THE RED SPIDER ON HOPS IN CALIFORNIA. 17 
NATURE OF DAMAGE. 
GENERAL EFFECT OF MITES UPON FOLIAGE. 
These mites feed upon the juices and cell contents which they 
suck from the tissues of the host plant. This extraction of cell 
contents usually results in the formation of a light spot at the point 
where the mite has fed. The presence of mites upon a vine during 
the early summer is readily detected by these yellow spots in the 
surface of the leaves. As the mites increase in numbers the leaves 
become more "speckled," turn yellowish, and when severely injured 
dry up and fall to the ground. Severely infested vines, at the time 
that the hop cones are forming, were observed to be very yellow and 
to lose many leaves. 
The decrease in the vitality of the vine, which is the direct result 
of the attack by mites, produces a premature ripening of the hops. 
The hop cones in infested yards were much further advanced than 
those in uninfested hopyards under similar soil and climatic con- 
ditions. This premature ripening results in a decrease in yield and a 
weakening of the roots, tending to decrease the crop of the following 
year. 
RELATIVE EFFECT OF MITES UPON MALE AND FEMALE HOPVINES. 
The male vines throughout the season showed more immediate 
and serious injury than the surrounding female vines. In many cases 
they were almost entirely defoliated (PL II, figs. 1, 2), while the 
neighboring female vines were but slightly injured. 
Some leaves which expanded late in the season on the lower parts 
of the pistillate vines were thin and papery, appearing identical in 
texture with the leaves of the male vine, and showed an injury similar 
to that of the male foliage. This severe injury to the male vines 
appears to be due largely to the nature of the foliage and not to the 
presence of a larger number of mites than are found upon the female 
vines. 
EFFECT UPON THE QUALITY OF THE HOPS. 
Although the foliage upon the arms of the vines was infested at 
the time the hops were coming out of the burr, the mites were not 
observed actually to feed upon them until they were nearly full-sized. 
No direct injury was noted when the mites were first found upon 
the cones, but soon reddish-brown spots appeared upon the scales 
and gradually the natural green and yellow turned to reddish brown, 
the scales became scraggly, and in severe cases the hop cones became 
so brittle that they could not be picked. (See PL V, fig. 1.) 
