December, ’24] 
severin: beet leafhopper fluctuations 
643 
Natural Enemies. —There are several secondary factors which 
reduce the number of beet leafhoppers in a natural breeding area. 
In all probability, the most important of these are the natural enemies 
of the leafhopper, a subject which has already been discussed in a 
previous paper (6). No information is at hand as to the value of egg 
parasites on the plains and foothills, and until more is known con¬ 
cerning these beneficial insects, Hartung (2) is hardly justified in stating 
that the problem of periodicity is associated with the fluctuation in the 
number of egg parasites. 
Spring Migrations. —Spring migrations reduce the number of beet 
leafhoppers in a natural breeding area. 
Summer Migrations. —Summer migrations may deplete to a large 
extent the natural breeding grounds of the beet leafhopper; the nymphs 
and adults which remain behind are then hard-pressed by parasites and 
predaceous enemies. The observations on the summer migrations from 
the San Joaquin Valley during 1919 have not been published and the de¬ 
tails follow. 
A trip was taken to the southern part of the San Joaquin Valley on 
July 24 to 25, with Mr. J. W. Hartung, and it was found that a summer 
migration of the pest had occurred. In the Connor and Corcoran beet 
districts the sugar beets not harvested were mostly dead owing to curly 
leaf, but the adults were scarce on the green innermost leaves with 
dried outer foliage of such beets as could be found. Sweepings were 
made on some of the most favorable host plants, such as the saltbushes 
growing in and along the margin of beet fields, but the leafhoppers had 
not assembled on these plants. During April and May, thousands and 
thousands of hoppers were found on various species of Atriplex, but in 
% 
July it was difficult to secure 100 adults on these same plants to de¬ 
termine the percentage of parasitism. 
In the middle portion of the valley the beet leafhoppers were ex¬ 
tremely abundant on Russian Thistles and various saltbushes during 
the spring, but on July 26, the adults were rare on these plants growing 
in irrigated fields. Along the roadsides and in the fields that had not 
been irrigated the Russian Thistles attained a few inches of growth and 
died, owing to a shortage of rain and probably also to the drain of 
enormous hordes of bugs. 
In the northern section of the San Joaquin Valley enormous numbers 
of adults had congregated on June 26, on Bractscale (.Atriplex bracteosa) 
growing among blighted sugar beets in the vicinity of Hatch Station. 
The foliage of these saltbushes was covered with droplets of clear 
