12 FARMERS’ BULLETIN 640. 
to pass the winter would, of course, depend upon later weather condi- 
tions. In the North these much-belated larvee are killed off by cold 
weather. 
The immunity of the late-sown wheat from attack by the fly is not 
due to frost, but to the fact that by the time severe frosts usually 
occur most of the flies have appeared and gone. 
The most marked influence of climatic conditions on this insect is 
seen in the effect of heat and drought, and especially of the two com- 
bined. In the South it is the long summer that so widely separates the 
two generations. Drought has a similar effect on the development of 
the insect as it has on the germination of the seed which produces its 
food; thus, dry weather in the late summer and fall tends to keep the 
insect in the flaxseed stage—a fact of special importance in the North 
where it is imperative to get the wheat sown early enough to enable 
the plants to stand the winter. Under exceptional conditions, such as 
in a dry room, flaxseeds may be kept for a year, or even two, but 
when moistened the flies will soon emerge. So in the fields they 
will, during a drought, remain in the flaxseed state for a considerable 
time after they would appear under normal conditions, and only 
appear soon after rains have moistened the soil. 
The first publication! dealing with the generally uniform retarda- 
tion of development of the fall generation from the north southward 
was based on work done by the author in Indiana during the years 
1886 to 1890, with the aid of experimental plats sown throughout the 
entire length of the State. 
Studies of the Hessian fly in Indiana, which were begun by the 
author in 1884, very soon indicated that information regarding the 
development of this insect based on information secured in the 
northern part of the State would not apply at all m the extreme 
southern section. So far as known to the writer, this publication is 
the first on record relating to this particular feature of the develop- 
ment of the pest. Later the same investigation was continued in 
Ohio2 The results of this work showed very clearly that the uni- 
formity of development of the fall generation, which is the one most 
to be feared by the farmer, was practically the same in Ohio as in 
Indiana. Even at that time there was an indication of what might 
be termed a variation from the normal condition in southern Michigan. 
Since 1904 similar wheat-sowing experiments have been conducted 
from northern Michigan and northern New York to central Georgia 
and across the State of Kansas. As a result of the sowings in Michi- 
gan it was found that owing to some influence, probably that of the 
1 Webster, F. M. Report on Some of the Insects Affecting Cereal Crops. The Hessian Fly. In U.S. 
Dept. Agr., Div. Ent., Bul. 23 (old series), pp. 63-79, 1891. ; 5 
2 Bulletin No. 7, Vol. 1V, af the Ohio Agricultural Experiment Station, pee in 1891; Bulletin 
No. 107 of the same institution, published in 1889; and Bulletin No. 119, also of the same institution, 
published in June, 1900. 
