44 BULLETIN 879, U. S. DEPARTMENT OF AGRICULTURE. 
this has resulted in the infection of all the plants on which the insects 
were placed (PI. II, B). Aphicls transferred from one healthy plant 
to another have never produced the disease. 
The high percentage of mosaic infection obtained from inoculation 
by means of aphids from diseased plants (Table XVII) is probably^ 
due to the fact that, being a sucking insect, it introduces the virus 
into those tissues which will distribute it most rapidly throughout 
the plant. The number of aphids necessary to produce infection 
is small, infection having occurred where only three of the insects 
were placed on each plant, but where only one to three individuals 
are used the percentage of infection is often lowered. 
TRANSMISSION BY DIABROTICA VITTATA. 
J 
Although the cucumber aphis may be responsible for severe epi- 
demics of mosaic, the striped cucumber beetle (Diabrotica vittata 
Fabr.) is probably the most important insect agent in the transmis- 
sion of the disease. While the aphis can cause extensive and rapid 
mosaic dissemination when present in large numbers, it appears 
only at intervals in most districts and is then often confined to a few 
fields. The striped beetle, however, is common to most cucumber- 
growing sections and usually occurs in considerable numbers through- 
out the season. It is a very active and voracious insect and attacks 
practically all parts of the plant, but more particularly the leaves. 
While aphids will nearly always produce infection when transferred 
from mosiac to healthy plants, the percentage of infection is much 
lower in the case of the beetle. Being a chewing insect, it trans- 
mits the disease only when mosaic plant juices are carried on its 
mouth parts to wounds in healthy plants and are then distributed 
throughout the tissues. Many of these insect wounds are slight, 
however, and are often located in the blossoms and at other points 
which are not favorable to the rapid distribution of the virus in the 
plant, so that it is probable that drying out often prevents infection. 
This low percentage of infection is offset by the fact that the beetles 
are numerous and active, constantly feeding first on one plant, then 
on another, with the result that the chances for infection are many. 
In localities where beetles have been numerous the disease has spread 
very rapidly and usually over a wide territory, as the insect often 
travels considerable distances. 
Experimental work has given definite proof of the agency of beetles* 
in mosaic transmission. Beetles captured directly from mosaic 
vines and placed on healthy plants under cages have produced the 
disease in numerous cases. Usually a considerable number of in- 
sects were placed ,in each cage, owing to the lower chances of infection 
before mentioned, but the numbers were no greater than were com- 
