40 
Journal of Agricultural Research 
Vol. XIV, No. i 
inoculated with M. solanifolii which had been feeding on a diseased 
plant for a period of 14 hours were infected, the positive symptoms 
appearing in an average time of 18.7 days. Thirty of R. persicae which 
had been feeding on the diseased plant for 14 hours were transferred to 
10 healthy plants and produced 6 infections. The symptoms appeared 
in an average time of 17.5 days. Similarly 30 of M. solanifolii which had 
been feeding on the diseased plant for 24 hours and 30 which had been 
feeding on the same plant for 48 hours were transferred to two lots of 10 
healthy seedlings each. The aphids which fed on the diseased plants for 
24 hours, produced 8 infections. The symptoms appeared in an average 
time of 17.2 days. The aphids which were allowed to remain on the 
plants for 48 hours produced 9 infections when transferred to the 10 
healthy spinach seedlings. Positive symptoms appeared in the average 
time of 18.1 days. Thirty of R . persicae allowed to remain on the diseased 
plants for 24 hours, when transferred to 10 healthy spinach, seedlings, 
produced 7 infections. The symptoms appeared in an average time of 
17 days. As a check on the foregoing experiments, 30 of M, solanifolii 
and 30 of R. persicae were allowed to feed on healthy spinach plants 
for 48 hours and were then transferred to two lots of 10 plants each, on 
which they remained for 24 hours. No infections resulted. 
The foregoing data indicate that a few aphids become virus bearers 
when they remain on diseased plants for 10 minutes. At the end of 14 
hours the individuals of M. solanifolii reached the maximum in their 
capacity to transmit the virus, whereas individuals of R. persicae of the 
24-hour group produced one more infection than did those which had been 
on the plants for only 14 hours. Probably under field conditions, where 
the insects are undisturbed, feeding takes place more readily than when 
they are more or less excited from artificial transfers, and they become 
virus bearers in much less time than experiments would indicate. Ac¬ 
cording to findings reported elsewhere, when virus-bearing aphids are 
transferred to healthy plants, there is no appreciable relationship between 
the number of infections produced and the length of time the aphids 
remain on the healthy plants, although in the present instance there is a 
distinct relationship between the time the aphids which have not previ¬ 
ously been associated with blighted plants remained on the diseased plant 
and the number of infections they subsequently produced. The longer 
the aphids remain on the diseased plants before being transferred, the 
shorter the time until visible symptoms of blight appeared on the in¬ 
oculated seedlings. 
DO IMMATURE APHIDS PROM DISEASED PLANTS BECOME NON-VIRUS-BEARERS 
AS A RESULT OP MOLTING? 
A number of Macrosiphum solanifolii and Rhopalosiphum persicae in the 
fourth instar were collected in the field from a diseased plant and placed 
in individual sterile vials for a period of 24 hours. During this time they 
