Si ANNUAL REPORTS OF DEPARTMENT OF AGBXCUL.TUBE, 1939 
[NVESTIGATIONS OF INSECTS AFFECTING MAN AND ANIMALS 
S4 io'.w WORMS AM) Hl.OW II II 3 
Several ne^ insecticides which are superior to pine-tar oil for pro- 
tecting animals against infestations by screwworms have been de- 
veloped. One of these, di phenyl amine, although not quite so effective 
as the others, now appears the most &use 
of it- availability and relatively low cost, o the 
treatment of animals with this chemical I i to the 
public. It has been found that the applical 
wounds every third day will pn from becoi 
with screwworms and other fly larvae. 
Laboratory tests with homologs of benzene as larvicides for the 
screwworm show that additions of 5 and 10 percent of thiophene and 
1" percent of naphthalene to benzene enhance the toxicity of the 
latter. In undiluted form toluene appears slightly superior to benzene 
as a larvicide for the screwworm (Y/<?< \ C. and P.), 
although the use of this material for destroying screwworms has been 
tried only under laboratory conditions. 
Progress has been made in the study of the immunity oi mammals 
to the screwworm. From a large number of tests it was determined 
that the number of screwworms constituting a maximum sublethal 
and a minimum lethal initial infestation in the average guinea 
was two and three, respectively, per hectogram of body weight. The 
maximum enhanced tolerance developed by guinea pigs to ( 'ocMiomyia 
americana larvae was found to be approximately 50 to 100 percent 
greater than that of unifested animal-. It was found that this en- 
hanced tolerance v. a- the result of the initial infestation. The toler- 
ance so developed was found to be systemic, but no substance dele- 
terious to the larvae was demonst rated in guinea pigs from a- many as 
four successive infestations. No enhanced tolerance was inherited by 
guinea pigs born of infested parents. 
Xo immunity to Cochliomyiu americana larvae was demonstrated 
by guinea pigs or sheep injected with six doses of vaccines com] - 
of (1) mature tub-reared V. americana larvae, (2) mature sterile-cul- 
tured 0. americana larvae, (3) pure cultures of P diem 
Sand., an organism commonly associated with screwv on si wounds, 
(4.) trypsin, (5) pepsin, and (<») the fluids in wounds infested with 
( . americana in guinea pigs, sheep, and goats. 
Preliminary studies on treatments of sheep and goats with a num- 
ber of organic chemicals dissolved in benzol indicate that these mate- 
rials may be of considerable value in the prevention of tl worm 
(Phormia regina (Meig.)) infestations in sheep and goats. T 
zol act- a- a larvicide and upon evaporation leaves ' the 
chemical on the w;»ol and infested areas t<> serve as a protector ag 
reinfestation. 
In Texas it has been fairly definitely established over :i three- 
season period by cage experiments and trapping surveys that t 
liomyia americana overwinters normally in restricted aria- south of 
30 X. During the coldest years the fly i- eradicated from T\ 
w'nh the exception of a -mall area in the lower Rio Grande Valley. 
Tie' overwintering area in Arizona is normally limited to the low 
valleys in the southern part of the State. In the Southeast during 
