394 
Journal of Agricultural Research 
Vol. XIV, No. 9 
lively, were transferred as they hatched to three healthy beet plants in 
separate cages. The insects were left off the plants until after they had 
become adults. All three plants remained healthy. On July 3 the 
insects of two of the lots were caged on two separate plants affected with 
curly-top. After 17 days they were again caged on two healthy plants. 
Both of these plants developed the disease. 
A similar test was begun on May 17, 1915. Three lots of approxi¬ 
mately 15 nymphs each were placed on healthy beets as before. At the 
same time two similar lots were transferred to diseased beets as controls. 
After the insects of the latter two lots were about half grown they were 
shifted to two healthy plants. These plants became diseased* while the 
three plants on which the first three lots were placed and kept remained 
healthy. 
Recently some work was performed jointly by the writers to verify 
the earlier results and to secure a supply of nonvirulent leafhoppers 
for laboratory experiments. Three lots of nymphs, numbering approxi¬ 
mately 50, 100, and 200, respectively, were lifted off in the manner 
described and placed on three healthy beet plants. The three lots were 
kept separate, and about 60 per cent of each grew to maturity on 
the original plants or on fresh healthy plants which were substituted 
as needed. In no case has a plant on which these insects were caged 
developed curly-top. During the same time the disease developed in 
other plants with which virulent insects had been caged under similar con¬ 
ditions. Single leafhoppers from each of the three lots have been caged 
on healthy plants without apparent effect, while at the same time viru¬ 
lent insects, caged individually on healthy plants, have quickly induced 
the disease. After having been caged on diseased plants, however, the 
nonvirulent insects have became virulent. 
These results show conclusively that uninfected insects placed on 
healthy beet plants will not produce curly-top. They are of further 
interest because the possibility of obtaining a supply of leafhoppers 
known positively to be nonvirulent opens up several promising lines of 
attacking the disease problem. For instance, nonvirulent leafhoppers 
may be used to determine whether or not other plants than beets harbor 
the virus of curly-top. The peculiar disease of the common mallow 
(Malva parviflora T.) wa?s thus shown by Boncquet and Stahl 8 to be 
caused by the same virus which causes the beet disease. 
3 Boncquet, P. A., and Stahl, C. P. wild vegetation as a source op curly-top infection op 
sugar beets. In Jour. Econ. Ent., v. io, no. 4. p. 392-397, pi. 17-18. 1917. 
