AN EXTREME CASE OF DELAYED FALL EMERGENCE 

 OF HESSIAN FLY.* 



(Phytophaga destructor Say). 



By Walter H. Larrimer, Scientific Assistant, 

 U. S. Bureau of Entomology, West Lafayette, Indiana. 



During June, July, and August, 1920, over the northern 

 portion of the east central states, with the exception of a small 

 isolated area in north central Illinois, the mean temperature 

 was from three to six degrees below normal and during Sep- 

 tember and October for this area the mean temperature was 

 from six to ten degrees above normal. Rainfall was more or 

 less general in occurrence over this area, but the amount 

 varied remarkably in different sections as well as in immediate 

 localities. It is to be expected, therefore, that such irregularities 

 in two of the most important of the controlling factors of Hessian 

 fly emergence would correspondingly influence the emergence 

 and activity of the fall brood of 1920. 



The emergence as observed at Lafayette, Indiana, may be 

 indicated by a graphic record of egg counts made daily from 

 September 23 to October 25. Fifty wheat plants of the two 

 leaf stage were marked and each morning the eggs were counted 

 and removed. When the plants grew to a size that was no 

 longer as attractive to the female flies for oviposition as the 

 smaller plants, the markers were transferred to a fresh lot of the 

 desired size from a later planting, care being taken to make 

 sure that these new plants were clear of eggs. In Fig. 1 the 

 graph shows the daily egg record for the period indicated. 



Very few wheat plants were available for oviposition at 

 the time of the first peak of emergence and the resulting 

 oviposition was of course concentrated on these few plants, 

 while an almost unlimited supply of wheat was available at 

 the time of the second peak on October 17. Therefore the true 

 relation as to the magnitude of the two waves of emergence 

 is not truly represented by the graph. However, the purpose 

 is not so much to show the proper relative proportion of the 

 waves as to mark the time of each. 



Published by permission of the Secretary of Agriculture. 



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