16 LEAFHOPPERS AFFECTING CEREALS, ETC. 
reticulata. For the grass crop, including timothy, brome grass, and 
bluegrass, the most important species are Deltocephalus inimicus, 
D. affinis, D. configuratus, Dreculacephala mollipes, and Phlepsius 
irroratus. For clover, alsike, alfalfa, soy beans, and leguminous 
crops the most important are Agallia sanguinolenta and Empoasca 
mali. 
The fact that in many parts of the country their injury is negligible 
for such crops as wheat, oats, rye, etc., is due to the rotation or alter- 
nation of crops in such manner as to make their rapid increase impos- 
sible. On the other hand, the conditions existing in permanent 
pastures and meadows or that prevail where wheat, oats, etc., are 
grown closely adjacent to considerable areas of permanent grassland 
furnish favorable opportunity for their multiplication and migration, 
and serious injury must inevitably follow. One of the strongest 
contrasts in this line is furnished by the methods of wheat culture in 
the North and South. Throughout most of the spring-wheat section 
of the Northwest and the winter-wheat section of the northeastern 
United States the complete system of rotation or the absence of 
adjacent grass areas at the time when wheat fields could be infested 
renders injury from these insects almost unknown. In a number ef 
the Southern States, however, the abundance of the grasses adjacent 
or the overlapping of the seasons permits a serious autumn infestation 
of the fields of winter wheat, rye, and oats and a consequent annual 
loss from this source. This is especially true of the Piedmont Plateau 
in South Carolina and Georgia, where the prevailing practice of ter- 
racing (see Pl. II, fig. 3) to prevent washing of the hillsides results in 
permanent strips of uncultivated and permanent grassland, including 
a mixture of many kinds of useless weeds. Furthermore, the size 
of the fields must be an important factor in the extent of infestation 
from adjacent fields and consequent injury. Where the fields cover 
hundreds or thousands of acres, opportunity for infestation is far 
less than where they cover but a few acres and are interspersed with 
permanent grasslands. 
In the extensive stock-grazing regions of the central-western and 
northwestern United States where there are extensive permanent 
pastures, and notably the great area of wild grazing land (see Pl. 
III, fig. 1), both prairie and woodland, these insects have the best 
opportunity for production of successive generations each season and 
their number is limited only by the ability of the plants to sustain 
them or by the control affected by natural enemies, such as the para- 
sitic or predaceous insects, spiders, birds, etc., that feed upon them. 
DERIVATION OF OUR LEAFHOPPER FAUNA. 
Inasmuch as several economic problems are dependent on a knowl- 
edge of the source of our present leafhopper fauna, it is desirable that 
this matter should be touched upon, although it must be admitted 
