THE RED SPIDER ON HOPS IN THE SACRAMENTO 
VALLEY OF CALIFORNIA. 
INTRODUCTION. 
The common "red spider" (Tetranychus himaculatus Harvey) has 
long been known as a pest in flower gardens and greenhouses, where 
it is often very injurious on ornamental plants. This damage, how- 
ever, becomes quite insignificant when the injury to hop vines is con- 
sidered. During the past few years this mite has become recognized 
as one of the most injurious of hop pests on the Pacific coast and 
especially so in the Sacramento Valley of California. 
The investigation upon which this bulletin is based was begun 
January 1, 1911, at the request of the horticultural commission of 
Sacramento County and in cooperation with the E. Clemens Horst 
Hop Co., of San Francisco, and extended over a period of 18 months. 
Observations were made throughout this period in the 'hop fields in 
all parts of the Sacramento Valley, and spraying experiments were 
conducted on both a large and a small scale. 
The recorded efficiency of the various insecticides tested during this 
investigation is based upon actual counts of mites present before and 
after spraying, as well as upon general observations under ordinary 
field conditions. 
ECONOMIC IMPORTANCE. 
The red spider was reported in injurious numbers in the hop fields 
at Wheatland, Yuba County, Cal., in 1902. Since then it has done 
some injury to hops in the Sacramento Valley every year and in 1910 
the hop crop was seriously injured in many localities. One company, 
the E. Clemens Horst Co., estimated their financial loss due to this 
mite in a few of their hopyards near Sacramento, Cal. (allowing 14 
cents per pound for hops), to be from $10 to $68 per acre. Other 
growers were not able to pick their hops so rapidly as did this com- 
pany, and their loss was necessarily much greater. In some yards 
near Sacramento the hops were so badly injured that they could not 
be picked at all and w.ere allowed to remain in the field. 
Note.— Obligations are due to the manager of the E. Clemens Horst Co. and to the superintendent of the 
ranches at Perkins and at Wheatland, who placed at my disposal every means possible to cany on the work; 
to Prof. W. B. Herms, for many valuable suggestions; to Mr. E. K. Carnes, superintendent of the State 
insectary, Sacramento, Cal., who generously provided me with desk room; and to Dr. F. H. Chittenden, 
under whose directions the investigation was carried on. — W. B. P. 
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