BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 
79 
resistance no fully immune plant has yet been found. Individ mil 
plants selected from the same parent stocks in 1938 for variations in 
aphid abundance were self- fertilized and the seed planted to individual 
rows in 1939. The selections for light aphid populations produced 
progeny in the ratio of 2 light infestations to 1 intermediate infesta- 
tion, while from the selections for heavy aphid infestation the ratio 
was 3 lightly infested, 7 intermediate, and 2 heavily infested. The 
variations in aphid populations appear to be associated with the 
physical characteristics of the plant rather than due to invisible factors 
of immunity. Studies of the physiological effects of calcium arsenate 
on the plant and on the aphids showed that dusting with calcium 
arsenate increased the pH value of the cell sap of the plant, and this 
in turn was positively correlated with the increase in aphid popula- 
tions. In the experiments with diluents, the effect on aphids of adding 
sulfur to calcium arsenate has not been definitely established. In some 
localities last season this mixture seemed to reduce the aphid popula- 
tion while in others the addition of sulfur had little effect on aphid 
abundance. Further investigations are needed to determine the causes 
for these variations and the effects of sulfur, since the sulfur-arsenical 
mixtures are being extensively used against several cotton insects. 
In experiments with aphicides added to calcium arsenate and 
cryolite to prevent the increase of aphids, better results were obtained 
with derris containing 0.5 percent of rotenone than with nicotine 
bentonite or nicotine bentonite-tannate. When enough derris was 
added to calcium arsenate, to mixtures of calcium arsenate and 
sulfur or diatomaceous earth, and to cryolites at the beginning of the 
season so that they contained 0.5 percent of rotenone, they remained 
sufficiently effective throughout the experiments to prevent a dam- 
aging aphid infestation from developing. 
ROOT APHIDS 
Experiments on control of cotton root aphids w T ere limited to the 
use of poisoned baits for the attending ants that place them on the 
roots of cotton. Sweetened baits made with thallium acetate and 
thallium sulfate distributed in small aluminum bait cans about 10 
feet apart on every third row gave effective control of the ants. A 
bait composed of y 2 of a pound of tartar emetic, 1 quart of cane 
sirup, and 1 pound of sugar to a gallon of water, absorbed by saw- 
dust or cottonseed hulls and distributed in small handfuls under the 
plants, likewise gave good control and was more economical and 
practical. Sawdust was less satisfactory than cottonseed hulls as a 
earlier for the bait. Twelve pounds of hulls will absorb about 2y 2 
gallons of bait and is sufficient for treating iy 2 to 2 acres of cotton. 
Best results were obtained by applying the bait just as the cot ion 
began to break through the ground and on warm sunny days follow- 
ing cool nights on ground that had been packed by rain. 
Root aphids feed on many of the common cultivated and wild 
plants and are more abundant on cotton following crops in which 
weeds develop during the fall and winter than in clean-cultivated 
crops. Studies were made of the abundance of root aphids on 
various soil-improvement crops and of crop rotations that will reduce 
their damage to cotton. 
