478 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 16 
period noted by the writers has been 20 minute for sugar beets and 10 
minutes for chickweed (Stellaria media). This indicates that only a 
small amount of virus may produce a typical case of curly-top. It has 
been reported that the larger the number of leaf hoppers feeding on a 
plant, the more rapid the development of the disease. This statement 
is contrary to the experience of the writers who have demonstrated many 
times that a single individual of this species, either nymph or adult, will 
not only produce as serious a case of curly-top as will a large number on 
a single plant, but also there is no difference in the rate of development 
of the disease. 
When once the insect is able to produce the disease, this ability is 
apparently never lost. Evidence to support this statement has been 
obtained by keeping individuals known to be viriferous on plants from 
which the virus has never been obtained and determining from time to 
time whether or not they were capable of producing the disease. In 
view of this finding, it is probable that hibernating adults might retain 
the ability of the virus transmission throughout the winter. This 
point, however, has not been tested by the writers. It is also known 
that nymphs do not lose the disease-producing ability during the molting 
process. There is apparently no evidence to support the theory that 
the nymph is more virulent than the adult. 
.An interesting fact encountered in the problem is the apparent 
inability of any species other than Eutetiix tenella to transmit the dis¬ 
ease. Although this possibility has been assumed for a number of years, 
little mention has been made of it in the literature. The writers have 
tested several species of insects from time to time and during the past 
season tests have been conducted with species commonly collected on 
sugar beets and related plants. In all cases the insects were forced to 
feed for some time on beets badly affected with curly-top. From these 
diseased plants they were transferred to young healthy beet plants 
growing under conditions especially favorable for the development of the 
disease. From one to five individuals were confined on a plant. These 
tests are still in progress but thus far all results have been negative. 
The following is a partial list of the species tested: 
Species Tested No. tested 
Cicadula sexnotata Fall. 17 
Lygus pratensis L. 12 
Nysius sp. 10 
Systena taeniata Say. 6 
Eutetiix strobi Fitch. 6 
Thamnotettix montanus Van D. 5 
Thamnotettix geminatus Van D. 6 
