BULLETIN OF THE 



IT 



No. 100 



Contribution from the Bureau of Entomology, L. O. Howard, Chief 

 August 31, 1914. 



(PROFESSIONAL PAPER.) 



WALNUT APHIDES IN CALIFORNIA. 



By W. M. Davidson, 

 Scientific Assistant, Deciduous Fruit Insect Investigations. 



INTRODUCTION. 



The study of walnut aphides dealt with in the following pages was 

 begun early in the year 1911 and continued until the summer of 1913. 

 The observations were at first made at San Jose, Cal., but after Sep- 

 tember, 1912, the work was done chiefly at Walnut Creek, Cal. Prac- 

 tically all the life-history observations were made at the former 

 locality, and much of the control work was done at Walnut Creek. 

 The habits of the aphides do not vary materially throughout Cali- 

 fornia. It was at first the writer's intention to confine his studies to 

 the European walnut aphis (Ohromaphis juglandicola Kalt.), as this 

 species alone infests walnuts of commercial value grown in California, 

 but latterly two native species of Aphididse were found to be pests 

 on native walnuts much used for stock on which to graft the European 

 or Persian nut, and thus the studies were extended so as to include 

 all three species. The two native aphides above mentioned are Monellia 

 caryse Monell, the American walnut aphis, which affects the eastern 

 black walnut (Juglans nigra) and Monellia caryella Fitch, the little 

 hickory aphis, 1 which affects the California black walnut (Juglans 

 calif ornica) . Both of these species infest the Royal Hybrid walnut 

 (a cross between the eastern black walnut and the California black 

 walnut), while the Paradox Hybrid walnut (a cross between the 

 European walnut and the California black walnut) is attacked by the 

 European walnut aphis and to a lesser extent by the little hickory 

 aphis. Both of these hybrids are rapid growers, and a certain per- 

 centage of the seedlings obtained from the crossings makes good 

 stock on which to graft the commercial varieties of nuts. The great 

 majority of European nuts and their varieties are grown in California 



1 This is the name Fitch gave to species which he found on hickory, and it seems best to retain it, 

 although rather an unfortunate title in so far as California is concerned, as the only wild member of the 

 Juglandacese in that State is Juglans californica. 



40859°— Bull. 100—14 1 



