11 



frosts would occur. Indeed, the writer has observed eggs hatching 

 during the daj^ in the fields when there were frosts nearh^ every night. 

 Whether or not the larvae from these would get sutficiently advanced 

 to pass the winter would, of course, depend upon later weather condi- 

 tions. In the North these much-belated larvae are killed off b}^ cold 

 weather. 



The immunity of the late-sown wheat from attack by the ll}^ 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 smnmer that so widely separates 

 the two broods. 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, dr}- 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 ma}^ be kept for a year, or even two, but 

 when moistened the flies will soon after 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 th(^ soil. 



NATURAL ENEMIES. 



There can be no doubt that parasites play a most conspicuous part 

 in the natural control of the Hessian fly, and if we onh' knew the whole 

 truth of the matter we should find that these minute friends of the 

 farmer are worth many times their weight in gold. Not infrequently 

 one species of these parasites will overcome the pest in a neighbor- 

 hood so effectively as almost to exterminate it. Several times the 

 writer has found, in attempting to breed Hessian fly from young wheat 

 plants that had been killed by the larvoe, that hundreds of these para- 

 sites would emerge from the flaxseeds, while onl}- an occasional fl}^ 

 could be obtained. Nearly all of these deposit their eggs in the bodies 

 of the maggots, but the fully developed parasites emerge from the 

 flaxseeds. 



Professor Osborn " has enumerated six species of these parasites, not 

 including the English species Entedoii ejngonu.s Walker. Poli/giiofns 

 iiiinutus Lindm., which occurs in Russia, France, and England, is in 



«1898: Bui. 16, n. s., Div. Ent., U. S. Dept. Agric, pp. 28-29. 



