20 THE RED SPIDER ON HOPS IN" CALIFORNIA. 
EXPERIMENTS FOR THE CONTROL OF THE RED SPIDER. 
METHODS OF EXPERIMENTATION. 
The sudden and widely distributed appearance of the red spiders in 
the hopyards, the rapidity of their increase, and the fact that the old 
remedy (sulphur) had not controlled the mites in the hopyards during 
the preceding season made the control problem appear a difficult one. 
As soon as the mites became numerous enough on the hop vines to 
warrant experimental work, the first of a long list of experiments 
which was planned during the spring of 1911 was applied. The lime- 
sulphur solutions, because of their efficiency and relatively low cost, 
soon became the most promising of the contact insecticides. They 
were therefore applied more extensively and were given more atten- 
tion than the other materials which were listed during the investiga- 
tion. 
The materials were applied, except in the small-scale work, with a 
power outfit (PL III, fig. 1) which maintained from 120 to 150 pounds 
pressure. Seven-foot rods tipped with a nozzle throwing a fine but 
washing spray were used for the work. 
In making the tests with the various insecticides it was found 
desirable to express their efficiency with as near an approach to 
numerical exactness as possible. Tag counts and field counts were 
employed in obtaining the percentage of mites killed and were found 
to be very accurate. In most cases only one of these methods was 
used on a plat, but occasionally both were employed. These counts 
were made both before and after spraying. 
Tag counts. — Twenty tags were attached to the petioles of as many 
leaves, and the numbers of mites found upon the several leaves, both 
before and after spraying, were recorded upon the attached tags. 
The percentage of mites killed by the spray was thus very accurately 
obtained. 
Field counts. — Field counts were made by picking 20 leaves from 
various parts of as many vines chosen throughout the plat. The 
numbers of mites found upon the leaves, which were taken b'oth before 
and after spraying, were compared and the precentage of mortality 
obtained. This method gave a good idea of field conditions but did 
not prove as effective as the tag count. 
SULPHUR. 
Sulphur in a dry and finely divided form has in past years been used 
very extensively against red spiders with supposedly good results. 
One experimenter states that although the sulphur does not affect the 
adult mites, it kills the larvae when they emerge from the eggs and 
thus checks the infestation. Several hop growers stated that the red 
