25 
evident from the fact that the apple-tree aphis and the wheat plant- 
louse belong to distinct genera. 
When it appears.—The wheat plant-louse appears on winter wheat 
in September in the form of wingless females, which rapidly reproduce 
themselves, going through several generations. It occurs about the 
base of the wheat and on the roots, remaining in evidence as late as 
September 30. During the fall this louse does little damage to wheat 
growing in good, fertile soil, and after the lice leave, the plants, as a 
rule, soon recover. On poor soil, however, wheat may be seriously 
injured at this season. The method of over-wintering has never been 
discovered, but it seems probable that it hibernates on the wheat in the 
egg stage. Atany rate, the wingless female lice reappear on the wheat 
early in April and remain 
in evidence, passing again 
through many generations, 
until harvest. Throughout 
the spring and early summer 
it works on the stems and 
leaves above ground. Later 
it moves to the wheat heads, 
and very frequently these are 
simply filled with clustered 
masses of lice, which now 
assume a brownish-orange 
color. 
Natural enemies.—Fortu- 
nately this species has many Fia. 11.—Wheat plant-louse | (Wecierophona eerealis)s a, 
° é arr: winged migrant; b, nymph of winged migrant; ¢c, wing- 
natural eneilmles, including less parthenogenetic female; d, wingless female, show- 
various insect-feeding bee- ease hole of parasite—all enlarged (adapted from 
tles and flies and also true 
Internal parasites (minute four-winged flies). These predaceous ene- 
mies and parasites, in connection with other natural agencies, particu- 
larly unfavorable weather conditions, are ordinarily suflicient to prevent 
undue multiplication. 
Cause of outbreaks. —The reasons for the periods of excessive abun- 
dance or occasional outbreaks of this insect are not always easy to 
point out, but as a rule such outbreaks are due to the occurrence of 
unusually favorable climatic conditions. A rainy and fairly cool spring 
and early summer are favorable to the plant-louse, because, while not 
checking its own multiplication to any degree, and, in fact, favor- 
ing it, the conditions described prevent its predaceous and parasitic 
enemies from operating to any extent. Asa rule, therefore, the drier 
and warmer weather commonly preceding harvest enables these natural 
enemies to gain the upper hand and quickly exterminate the lice, and 
this is commonly accomplished soon enough to prevent material dam- 
age to the crop. 
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