December, ’23] 
STAHL AND CARSNER: EUTETTIX AND CURLY-TOP 
477 
emerged from the egg; this has been demonstrated experimentally in 
the laboratory as well as by field observations. Nymphs which were 
removed immediately after hatching from diseased plants and before 
they had fed on them, were placed on healthy plants and in no case 
were they able to produce the disease. After feeding on an infected 
plant, however, they were able to transmit the disorder. Important 
additional evidence bearing on this phase has been obtained in the 
field. It is often possible to find in cultivated areas and in beet fields 
in the spring large numbers of adults of this insect which will not produce 
the disease. Fields heavily infested by this jassid have been noted in 
which the disease was either absent or present only in scattered cases. 
These facts suggest strongly that the leafhopper is normally unable to 
produce the malady and that there must be a source, continuous from 
year to year, from which the virus can be obtained. 
No evidence has yet been gained to support a theory that this ac¬ 
quired ability is of any advantage to the insect. 
When a nonviriferous insect, i. e., one which does not carry the virus, 
is allowed to feed upon a plant affected with curly-top, in our experi¬ 
ments it invariably becomes able to transmit the disease to healthy 
plants. The length of the feeding period required for the insect to 
obtain the virus is evidently very short. Reports regarding this point 
vary, intervals as short as one or two minutes having been indicated 
as sufficient in some instances. The writers have produced curly-top 
in healthy plants with insects which had fed on diseased plants for 10 
minutes only. 
After feeding, a short interval apparently must elapse before the virus 
can be transmitted to a healthy plant and produce disease, four to six 
hours under high temperatures having been reported. The writers 
have been able to secure such a transmission of the disease bv means of 
j 
Entettix tenella within a period of 15 hours after the insect had fed on a 
diseased beet plant. It has been noted that at least for a certain period, 
a higher percentage of insects is able to produce the disease as the length 
of the interval between feeding and inoculation increases. The in¬ 
crease in number is greatest during the interval between 24 and 72 
hours. The evidence available at the present time, therefore, indicates 
that the transmission is not mechanical, that is, simply by the con¬ 
tamination of the mouthparts. 
When the insect is able to transmit the virus of curly-top, only a short 
feeding period on the healthy plant is necessary. A period of 5 minutes 
has been reported as sufficient for transmission but the minimum 
