Notes on the Bionomics of Eugonia 

 (Vanessa) Californica 



By F. Grinnell, Jr., Pasadena. 



Our interest in the California Tortoise-Shell Butterfly, has 

 ])een heightened just now by the immense swarms which have 

 occurred all over the Pacific Coast, especially in California, 

 during the year 1911. We have records of it from 1852 to date 

 whiclf give' it unparalleled importance to students of insect 

 periodicity. Mr. Ilaskin's brother reported it in great numbers 

 at Spokane. Washington; and :\Ir. C. W. Ilerr found it com- 

 mon in the adjoining parts of Idaho, at Priest River. 



In California it was especially noted and studied in North- 

 ern California, Siskiyou and Shasta counties; an extended and 

 important article by Bryant (6) was the immediate impulse 

 to write this article, and may profitably be referred to and 

 quoted from briefly. This paper by Bryant was an account 

 of work devoted especially to a study of the part played by 

 various birds in the destruction of these butterflies. The de- 

 foliated buck-brush (ceanothus) throughout Siskiyou County, 

 indicated the large extent of the depredations of the larvae. 

 The vicinity of :\It. Shasta was most afi:'ected, it being particu- 

 larly abundant at Weed, Igerna, and Sisson on the western 

 base of the mountain; the larvae were reported as abundant 

 at Marble Mountain in western Siskiyou County, and at Wea- 

 verville, Trinity County, and 35 miles east of Redding. In 

 Tuolumne County it was abundant (J. B. Curtin.) Bryant 

 spent a week during the latter part of August, 1911, at Sisson, 

 Siskiyou County, collecting data by field observation on the 

 relations of various birds and these butterflies and collections 

 of birds for analyses of the stomach contents. One hundred 

 and fifty individuals of Eugonia were counted in one square 

 foot where they had congregated to drink in some damp places. 

 "Often the ground would be blackened by them for many 

 square yards. ... In order to estimate the numbers flying, 

 counts were made of the individuals passing between two fir 

 trees al)out twenty feet liigh and standing about thirty feet 

 apart. The counts for ten successive minutes lietween -i-AO 

 and 4:50 P. :\I. on August 20, 1911," averaged 108 per minute. 

 This was going on all over Siskiyou County, and they were all 

 migrating soutliward. By half past nine they were in full 

 migration. At night they rested on trees, shrubs, buildings or 

 other convenient places. "The species of birds plainly seen to 

 eat these butterflies were the Brewer blackbird. Western King- 

 l)ird. and Western Meadow-lark," of which the most efficient 

 destroyer Avas the Brewer l)lackbird. which fed entirely on 

 this butterfly; several birds were seen to take an average of 



