44 
CHARACTER OF ATTACK. 
The actual effect upon the plant, whether chemical or physiological, 
is not clearly understood. If a few Toxoptera be placed upon a per- 
fectly healthy plant, in a few days the tissue in the immediate 
vicinity of the aphidids will take on a yellowish tinge; if the aphidids 
remain in one place for a considerable time and increase in numbers, 
the whole plant gradually turns yellow and dies, the leaves changing 
to reddish brown. 
"When the original source of infestation arises from some one or 
more points within a field, as described elsewhere in this paper, the 
plants take on a yellowish color in small, almost circular areas, (PL 
I, fig. 2) and as the Toxoptera increase in numbers the plants in the 
center die, becoming reddish brown, and the aphidids work outward 
in every direction from the center, gradually enlarging the spot until 
it may cover many acres. When a field is infested from without by 
migrating forms, the aphidids appear to spread evenly over the entire 
field and the whole gradually turns yellow, and in cases of severe 
outbreaks a whole field may die simultaneously. (See PL I, fig. 1.) 
These aphidids are essentially leaf -feeders, rarely if ever being found 
injuring the heads or fruiting parts of the plant. 
Toxoptera appears to have a more strildngly disastrous effect upon 
wheat or oats plants than any of the other common grain aphidids. 
Seemingly when in no greater numbers than other species the plants 
will succumb more quickly to the attack of Toxoptera. 
VIVIPAROUS DEVELOPMENT. 
Toxoptera graminum, as already shown, has been found to breed 
over a. wide range of country, and its behavior, under the varying 
temperatures and climatic conditions prevailing over this vast terri- 
tory, presents and opens up a broad field for investigation. 
IN THE SOUTH. 
In northern latitudes the normal manner of reproduction among 
the Aphididge is both sexually and asexually. In southern latitudes 
hese conditions, apparently, do not obtain, as here the normal means 
of reproduction seems to be asexually, each generation being com- 
posed entirely of viviparous females. 
South of about the thirty-fifth parallel, except in high altitudes, it 
appears that Toxoptera breeds continuously throughout the year 
without the appearance of the true sexes. April 6, 1906, Mr. George I. 
Reeves, of this bureau, found the eggs of a plant-louse on wheat at 
Nashville, Tenn., and Mr. Kelly found males (fig. 6), females, and 
eggs of Toxoptera at Knoxville, Tenn., in December, 1908. The 
eggs found by Mr. Reeves may have been those of Toxoptera, but we 
