74 ANNUAL REPOBTS <>i DEPARTMENT OB AGBXOULTUKE, 1939 
reported from Alabama, Tennessee, and South Carolina, and during 
September from Georgia, Florida, and Missouri Although leaf 
worms reached cottonfields of all cotton-growing States except Cali- 
fornia in 1988, the infestations were generally rather Bpotted until 
late in the season. In the Southeastern Mate- the worms appeared 
late, the infestations were light, and few growers used control meas- 
ures. However, in the Delta section of Mississippi and in the 
States west of the Mississippi River many growers applied arsenical 
poisons to control the leaf worm. During L938 the leaf worm moth- 
apparently entered this country from the south only through Texas, 
tne evidence indicating that they did not enter also through Florida 
as in some years. All the arsenical insecticides used for spraying or 
dusting plant- appear to l»e effective against the leaf worm. Experi- 
ments at Tallulah indicate the value of arsenicals high in water- 
soluble arsenic pentoxide for use against the leaf worm. There was 
a direct correlation between the amount of water-soluble arsenic pen- 
toxide in calcium arsenate- and their effectiveness in causing mortality 
of leaf worms. Those highest in water-soluble arsenic pentoxide pro- 
duced mortality with, the smallest doses, and those lowest in water- 
soluble arsenic pentoxide required the largest do-*-. The cryolite- 
tested were less effective against the leaf worms than were the calcium 
arsenates. 
VARIETAL RESISTANCE OF COTTON TO INSECTS 
Work is being continued at Stoneville, Mi—.. t<» secure basic infor- 
mation for use by plant breeder- in developing varieties of cotton that 
may he tolerant or immune to the boll weevil, aphid-, and thrips. 
More than 2,000 species, varieties, and strains of cotton are available 
for study. From the 44 varieties previously studied, 12 commonly 
grown varieties were selected, the seeds of which had been self-ferti- 
lized for 4 years. Bolls 20 days old collected from each of these 
varieties which were intermediate in thickness and showed the maxi- 
mum toughness of carpel lining are being further tested to determine 
whether this type of boll continues to show the greatest resistance to 
the boll weevil. From each of these variei ies -election- have been made 
of the plants which were determinate in growth, had heavy leaf drop, 
and maintained li^rht aphid populations. Under natural conditions 
the plant- with the lightest aphid population- have been tin 1 poorest 
in producing fruit . The positive correlation between pilosity of the 
plants and aphid populations on undusted cotton ha- been further 
confirmed. Dusting with calcium arsenate increased the aphid popu- 
lation on all types of cotton. Thrips were most abundant on the pilose 
cottons, which have large cluster-tyne laid- in which the insects are 
sheltered. 
PINK BOLLWORM CONTROL 
The most outstanding developments in the pink bollworm situation 
for the 1938 crop season were the discovery of a light pink bollworm 
infestation in the Coastal Bend section of Texas, the finding of rein- 
fe-tation in the Salt River Valley of Arizona following several years 
of freedom from infestation, and the disclosure of a heavier infestation 
in t lie lower Rio Grande Vallev of Texas and Mexico. 
