PARASITE, APHIDIUS TESTACEIPES. 121 



present for weeks in a field with no visible indication of their presence. 

 Yet only a few warm days are required to bring about their final 

 development, whereupon the presence of the more or less globular, 

 leathery, brown bodies of the parasitized host first begin to attract 

 attention and thus actually reveal the presence of the Aphidius, 

 which has already been established there. 



An excellent illustration of this is afforded by an occurrence of 

 Toxoptera in eastern North Carolina, observed by Mr. L. M. Smith. 

 In a small field of oats near Newport, wingless viviparous female 

 Toxoptera and young were found in destructive abundance with no 

 indication whatever of the presence of Aphidius. Yet when speci- 

 mens of the pest submitted by Mr. Smith reached Washington, some 

 of them were beginning to change color from the presence of Aphidius 

 larvae within their abdomens. Again, when Mr. C. N. Ainslie visited 

 Wellington, Kans., April 1, 1907, he observed no trace of the presence 

 of Aphidius, but upon returning to this same locality on April 10 he 

 found them present. Only a few of the Toxoptera had yet become 

 dark brown, but a large number showed the orange color that told 

 the story of their parasitism. Therefore all statements made in 

 previous publications relative to the lack of parasites, or to the 

 extent to which they occurred in any field or locality, must be under- 

 stood as applying only to either the adults or to the browned cocooned 

 bodies of the host insects, and are not in any sense to be considered 

 as indicating the extent to which these host insects were carrying 

 obscured Aphidius larvae about with them in their bodies to develop 

 adults whenever there were a few sufficiently warm days. 



EFFECTS OF WET WEATHER ON THE DIFFUSION OF APHIDIUS. 



There is another element affecting the diffusion of this most 

 efficient of natural enemies of Toxoptera, namely, protracted rains. 

 When it is raining the parasite simply will not take wing at all or 

 move about in a way to be affected by winds. This element will 

 not admit of tabulation for the reason that a thunder shower followed 

 by warm, bright sunshine tends to make these, as well as all winged 

 insects, more active after the storm has passed. Thus, the amount 

 of precipitation really means little, while a slow, drizzling, protracted 

 rain (though the total precipitation may be much less) will keep the 

 parasite in seclusion much more effectively. Hence it is that not 

 only a comparatively high temperature accompanied by winds is 

 essential, but the weather must also be fair and sunny. 



In British East Africa Toxoptera is worse during seasons when 

 there is much wet weather, and in the Orange Free State outbreaks 

 of the pest seem to be also associated with similar meteorological 

 conditions during spring. 



