LETTER OF TRANSMITTAL. 
United States Department of Agriculture, 
Division of Entomology, 
Washington, />. C, February 1, 18!)7. 
Sir: I have the honor to submit for publication, under the title a A 
Study in Insect Parasitism," an account of the principal factors which 
brought about the almost total disappearance of tussock-moth cater 
pillars on the shade trees in the city of Washington during' the summer 
of 1896, after an extremely injurious outbreak of this insect which took 
place during the summer of L895. The details of this study may be con- 
sidered in a degree typical of many sudden interruptions of the rapid 
multiplication of injurious species of insects. On account of the fact 
that the detailed consideration of the problem involves the close rec- 
ord of many details of little popular interest, as well as the description 
of a certain number of new species, it seems best that the paper should 
be published in the Technical Series of bulletins of this division. 
Respectfully, 
L. O. Howard, 
Entomologist. 
Hon. .1. Sterling Morton, 
Secretdry of Agriculture. 
