PARASITE, APHIDIUS TESTACEIPES. 119 



relative to this matter cleared away, it will be observed that it is 

 entirely possible for great numbers of the adults, or those that are 

 nearly mature, to become parasitized in a southern locality, the 

 latter to develop to winged females under a more or less high 

 temperature, and for both to be carried many miles to the north- 

 ward, and then settle down and begin to reproduce, the Aphidius 

 becoming adult and issuing later from the dead body of its host. In 

 the meantime the offspring of the host Toxoptera would, of course, 

 develop and themselves reproduce, some of them, without doubt, falling 

 victims to the very parasite brought along by their parent. While 

 this may not be the chief factor in the dispersion of this parasite, it 

 probably enables it to follow along with the host insect and become 

 diffused with it, although if low temperatures prevail after the time 

 the migrating female settles in her new home there may be consider- 

 able delay in the issuing of the adult parasite without to any great 

 extent delaying the development and preventing the increase of 

 Toxoptera. 



With the temperature at a point which enables Aphidius to become 

 active there is no doubt that the parasite follows with the host insect, 

 and, indeed, these parasites are usually found on the wing in the com- 

 pany of their hosts during warm sunny days. With high cold winds, 

 which usually come from the northward and would tend to drive the 

 parasites back over territory to which Toxoptera has already come 

 and from which it has now largely disappeared, the adult Aphidius 

 is observed to nestle down among the infested plants and not to 

 venture abroad. Thus it is that this parasite is doubtless usually 

 present in some form in the grain fields with the Toxoptera, though 

 critical examinations of such fields may fail to reveal them until the 

 temperature reaches a point that enables them to become active. 



All of this is applicable to the insect in southern territory where no 

 egg stage is yet known to occur. Aphidius occurs all over the coun- 

 try, and we have learned that in the North it winters as fully devel- 

 oped larvae and pupse within the "cocooned" bodies of its hosts, its 

 emergence and activity in spring being controlled by the temperature 

 and its dispersion influenced by the same forces and in much the same 

 manner as in the South. 



TEMPERATURE INFLUENCES ON APHIDIUS. 



Probably the whole secret of these disastrous outbreaks of Tox- 

 optera lies in the fact that this parasite is not active in a tempera- 

 ture much below 56° F., while, as has already been shown, the aphis 

 begins to reproduce in a temperature at or slightly below 40° F. — 

 a probable difference of at least 16° F. Therefore the situation in a 

 field of wheat in the South in early spring may be described in this 

 way: There are present many Toxoptera of all ages, with viviparous 



