BULLETIN OF THE 
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 (Chromaphis juglandicola Kalt.), as this 
species alone infests walnuts of commercial value grown in California, 
but latterly two native species of Aphididaa 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 
Juglandaceae in that State is Juglans californica. 
40859°— Bull. 100—14 1 
