time is therefore neces- 

 sarily required for their offspring to become excessively abundant. 

 For this reason parasitism, over the section where the host insects 

 pass the winter in the egg, begins later, and, at the start, proceeds 

 necessarily much slower than in the South, but on the other hand 

 Aphidius, unless the winter be an exceptional one, must of necessity 

 winter over in the "cocooned" bodies of its numerous hosts, as 

 mature larvae or pupae, and would therefore promptly respond to 

 the warm days of early spring, although delayed somewhat by 

 low temperatures that might not retard the host insects. 



There is one point in connection with parasitism by Aphidius that 

 must be always kept in view, particularly to the southward, in order 

 that mistakes and misstatements may be avoided regarding its 

 actual occurrence in any particular locality. While the larva is 

 contained within the still living body of its host its presence there is 

 not easily detected. Indeed it is not until the larva becomes nearly 

 full grown that it can be detected even by an expert. Therefore, 

 in the light of what has previously been stated concerning the situa- 

 tion in milder latitudes, there may be millions of living larvae 



