298 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 16 
The first factor to attract attention was silica, since it occurs in large 
quantities in the ash of wheat straw. While the silica content of the 
different varieties of wheat is not known, it has been found in other 
plants that there is a marked difference, even between strains of the same 
variety. Silica has also been considered as the basis of resistance in 
the case of aphids on Lithospermum arvense , and of certain plant dis¬ 
eases. 
A rather extensive series of experiments has been conducted by grow¬ 
ing the different varieties of wheat in Pfeffer’s culture solution to which 
varying amounts of sodium silicate are added. While this phase of the 
work has been underway for less than a year, and there is still much to 
do toward developing the technique, certain striking results have been 
obtained which indicate that silica has an important part in resistance. 
Several of the very susceptible varieties have shown marked resistance 
when grown in Pfeffer’s solution containing a small amount of sodium 
silicate and the degree of resistance has varied with the amount of silica. 
The larvae reach the base of the plant, but as in the case of the resistant 
varieties discussed above, they do not develop. The data also indicate 
that different varieties of wheat respond differently to varying amounts 
of silica. The results have been of such significance that a thorough 
study of the utilization of silica by the different varieties of wheat is 
now under way. 
Summary 
This paper, as the title indicates, is a progress report of the investi¬ 
gations of the resistance of wheat to the Hessian fly. The problem is 
far from being solved, but the results obtained are encouraging and are 
presented at this time in the hope that they will be of service to other 
workers in this field. The results of the experimental work discussed 
may be summarized as follows: (1) Evidence of resistance has been es¬ 
tablished in the field and this resistance is fairly constant. (2) The soft 
wheats as a class are more resistant than the hard wheats. (3) The 
cause of resistance has not been determined, but many factors have been 
eliminated. (4) Resistance in young plants is apparently located at 
the base of the plant. (5) Resistance in wheat cannot be explained by 
any selective action of the adults in ovipositing. (6) The fact that a 
sufficient number of larvae get down on all varieties to seriously injure 
the plant indicates that resistance is not due to gross morphological 
characters of the plant. (7) The evidence indicates that resistance is 
due to physiological causes and that silica is in some way associated with 
it. 
