erent
species vary greatly but for these studies any available species will be
satisfactory.

The plant-louse or aphis is a sap-sucking insect which feeds and
multiplies rapidly often seriously injuring crops. The loss of sap
together with the poisoning effect of the bite causes the weakening of
the plant or leaf with its ultimate death if feeding continues. The
greatest damage is usually done during cold springs or during a cool
rainy period. This prevents the enemies of the louse from increasing and
attacking it while the weather may not be too severe to prevent the
louse from working. Under favorable climatic conditions the natural
enemies of the louse as a rule are able to hold it in check. The
principal enemies of the louse are certain small insect feeding birds,
lady-beetles, syrphid-flies, lace-wings and tiny wasp parasites. The
beneficial work of the lady-beetles is discussed in an earlier chapter.
The birds and lady-beetles devour them bodily, the larvae of the
lace-wings and syrphid-flies extract their blood while the wasps live as
internal parasites.

In the latitude of Missouri the plant-lice as a rule live thru the
winter in the form of a fertile egg attached to the twigs of trees and
shrubs. The winter egg is produced by a true female plant-louse. As a
rule there is only one generation of true males and females produced
each year. This brood develops late in the fall to produce the
fertilized winter eggs. In the spring these eggs hatch and the tiny
nymphs begin to extract sap. On maturing they begin to give birth to
young lice. Throughout the summer this method of reproduction continues.
These summer forms are known as the stem mothers or agamic females.
These are not true females for they produce living young in place of
eggs and during the summer no male lice are produced at all. This is
nature's way of increasing the race of plant-lice rapidly. Late in the
fall again a brood of true males and females is produced. During the
summer the plant-lice increase more rapidly than any other type of
insect.

[Illustration: Black winter eggs of Aphis showing how they are deposited
in masses on twigs of apple. (After U. S. Dept. Agri.)]

Plant-lice vary in size, color and general appearance. Many are green
while some are red or black or covered with a cottony secretion.

OBSERVATIONS AND FIELD STUDIES

Plant some melon, radish or other seeds in fertile soil in pots for use
in this study. When lice appear