8 BULLETIN 1008, U. 5S. 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 fiy 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 Lier, 
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 
