68 



The wet weather theory here appears broken, and a low temperature- 

 is the only element which appears uniformly through the months dur- 

 ing which the Grain Aphis was, in all probability, increasing with the 

 greatest rapidity. That cool weather should favor the development of 

 these insects would, if true, be a new factor in the problem, not only 

 of this, but other species also : and before leaning too heavily upou 

 this evidence we should cast about for good reasons for this apparent 

 ambiguity. 



There is one very important element in this whole problem which we 

 have so far left out of consideration, viz, natural enemies. While low 

 temperature might not favor the development of the grain Aphis, or in 

 fact, if the effect was slightly adverse, if the outcome was to destroy or 

 retard the development of parasites, the ultimate result would be to 

 favor the Aphis. 



For myself, I can not get rid of the feeling that the indirect action of 

 the weather of May and June — the action upon the parasites — was 

 much greater than the direct effect upon the Aphis itself. 



According to my field-notes, my earliest observation of the grain 

 Aphis about La Fayette, lat. 40° 27' X., during any year, was on April 

 27, and we have observed them during other years on grain early iu 

 May, in greater abundance than they were the present year on the 1st 

 of June ; yet in the former case no outbreak occurred. Up to the 1st 

 of June, the Ax>his was not exceedingly abundant on grain about La 

 Fayette. 



Even as late as the 7th their numbers on the heads of wheat were 

 not so much greater than they had occasionally been in former years 

 as to cause alarm; yet within ten days they were swarming in these 

 same fields in myriads. This certainly bespeaks more of the effects of 

 relief from the pressure of parasitism than from the effect of meteroiog- 

 ical conditions, especially a change from dry to wet weather. 



The question may be asked, why, if this be true, were not the para- 

 sites destroyed in the southern portion of the State, late in May, thereby 

 relieving the Grain Aphis from this check on their increase, and why 

 the latter by reason of this relief did not, as the wheat became too ad- 

 vanced, overrun the oat-fields, as would have at that date naturally 

 followed. The reply is that such results did follow to a limited extent, 

 the oats being rather more seriously infested by the Aphis than farther 

 northward, and the reason why this feature was not more marked was 

 doubtless owing to the fact that the cold waves of the first and last of 

 May, especially the latter, were less severe than farther north, and the 

 effect on the parasites correspondingly less fatal. 



The records of the State weather service show that the minimum 

 temperature of the first four days of May at La Fayette was below the 

 freezing point; and on the 22d, 23d, 30th, 31st, from 31° to 39° Fahr. 

 The mean miuimum for the entire State for the same month, according 

 to the same authority, was, for the southern portion, 36° Fahr., for the 

 central 32° Fahr.. and for the northern 30- Fahr. 



