June, ’18] DAVIDSON: ALTERNATION OF APHID HOSTS 289 1 
the white-fly, but they do not effect commercial control in the high 
pine lands. At Datonia the fungus was sprayed on to the trees, so that 
it was a case of the artificial use of the fungus. 
R. S. Woglum: I understand that the lime sulphur which is used in 
Florida prevented the development of the fungus. In your paper the 
lime sulphur did not appear to have any effect on the fungus in this 
State. 
0. F. Burger: As I recall the experiments, lime sulphur did check 
the fungus, as well as the insects, but where the trees were sprayed 
with Bordeaux, the fungus was unable to thrive, but the insects multi¬ 
plied rapidly. 
G. P. Gray: The difference in the effect of sulphur may be different 
according to the fungus in question, as for instance, the downy mildew 
and the powdery mildew. 
R. S. Woglum: How does the Entomophthora fungus enter the 
insect? 
O. F. Burger: The spores of the fungus are discharged from the 
conidiophore with considerable force. Adhering to each spore is 
considerable protoplasm, so whatever object the spore strikes it be¬ 
comes glued to it. If the spores happen to hit an insect it becomes 
glued to the body. The spore then germinates and the hyphae enter 
the insect. 
President G. P. Weldon: The next.paper is by William M. 
Davidson, on “ Alternation of Hosts in Economic Aphids,” which 
will be read by A. F. Swain. 
ALTERNATION OF HOSTS IN ECONOMIC APHIDS 
By W. M. Davidson, U. S. Bureau of Entomology 
In recent years considerable strides in forwarding our knowledge of 
the peculiar habit of many of the Aphididce to the alternation of host 
plants have been made. 
This work has centered mainly about plant-lice of economic interest. 
True alternation of hosts implies a summer host or hosts and a 
winter host or hosts, the latter harboring the egg or dormant stage and 
a series of actively feeding spring generations and the former support¬ 
ing only actively feeding generations through the summer and fall. 
Almost invariably the summer and winter hosts are widely separated 
in a botanical sense. Frequently there is more than one summer host 
and these are not necessarily close botanical relations. On the other 
hand the winter hosts are generally few and always, when more than 
one, closely related. Thus we find the Hop Aphis (Phorodon humuli 
