LETTER OF TRANSMITTAL. 
U. 8S. DEPARTMENT OF AGRICULTURE, 
DIVISION OF ENTOMOLOGY, 
Washington, D. C., July 16, 1901. 
Str: I have the honor to transmit herewith the manuscript of a 
short but full paper entitled ‘‘ Life history of two species of plant- 
lice inhabiting both the witch-hazel and birch,” by Theodore Per- 
gande, an assistant entomologist in this Division. I recommend that 
this paper be published as Technical Series No. 9. As explained in 
the letters transmitting earlier bulletins of this series, papers of this 
character, while only indirectly of interest to farmers, fruit-growers, 
and foresters, are of especial value to economic entomologists who are 
engaged in making practical applications of scientific entomology. 
In the present case plant-lice form a group of insects which are very 
destructive, economically considered. Their life histories are very 
remarkable and little understood. This is especially the case with 
those forms (and they are many) which have alternate food plants. 
The practical value of this kind of work was admirably illustrated in 
the investigation of the hop plant-louse, carried on under this Division 
some years since. In the discovery of the alternation of the food 
plants and the exact details of the life history was found a suggestion 
for aneasy and practical remedy. The present paper well illustrates 
the remarkable phenomena which are to be ascertained in this group 
of insects and will be a guide to methods of investigation and to the 
results to be expected from the study of forms of greater economic 
importance, although one species here considered has in fact been 
known to kill young birch trees. 
Respectfully, L. O. HOWARD, 
Entomologist. 
Hon. JAMES WILSON, 
Secretary of Agriculture. 
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