8 BULLETIN 1008, U. S. DEPARTMENT OF AGRICULTURE. 



Correcting the figures to correspond with what actually happened 

 in the field and calculating in the same way as before, it is found that 

 this extremely small percentage of flies transforming in this stubble 

 field in the fall would amount to 68,600 flies per acre, with 60 per cent 

 or 41,160 females. These females would lay enough eggs to develop 

 into 9,466,800 flies per acre in young wheat sown in the fall of 1919. 

 This is a much smaller number than had been anticipated, but it will 

 be readily seen that even this is an enormous infestation. If the 

 farmer sows about the same acreage from year to year and no flies 

 are lost by migration, then the flies from an acre of stubbles would 

 proceed to an acre of new wheat. It is calculated that 1,000,000 

 plants per acre is a good stand, and in that case this farmer would 

 have over nine flies developing on every wheat plant. It will be 

 readily seen from this that his chances of securing a wheat crop 

 under the conditions named are practically zero. It is also evident 

 that anything which helps to cut down this rate of reproduction is 

 of enormous benefit. The principal checks to the multiplication of 

 the Hessian fly are its parasitic enemies and unfavorable weather 

 conditions, and if it were not for them it probably would be im- 

 possible inside of two years to grow wheat at all. This illustration, 

 it is hoped, will also help to explain how an outbreak may seem to 

 develop suddenly from a very low infestation of stubbles. 



CONCLUSIONS. 



The principal points made in this bulletin may be summarized as 

 follows : 



1. The rate of multiplication of the Hessian fly is much higher 

 than has been realized. 



2. The rate of multiplication is quite different in the two principal 

 broods of the Hessian fly, the spring brood laying on an average 

 only about 230 eggs per female, whereas the fall brood lays about 285 

 eggs per female. 



3. The capacity for reproduction also varies with a number of 

 other factors, such as date of sowing, number of puparia per tiller, 

 etc. 



4. Because of these various influences, the actual rate of multipli- 

 cation will be found to vary from year to year and even from field to 

 field, and in years of light infestation the figures will prove too low. 



5. The proportion of males to females varies in the two principal 

 generations. In the spring generation about 60 per cent of the flies 

 are females; in the fall generation the sexes are approximately equal 

 in number. 



6. By applying these figures, however unsatisfactory the basis may 

 be, it is believed that entomologists will be better able to appreciate 

 how a Hessian fly outbreak may develop very suddenly, and to pre- 

 dict in a more accurate manner the approach of a dangerous outbreak. 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1921 



