65 ANNUAL REPORTS OF DKI'AK TMKNT OF AGRICULTUKE. 1937 
exception of a narrow strip along the lower Cnlf coast, the maximum lly 
abundance on the Gulf Coastal Plain was never more than 50 percent of that 
in the area along the escarpment. 
Cochliomyia amcricana C. and P., the primary screwworm fly. overwintered 
in Texas south of the Edwards Plateau escarpment and as far eastward as San 
Antonio. Overwintering also occurred under experimental conditions at Val- 
dosta, Ga., and Gainesville, Fla. Results of hil)ernation studies indicate that 
the tiy is unable to survive the winter in Arizona or at Menard or Dallas, Tex. 
These studies also showed during the winter of lU.Sr*-.*;? a i)ossible maximum 
"carry-over" period, that is, from the egg to the death of the last fly in the 
same generation, of approximately .S months at Uvalde, Tex., and somewhat 
longer at Gainesville, Fla., and Valdosta, Ga. 
Other investigations on the biology and habits of the .screwworm indicate 
that in certain areas the percentage of infestation in wild animals is as higli 
as or higher than in livestock ; that the greatest increase in numbers of this 
species of fly is in areas having a high wild-fauna population ; and that in 
areas where control measures are efficiently carried out for livestock the fly 
pop\ilation is maintained by the breeding of the parasites in wild animals. 
Studies on the elTect of acidity and moisture content of the .soil on the 
emergence of Cochliomi/ia amcricava have .shown that grown larvae are able 
to withstand greater extremes of acidity and alkalinity than exist naturally 
in the soil. Nearly 50-percent emergence was obtained in sand one-fourth 
saturated with 0.2N sulphuric acid, and al.so in .<and one-fourth saturated 
with l.ON potassiimi hydroxide. Decreasing tolerance for acid or alkali media 
occurred with increasing .saturation. 
A distinct forward step in the control of screwworm flies has been indicated 
by the results obtained in experiments with phenothiazine as a larvicide. 
This material when applied to a wound prevents the establishment of newly 
hatched larvae in the tissues of the host but does not rep<*l flies from the 
wound or prevent oviposition. Thus injury to the animal by the parasite is 
prevented and at the .same time the breeding of the flies is greatly reduced. 
Considerable attention has been given to the testing of numerous other or- 
ganic and inorganic substances producing the .same efi'ect as phenothiazine, 
and some of these are distinctly promising and warrant further investigation. 
Other experiments for the development of larvicides and repellents for 
blowflies have shown that a mixture of 73.5 percent of a pine oil. 1.5 per- 
cent of 05-percent nicotine, and 25 percent of a wetting agent gave satisfactory 
control of all stages of the larvae of Corhlionnna amcricann in wounds. Benzol 
made miseible with water by the addition of 10 to 25 i)ercent of a wetting 
agent was much more elTective in killing screwworm larvae in wounds than 
was benzol alone, when no cotton packing was used in the wound. 
Preliminary tests were made of the following larvicide-repellent combina- 
tions: (1) A pine oil 73.5 percent, nicotine (95 percent) 1.5 percent, and a 
wetting agent 25 percent; and (2) benzol 55 percent, diphenylene oxide 20 
percent, and a wetting agent 25 percent. The tests indicated that as compared 
with benzol, when tlie latter is u.sed with cotton ]iacking and followed by 
applications of pine-tar oil. they are superior in killing the larvae, reducing 
the number and severity of infestations, and reducing the healing period 
of woiuids. 
Experiments to determine the fraction or fractions of pine-tar oil responsible 
for its fly-repellent property and its injurious elTect on the skin of animals 
have shown that a certain fraction of pine-tar oil appears to be more repellent 
to blowflies than pine-tar oil. This material, however, causes more severe 
injury to the tissues of animals. Other fractions of iiine-tar oil tested showed 
no value as fly repellents. Fraction 1 of retene oil was about as reix^llent as 
pine-tar oil but was more injurious than the latter when applied to the normal 
skin of animals. 
Attempts to infect Corhlioniuin nmcrirnnn experimentally with a species 
of KfujuiKfi, a parasitic fungus fatal to (\ wnccllaria and Phormia spp.. were 
unsuccessful. 
Sodium cyanide tone-fourth ounce per gallon of water) and carbon disulphide 
(one-eighth i>int (tf a 1 : 1 emulsion with ncMitral Turkey Red oil to 1 gallon of 
water) were both KK) percent effective in destr<\ving the larvae of CocMiomyUi 
aiiKricnvn and other blowflies in the soil when applied at the rate of 2 gallons 
]K'r .square yard of soil surface. The former .»iohition was also eflfcx'tive in 
killing pujiac of ('. (imrricnud in the soil. 
