32 
Journal of Agricultural Research 
VoL XIV, No. i 
the first or second instars are used. The percentage of infections also 
is increased according to the age of the aphids when they are transferred 
from disease to healthy plants. 
TablS VI .—Infectivity of mature and immature aphids 
Species. 
Instar. 
Num¬ 
ber of 
insects 
to 
plant. 
Num¬ 
ber of 
plants 
inocu¬ 
lated. 
Num¬ 
ber of 
plants 
in¬ 
fected. 
Aver¬ 
age 
length 
of incu¬ 
bation 
period. 
Num¬ 
ber of 
plants 
remain¬ 
ing. 
healthy. 
Macrosiphum solanifolii . 
First ... 
4 
IO 
1 
Days, 
22 
9 
6 
Do! .. 
Second . 
4 
IO 
4 
19 
18.5 
17. 6 
17. 2 
14 
Do . 
Third . 
4 
IO 
4 
6 
Do . 
Fourth . 
4 
IO 
8 
2 
Do . 
Fifth alate . 
4 
IO 
9 
7 
1 
Do . 
Fifth apterous . 
4 
IO 
3 
IO 
Rhopalosiphum persicae . 
First. . ! . 
4 
IO 
Do. / . * . 
Second . 
4 
IO 
3 
4 
18. 2 
7 
6 
Do . 
Third . 
4 
IO 
19 
16. 2 
Do . 
Fourth . 
4 
IO 
6 
A. 
Do . 
Fifth alate . 
A 
IO 
7 
8 
18 
% 
Do .... 
Fifth apterous . 
4 
IO 
15*5 
0 
2 
The results obtained with Rhopalosiphum persicae were similar to those 
obtained with Macrosiphum solanifolii, especially in regard to the in¬ 
creased number of infections obtained with mature aphids over those 
resulting from the transfers of the immature stages. The length of the 
incubation period of the disease is also decreased where the older aphids 
are used. No marked differences could be observed in the ability of 
the two species to produce infections. 
ABILITY OF APHIDS TO CARRY INFECTION TO MORE THAN ONE 
HEALTHY PLANT 
A series of experiments were started to determine whether an aphid 
can carry the virus of the disease to more than one healthy plant after 
leaving the original diseased plant. Three series of inoculations were 
made. In the first series the insects were allowed to remain on the 
plants for 24 hours; in the second, 14 hours; and in the third, 2 hours. 
The diseased plants from which the aphids were obtained were carefully 
checked out by means of prick inoculations with virus, and positive 
proof of their being affected with the disease was thus obtained. The 
healthy plants used in the inoculations were 4 weeks old at the time the 
aphids were placed on them. Three or four plants were allowed to 
grow in 4-inch pots and the aphids were placed on these for each period. 
This gave a record of several plants in each series. The results of this 
experiment are given in Table VII. In comparing the results obtained 
with the transfers for the several periods it will be seen that where the 
insects remained on the plants for 24 hours the incubation periods were 
