■Feb. 16, 1924 
Effect of Light Exposure on Migration in Aphids 521 
produce eggs in the fall would argue for a chemical change in the sap of 
the host. Garner and Allard (7) have obtained correlation between the 
observed responses of the plant to change in the length of day and hydro¬ 
gen-ion concentration of the cell sap, as well as between the content of 
“available” carbohydrates. 
The experiments with Aphis sorbi would indicate that just as the 
short days of fall stimulate the production of fall migrants, so the length¬ 
ening days of spring stimulate the production of spring migrants to take 
their abode on the plantain. The length of time that this pest remains 
on apple or the production of an added generation is of extreme impor¬ 
tance in several noted apple sections. To have the spring migrants 
produced early is what the apple grower is desirous of, and if cheap light 
were available he could easily do this by artificially lighting the orchard 
for a few weeks. Orchard heaters are now in use in many sections. 
Why not lighted orchards? 
SUMMARY 
(1) The production of the sexes of several of the Aphididae under 
observation has been produced experimentally in early summer and 
appears to be governed by the length of day to which they are exposed, 
regardless of the temperature. 
Garner and Allard (6), who first made use of the principle of “photo- 
periodism,” that is, the response of a plant to the relative length of day 
and night, were able to make ordinary fall-flowering plants blossom in 
summer or at any other season by the employment of a short day. 
Both plants and plant lice appear to respond to the same stimulus as 
far as sexual reproduction is concerned. 
(2) The exposure of strawberry plants bearing Aphis forbesi to a 
short day of 7^ hours, beginning May 23, 1922, resulted in the produc¬ 
tion of both males and oviparous females in abundance by September 
18. In 1923 strawberry plants were given the same exposure at about 
the time the eggs were hatching, starting February 23. The first ovi¬ 
parous females were observed on May 7 and the first eggs were laid 
May 22. The normal appearance of the sexual forms of A. forbesi in 
Tennessee is in November. 
(3) Conversely, under a long exposure, beginning September 4 out of 
doors, the sex forms of Aphis forbesi were inhibited from appearing and 
viviparous reproduction was still taking place when examined Decem- 
ber 3. 
(4) The stimulus to migration of those plant lice having an alternate 
host was also found to be the relative length of day. By subjecting the 
secondary or summer hosts of several well-known migratory species, 
such as Aphis rumicis t Aphis sorbi , and Capitophorus hippophces t to 
short days of about 7 hours, it was possible to produce die fall mi¬ 
grants and males in the latter part of May and early June. Oviparous 
females of A. rumicis matured on snowball June 20, at the hottest time 
of the year, when the days are longest. The normal appearance of the 
sexes of these species is the latter part of October and November. 
(5) Ten generations of Aphis sorbi were produced on apple by June 
12, with the possibility of many more, as the result of keeping the apple 
seedlings in the laboratory under subdued light and giving each genera¬ 
tion a new plant. Winged forms appeared in each generation from the 
third to the tenth very sparingly, with no tendency to increase. 
