308 
Journal of Agricultural Research 
Vo]. XXVIII, No. 4 
Table VI.— Length of incubation period in the leaf hopper before becoming viru - 
liferous 
Date started 
Experi¬ 
ment 
No. 
Length of time 
Number of in¬ 
sects 
On 
curly- 
top 
plant 
On 
healthy 
plants 
Total 
Tested 
Produc¬ 
ing the 
disease 
Season of, 1918: 
Hours 
Hours 
Hours 
July 17 .. 
la 
24 
24 
48 
20 
1 
Do. 
16 
24 
72 
14 
5 
Do_ 
2 a 
24 
24 
48 
20 
5 
Do. 
26 
24 
72 
20 
9 
August 14_ 
3a 
24 
24 
48 
30 
5 
Do. 
36 
24 
72 
30 
6 
September 2__ 
4a 
24 
48 
30 
10 
Do_— 
46 
24 
72 
28 
22 
Do.. 
4c 
24 
96 
26 
15 
November 8.. 
5a 
5 
18 
23 
26 
11 
Do.- 
56 
73 
96 
24 
22 
Season of 1920: 
August 23- 
6 a 
3 
19* 
22 * 
5 
Do. 
66 
9 
31* 
5 
Do. 
6 c 
15 
46* 
5 
1 
Do. 
6 d 
72 
120 
5 
1 
Do.- 
7a 
■ 7 
15* 
22 * 
4 
1 
Do_ 
76 
4 
31* 
4 
3 
Do. 
7c 
15 
46* 
4 
Do. 
7 d 
72 
120 “ 
4 
2 
September 13. 
8 a 
51 
17 
22 * 
10 
Do_ 
86 
7* 
30 
10 
1 
Do. 
8 c 
17* 
47* 
10 
3 
Do. 
8 d 
144 
192 
10 
4 
September 14. 
9 a 
4* 
17* 
21 * 
10 
2 
Do_ 
96 
74 
29* 
10 
Do. 
9c 
72 
96 
10 
3 
September 22. 
10 a 
6 
2 * 
240 
10 
Do.. 
106 
15 
23* 
5 
Do. 
10 c 
8 
31* 
10 
1 
Do. 
10 d 
6 
47* 
10 
2 
Do. 
10 c 
8 
55* 
10 
1 
Do. 
10 / 
16* 
72 
10 
Do. 
10<7 
240 
312 
10 
4 
Remarks 
Inoculated plants in 5-inch pots. 
Plants in bed in green-house. 
More favorable conditions for 
growth may have been a factor. 
One insect which produced the dis¬ 
ease in the first period failed to 
do so in the second period, so 
that five more than in the first 
period were then effectively 
viruliferous. 
Stellaria media plants were used 
instead of beets. 
One insect which caused disease in 
the first period did not cause it in 
the second, so there was in the 
second period an increase of 13 
that were effectively viruliferous. 
SteUaria media plants were used 
instead of beets. 
The second period was so much 
longer than the first that a fair 
basis of comparison is hardly 
afforded. 
One of the four insects did not pro¬ 
duce the disease at all. 
Insect No. 8 produced the disease. 
Nos. 5, 7, and 10, produced the 
disease. 
Nos. 6, 7, 9, and 10, produced the 
disease; four of the insects did 
not produce the disease at all. 
Nos. 2 and 7 produced the disease. 
Nos. 2, 5, and 7 produced the 
disease; seven of the insects did 
not produce the disease at all 
No. 10 produced the disease. 
Nos. 1 and 6 produced the disease. 
No. 2 produced the disease. 
No. 1, 4, 6, and 10, produced the 
disease; five of the insects did 
not produce the disease at all. 
VARIATION OF INDIVIDUAL INSECTS IN REGARD TO INFECTION 
In the attempt to determine whether or not there is any uniformity of behavior 
among individual leafhoppers in regard to producing the disease, a considerable 
number of insects were caged singly on individual plants, under similar conditions, 
on beets or other susceptible plants for periods of 24 hours or longer on consecu¬ 
tive days. Tests of this nature were repeatedly made, using 3 to 30 individual 
insects in a test. The results of two of these tests or series are brought together 
in Table VII. 
