68 
The wet weather theory here appears broken, and a low temperature 
is the only element which appears uniforinly through the morths 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, bea new factor in the problem, not only 
of this, but other species also; and before leaning too heavily upon 
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’ N., during any year, was on April 
27, and we have observed them during other years on grain early in 
May, in greater abundance than they were the present year on the Ist 
of June; yet in the former case no outbreak occurred. Up to the Ist 
of June, the Aphis was not exceedingly abundant on grain about La 
Fayette. ; 
ven 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 meterolog- 
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 toa 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 34° to 39° Fahr. 
The mean minimum for the entire State for the same month, according 
to the same authority, was, for the southern portion, 36° Fahr., for the 
central 329 Fahr., and for the northern 30° Fahr, 
