Development of insect resistance to insecticides

Material Information

Development of insect resistance to insecticides
Portion of title:
Development of insect resistance to insecticides a critical review of the literature up to 1951
Babers, Frank H
Pratt, John J
United States -- Bureau of Entomology and Plant Quarantine
Place of Publication:
[Washington, D.C
U.S. Dept. of Agriculture, Agricultural Research Administration, Bureau of Entomology and Plant Quarantine
Publication Date:
Physical Description:
45 p. : ; 27 cm.


Subjects / Keywords:
Insecticide resistance ( lcsh )
Insecticides ( lcsh )
bibliography ( marcgt )
federal government publication ( marcgt )
non-fiction ( marcgt )


Includes bibliographical references (p. 30-45).
General Note:
Caption title.
General Note:
General Note:
"May 1951."
Statement of Responsibility:
by Frank H. Babers and John J. Pratt, Jr.

Record Information

Source Institution:
University of Florida
Rights Management:
This item is a work of the U.S. federal government and not subject to copyright pursuant to 17 U.S.C. §105.
Resource Identifier:
030323703 ( ALEPH )
780436671 ( OCLC )

Full Text

May 1951

United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant (u'ixrantine

A Critical Review of the Literature up to 1951

By Frank H. Babers and John J. Pratt, Jr.
Division of Control Investigations

In May 1949 the senior author (Babers 9) published a review
covering the literature up to the fall of 1948 on the subject of the
development of resistance by insects to insecticides. At that time
only a few cases of the failure of DDT to control house flies in the
field had been reported from this country, and the potential signifi-
cance inherent in these failures was apparently not universally recog-
nized. Recently, however, these failures have received great publicity
in scientific publications and in the press.

In the earlier review it was noted that the following species had
been reported as having developed resistance to insecticides:
California red scale (Aonidiella aurantii (Mask.)), black scale
(Saissetia oleae (Bern.)), citricola scale (Coccus pseudomagnoliarum
(Ku.), 7 coing moth (Carpocapsa pomonella (L.)), vinegar fly
(Drosophila melanogaster Meig.), cotton aphid ( Aphis gossypii Glov.),
confused flour beetle (Tribolium confusum Duv.), blue tick (Boophilus
decoloratus (Koch.)), citrus thrips (Scirtothrips citri (Moult.)),
screw-worm (Callitroga americana (C. & P.)), an Italian subspecies of
the northern house mosquito (CUiex pipiens autogenicus), house fly
(Msca domestic L.), and San Jose scale (Aspidiotus perniciosus Comst.).

It is the purpose of the present review to bring the subject of
the development of insect resistance to insecticides up to date and
to include a critical discussion of some of the causes of the various
discrepancies in results reported by the several observers.

California Red Scale

Several papers overlooked in the earlier review have been noted.
Busbey et al. (35) found that second-stage nymphs of the strain of red
scale (Aon-dielTa aurantii (Mask.) resistant to hydrccyanic acid were
more susceptible to trichloroacetonitrile than were the adult ilisects.
The reverse is true when the insects are tested against hydrb-:.,yanic
acid. The normal strain was not tested. Yust, 'elson, and Fusbey (171)
studied the effect of inbreeding on the susceptibility of the red scZ-e
to hydrocyanic acid. After several years no difference in susceptibility
attributable to the inbreeding was noted.

Gressman (40O) found large differences in the mortalities of scales



from various groves when the insects were sprayed in the field with
comparable dosages of oil, or collected in the field and sprayed in
the laboratory. Scales collected from groves in which extreme dif-
ferences were noted arid reard- in the laboratory for several genera-
tions showed almost no differences in susceptibility when tested at
intervals over a period of about 9 months. Cressman concluded that
most of the observed variations in the susceptibility of field stocks
of red scales to oil sprays were due to other than genetic differences.
Whether or not any of the strains tested were those considered to be
resistant to hydrocyanic acid was not stated. It is not clear how
Cressman reached his conclusions concerning lack of genetic differences
from the type and small amount of data presented. In the previous
review (9) it was mentioned that the resistance of red scale to hydro-
cyanic acid had been shown by several investigators to be inherited.
According to Lindcren and Dickson (91) there was no difference between
the susceptibility of laboratory-reared stocks of the two strains to
oil sprays. Cressnan did not refer to this earlier work. Munger (119)
compared the rate of development of red scales resistant to hydrocynic
acid with nonresistant insects and found no significant differences.
He also (12C) determined the effect of temperature on the biology of
the two strains, and again no significant differences between the
strains were noted.


Kalina (70) tried to develop a resistant strain of Drosophila
melanogaster by rearing larvae produced from eggs laid by normal
females in a medium containing 5 parts per million of DDT. Larval
development was somewhat slower than normal but up to the time of early
pupation no real difference was noted. In the later stages of pupation,
development was abnormal. Adults usually failed to emerge, and those
that did exhibited signs of DDT poisoning and soon died. The specula-
tion was made that DDT was stored in the larval fat and was released
during histolysis in the pupae. Whether or not experiments were per-
formed using lower concentrations of DDT was not stated.

A strain of Drosophila resistant to DFDT (l,l,l-trichloro-2,2-bis
(p-fluorophenyl)-ethane) was developed by Reimschneider and Rohrmnann
(T37). After 32 generations of selection, resistance had developed so
tat an exposure to deposits of DFDT that caused 70 percent mortality in
the normal strain caused 30 percent mortality in the resistant strain.

Resistance to carbon dioxide by Drosophila is apparently a normal
condition, whereas susceptibility is abnormal. The sensitivity seems
to depend on the presence in the cytoplasm of an agent called a Fenoid.
Lnieritier and his .isociates (8E6,8.889.`C) shower' that the siceutibilit"
to carbon dioxide could be transmitted by transplanting from susceptible
to normal individuals ovaries, brain tissue, or imaiinal discs of the
eyes, lefs or wxi;s, or portions of the gut. Hernolymph as well as the
centrifuged supernatant of crushed susceptible flies at any stage was
si coTrJ to be infectious. Under certain conditions the exposure of sus-


ceptible flies to heat will "cure" their germ cells, and it is thus
possible to obtain a genoid-free stock from any sensitive stock. Th-iey also
found that X-rays would inactivate the genoid (87). This and further work
by L'Heritier raised the question as to the possible similarity of the
genoid to a virus, and L'Heritier and his collaborators are now a.pcroach-
ing the subject by using techniques applicable to virus research. Such
an approach will, if successful, throw light on the relation between
viruses and genes and will have far reaching applications.

According to Goldstein (58, $9), if "nonnrmal" Drosophila males are
made "sensitive" by the injection of sensitive hemolymph, they transmit
the sensitivity to only a small percentage of their progeny, whereas if
the males are normally sensitive a higher percentage of transmission
occurs. It was found that the two types of males could be obtained from
any stock. To explain this, it was siIvested that the genoid of the
synthetic sensitive stock had undergone a mutation limiting strongly
its capacity to infect the male germ. Plus (133) determined the time
required (incubation time) for Drosophila to become susceptible to
carbon dioxide following injection with a suspension of genoids. If
moderately high concentrations of genoids were used, a linear relation
was observed between incubation time and the log of the number of in-
fectious units injected. As an appendix to Plus's paper L'Heritier
presents a mathematical treatment of these results. A review by
L'Heritier (85) covers the subject of carbon dioxide sensitivity up
to about 19h7.

There is considerable variation in the susceptibility to ether
among Drosophila virilis, D. americana, and D. texana. Crow (1hl) found
that when the dosage was varied and the time of exposure held constant
virilis was the most resistant, followed by americana and texana. These
observed differences were small. When the dosage was constant and the time
of exposure varied, virilis was the most resistant, then texana, and
finally americana. These differences were large. Fox (53) prepared
rabbit anti-sera from three mutant strains of lyophilized insects. By
complement fixation tests, no differences in the anti-sera were noted.
By precipitin ring tests, however, certain differences were noted. These
differences were believed due not to the single major genic difference
but to the multiple genic differences that must exist between such unrelated


No reports of insecticide-resistant ticks in the United States have
been noted. In sections of South Africa and to a lesser extent in
Argentina, the problem is of considerable importance. Resistance to
arsenic dips by the blue tick was first noted in 19O40 in a small coastal
area of South Africa. The resistance spread rapidly. It seemed to be
inherited but other factors were definitely involve-.. 1Niitnall and
Bradford (165) reported that if cattle infested with resistant ticks
were transfered from the coastal area to an inland area, the ticks
either disappeared because of their inability to survive in the new

environ.mrent or they succumbed to the normal arsenical dipping. 'Thitnall
and Bradford in 197 (165). and again in 19h9 (166), and Thorburn. (157)
in 1947 reported that the resistant ticks were readily controlled with
Gammnexane dips. Whitnall anri Bradford (1E6),also reported that the tick
that caused sheep paralysis, Ixodes pilosns Koch, seemed somewhat+ revis-
tant To tne Gammexane but succumbed to the usual arsenic dips. Later in
19h9 Whitnall and others (1F4, 167") and Haarer (60) reported that the
arsenic-resistant tick had become resistant to gminma benzene hexachloride.
In Argentina control of the blue tick with benzene hexachloride was still
being obtained (51). Haarer (60) in discussing some of the economic
aspects of the resistance of the blue tick stated, "On one ranch alone,
600 head of stock died in one year because of the failure of arsenic to
control the blue tick. The first year Gairmexane was used, the figure
dropped to 70 head. It was no wonder the cattlemen were jubilant with
the material and it is also easy to understand their perturbation at the
fact that resistance to benzene hexachloride has now developed."

Red Spider Mite

.-tth, Fulton, and Lung (150) reported excellent control of many
c.) ou-e pests, including the two-spotted spider mite (Tetranychus
bimacul ''.s Harvey), *ith parathion aerosols in 1948. Complete control
of the -A: mite with parathion was also reported by Blauvelt and
Hof-,-.r. (22) about the same time. They state "Parathion shows promise
of bc--., The most important material yet discovered for greenhouse pest
.:r,-'';ol.! Parathion was effective in greenhouses for only about a
year, -when Smith and Fulton (147, 1148, i49) and Garman (56) found that
resistance to the insecticide ad developed.

Garman (56) found that if resistant mites were transferred from
greenhouses whe-re they had infested roses to the laboratory where they
were reared on bean plants, the resistance was lost in about 4 months.
On the other hand, Smith and Fulton bad reared resistant mites on beans
for many months without exposure to insecticide and without any seeming
loss of resistance. Garman found that the parathion resistance did not
include resistance to tetraethyl pyrophosphate, p-chlorophenoxy methane,
di(p-chlorophenyl) methyl carbinol, or'alkyl sulTite" (trade product of
unsTated chemical composition). Except for tetraethyl pyrophosphate,
viich is distantly related chemically to parathion, the other materials
are not structurally related to that compound. Smith's colony, however,
showed considerable resistance to the following other insecticides:
Hexaethyl tetraphosphate, tetraethyl pyrophosphate, tetraethyl dithio-
pyropho s.h a te, 1, 1-bi s ( p- chlorophenyl ) ethanol, 2- (p-tert-butylphenoxy-
l-methylet}b, 1 2-chlo,.eT yl sulfite, and p-chloropFenyl p-chlorobenzene-
sulfonate. rDo of th, above compounds were also tested by Garman, who
reported his strain was not resistant to them. Fair control was still
obtained .' Smith sin-g several of the above although resistance
was evidlrt. Oct.oi tthyl pyrophosphoraride gave excellent control of
resistant mites but whether or not there ti-s any resistance to the com-
pound was not determined.

- h -


In two greenhouse ranges in which Smith found "parathion-resis tant"
spider mites, resistance to hexaethyl tetraphosphate had been noted in
19h7 when it was first used as an insecticide. Repeated applications
and heavy dosages had, however, maintained control. The development
of resistance then could not be attributed to the use of either hexaethyl
tetraphosphate or parathion. The history of these greenhouse ranges
prior to the first use of hexaethyl tetraphosphate is not known, but
undoubtedly the usual insecticidal practices for the control of green-
house pests had been followed.

E. W. Baker of this Bureau's Division of Insect Identification
(personal communication) has been unable to find any morphological
differences between the susceptible and resistant strains.

Palm (126) reports that the red spider mite in New York State de-
veloped resistance to ammonium potassium selenosulfide when this compound
was used several years ago, and that cases of resistance to parathion
are now occurring.

Neiswander and Morris (121) after a period in which excellent
control of two-spotted spider mites on roses grown in a nutrient solu-
tion containing selenium was obtained, reported that a severe infesta-
tion developed on the plants that had received a high dose of selenium.
At that time, the infestation was not attributed to resistance. Re-
cently, however, Neiswander, Rodriguez, and Neiswander (122) have
decided that the insects had become more resistant to the insecticide.
They further conclude that wide variations between populations of the
mite exist. The rate of development as well as the resistance to
acaricides was influenced by the host plant. A mite population reared
on roses is generally more resistant than one reared on beans. A
population subjected to an acaricide for three or more generations may
develop a partial immunity to that material. Neiswander et al. (122)
state, "It appears that such a heterogeneous organism as the two-spotted
spider mite may require that acaricidal materials be changed at frequent
intervals in order to insure effective control."


Salt-marsh areas along the east coast of Florida have been treated
with oil solutions of DDT at regular intervals since 194h5. Before this
period of treatment salt-marsh mosquitoes occurred in such numbers as
to prevent full development of the area. After the treatment, freedom
from mosquitoes was considered one of the shining examples of modern
insect control measures. Recently the two most prolific insects in-
volved, Aedes sollicitans (Wlkr.) and A. taeniorhynchus Wied., have
been reported as developing resistance to DDT (17, ?7. 77, 47, 48, 73, 10

Deonier and his associates (h7, h8) found that the DDT resistance
was also apparent in the larvae. In addition to DDT the authors tested
other insecticides--TDE, toxaphene, lindane, technical benzene hexa-
chloride, and chlordane. They conclude, "Like DDT, TDE was much less
effective against the adults from the treated area than it against
those from untreated areas, an indication that the adults were about


equally resistant to TE and DYI'. At the lower concentration (0.5
percent) the other materials also showed less toxicity to the mos-
quitoes from the treated area, but at the higher concentration
(2 percent) this difference vas not in evidence except possibly with
toxcf,'rene.' The authors do not coiuient on the statistical significance
of their data. In California, DDT has been reported as failing to
control larvae of Culex spp. (72) and of Aedes nigromaculis (24).

Fay, Baker, d.-nd Grainger (50) attempted to develop resistance by
e,.',-n the common malaria mosq-uito (Anopheles quadrimactilatus Say)
to low concentrations of insecticides in the laboratory. After one
;-i .-*.ation, soe resistance had apparently been developed, but after
several succeediLng generations the resistance had not appreciably in-
creafed and the results did not seer. to be conclusive. The experiments
'. hen a Lndoned. The Kalaria Eradication Prograri. of the U. S.
I Ic c. nh Service (26) has recently completed 5 years, operational
woork. T' Lsands of houses have been sprayed in the southeastern United
States zi an attempt to eradicate malaria by control of the mosquito
vector. D'L-ring this 5-year program, apparently no resistance to DDT
has been vJevirIent. During the course of each season, however, there was
a gradual decrease in the effectiveness of residual DDT. No indication
as to the reason for this decrease was given but it is probable that loss
of effectiveness was due to the normal aging of the deposited DDT.
.. (ll11, 115) has reported that the DDT-resistant mosquito, Culex
pipiens autogenicus (113, ll8),was effectively controlled with OcETa4klor
S' ]ordan) and hexachlorocyclohexane (gamma benzene hexachloride).
Only control dosages of these chemicals were used and no effort was
nade to del-7m ine whether any resistance to them was apparent. Mosna
noted that in DDT-treated houses, although there was an increase in the
.UTIm.r of live insects, there iias also an increase in the number of dead
ones. The resistant insects were able to transmit the resistance through
the eight generations tested without exposure to DDT. According to
Verolini (161) the adults of C. pipiens from Latina were resistant to
DDT, but the larvae showed high mortality from exposure to DDT. No
comipnarison was made to determine whether or not any resistance was
present. Starting a laboratory colony of C. fatigans Wied. from a
single egg mass, Newman, Aziz, and Koshi (124) found that successive
generations showed greatly increased resistance to DDT and gar-mnexane.
The authors suggested that the increased resistance might be due to a
rhythmic variation in the resistance of C. fatigans.

Gahan and others (54) compared the susceptibility of Anopheles
psuedopunctipennis TheoV. from areas in Mexico that had received four
annual treatments with DDT to the susceptibility of mosquitoes from
untreated areas. No difference was noted.

Knipling (80) reported that there is no evidence that mosquitoes
in the Pacific Northwest have developed resistance although resistant
Sii:'.i :'.'.'i been suspected in Oregon.


House Flies

Development of resistance in the field and in the laboratory

House flies that have developed resistance to insecticides havt
been collected in the field in Italy (110), Greece (61), Egypt (55),
Venezuela (160), Sweden (168). Denmark--_l), England--U), Sicily-(
Peru _Ll, Mexico (160), and the United States (L2). Strains with
acquired resistance to various insecticides have been developed in
several laboratories in the United States (15, 23, 29, 94, 103,
134), Sicily (42), and Great Britain (65). Busvine7-36--collected
several strains of Musca vicina YMacq., the house fly of Ceylon, and
found no resistance although the area had been sprayed with DDT for
2 years. Surveys have been made in several sections of this country
(12, 30, 33, 64, 72, 74, 81, 99, 126, 1:4a, 139, 160) and in Italy
(17I2, 1437, and many strains of house flies resistant to insecticides
found. Indeed, Upholt (160) states that his coworkers were unable to
find a nonresistant stra-n in this country and contiguous areas of
Mexico in 1949.

La Face (83), in a study of the biology of the fly and its impor-
tance as a vector of various diseases, discussed resistance to insec-
ticides. DDT-resistant flies--Musca domestic and Stomoxys calcitrans
(L.)--in Sweden were discussed by Kjellander (75, 76). No report of
insecticide resistance in Russia has been noted. However, Rubtsov (14(
after a visit to Kissirolis' laboratory in Italy, sunmarized LissiroTh'
results and speculated somewhat as to the probable causes for the
appearance of resistant insects.

Morrison and coworkers (112) reported that in a dairy barn in which
DDT has been applied as a space spray since 1945, resistant flies
have not developed. In contrast, a hog barn about 0.8 mile from the
dairy barn has been repeatedly sprayed with DDT for residual treatment
and here a decided resistance has developed. Pimentel and Dewey (130)
found that the larvae of a field-collected resistant strain were also

Barber and Schmitt (13) and Hansens and Goddin (63) found that in
two colonies from the strain of flies used as a standard by the members
of the Chemical Specialties Manufacturers Association (then the Nationa
Association of Insecticide and Disinfectant Yanufacturers) there appear(
to be resistance to methoxychlor. Such resistance has been reported by
none of the many other laboratories using this strain.

The list of insecticides used in the laboratory to develop resistar
strains of flies includes DDT (9_4), chlordane (99), gamn.tia benene hexa-
chloride (23, 131), parathion (103), pyrethrum, toxaphene, methoxychlor
paraoxon, and dieldrin (33). Early- in 1946, long before resistant flies
were reported from the Tield, Lindquist and gilson (94) began the de-
velopment of the Orlando resistant strain. Flies from the regular
laboratory colony were exposed to such a dose of EDT that about 90 per(


v'.ere !-illed. The survive- 's were used to carry on the colony. After
three peneraticns of snch ex"osure, resistance was f'-nrl. Essentially
their technique has been use& in the development of all the other lab-
orati r.--esistant strains reported in the literature.

In all but two cases notedatter:pts to develop resistance in house
flies .v exposure to a given insecticide have been successful. The
exceptions are the failure by D'Alessandro et al. (h2) snl the
failure reported by Harrison (65) of attempt Eto develop a strain re-
sistc-,rt to gamma benzene hexachloride from an Italian strain of resistant
flies, In Harrison's case, only five generations were selected and
this .n-mall nui-m er probably accounts for her failure. The development
of strains resistant to benzene hexachloride, in the laboratory as well
as in the field, is well known.

Atter., ts by Bucher, Cameron, and Wilkes (3h) to develop a strain
of house flies resistant to cold were unsuccessful.

Loss of resistance

Barber and Schmitt (lh) tested the Ellenville line of resistant
Flies after 10 and 11 generations in the laboratory without exposure
to TP. These flies were still resistant to DDT, but not to the same
degree, as the third generation. After 2 hours' exposure to il1 mg. of DDT
per square foot, the 2A-hour -iortality reporteO for the third r-prra-
tion -,s 10.9 percent and for the "10th and llth" (which one was not
-r..:-iPied) 59.2 percent. March and Metcalf (100) reared the Bellflower
rc- -'- rt strain for 35 g~erierations without exposure to DDT and without
loss of resistance. They state that "At present no indication has been
found that resistant strains will revert to more susceptible strains
following non-exposure to the insecticide." More recent tests, however,
(1C2) "have shown a trend toward an increase in the number of more sus-
ceptible flies in comparison with the number of highly resistant indi-
viduals." In the writers' laboratory a colony started from pupae
furnished by Metcalf and March in November 19h9 showed considerable
resistance to insecticides in the first two generations. Within 10
generations without exposure to insecticide, however, the resistance
had fallen to the level of that of the normal laboratory strain (un-
published results). At the Orlando, Fla., laboratory of this Bureau
tests on the Bellflower strain showed that after 15 generations with-
out exposure to insecticide a marked loss of resistance had occurred.
Ihe ratio, however, was still 10 times that of normal flies compared
with 80 times when the strain was received (personal communication from
.. V. King). The Orlando resistant strain loses resistance slowly but
steadily if exposure to DDT ceases (72). Keiding and Van Deurs (71)
found that the resistance of their str-ain decreased considerably after
7 to 9 generationss although it was still remarkably high.

Knipling (77), speaking of work done at the Orlando laboratory,
stated that 6 colonies of resistant flies collected in nature showed
20 to 50 Lijie-; the resistance of normal flies to DDT. After 6 genera-


tions without exposure to DDT, some loss of resistance was apparent but
the extent of loss was extremely variable and most of the flies still
showed some resistance. King (72) reports that nearly all wild resistant
stocks tested showed some loss of resistance when exposure to DDT was
stopped. The rate of loss of resistance was very inconsistent. Bruce
(30) on the other hand states "The several DDT-resistant strains of flies
have shown no tendency to revert or lose their DDT tolerance through 3h
generations of inbreeding." Bruce and Decker (33) state "All strains of
DDT-resistant flies studied have retained their respective levels of
tolerance when placed in a toxicant-free environment." They further
state "It would appear by inference that if the chemical was entirely
removed in the field, then the resistant strains might be effectively
diluted by interbreeding with susceptible wild strains. This, of course,
would be possible only if done before the entire population acquired a
high degree of tolerance. Data obtained in a field survey may indicate
this opportunity is rapidly passing if not gone." According to a later
report, however, Bruce (6) now has experimental evidence that "dilution
of one resistant strain With susceptible flies does not result in a
strain with reduced resistant qualities." If this report is substan-
tiated, the genetics involved are indeed unusual.

Specificity of resistance

Wilson and Gahan (170) tested the resistance of the Orlando resis-
tant strain to several insecticides other than DDT,which was the insec-
ticide used in the selection of the strain. The strain was more resistant
to all the materials tested than u.,as the regular colony. The materials
were DDT, chlordane, pyrethrins plus 5 percent of piperonyl butoxide
(for composition see 162), chlorinated camphene (toxaphene), rotenone,
and thanite. Miller T709) compared the resistance to pyrethrins and
pyrethrin mixtures of several strains of flies whose resistance had been
developed by exposure to insecticides other than pyrethrins. No resis-
tance to pyrethrins was found.

By exposing DDT-resistant flies (Orlando-Beltsville resistant strain)
to a mixture of DDT, chlordane, toxaphene, lindane, methoxychlor, and
pyrethrins, Pratt and Babers (134) developed a strain of flies that was
resistant to these insecticides and to some extent to parathion as well.
However, the resistance of this strain to the above insecticides was not
statistically different from that of the Orlando-Beltsville resistant
strain, whose resistance had been developed by exposure only to DDT.

Bruce (29) reported that strains can be developed to be more or less
specifically7-esistant to one chemical or to closely related chemicals,
but presented no data from which the degree of specificity could be de-
termined. He found his resistant flies had a high tolerance for DDT and
Rhothane D-3 (l,l-bis(p-chlorophenvl)-2,2-dicllo-oethane). A small
amount of tolerance was evident for methoxychlor, and no "practical
amount" for chlordane, heptachlor, toxaphene, aldrin, dieldrin, pyrenone,
(piperonyl butoxide-pyrethrins mixture), or benzene hexachloride. He
gave no data supporting these results. In his next paper 3ruce (30)


- 10 -

presents data which indicate that the LD-50 of the most resistant
strii-n is 776 times that of the normal strain for DDT, 1-.4 times
for methoxychlor, 3.2 times for lindane, 1.5 times for chlordane,
2.1 times for dieldrin, 1.6 times for piperonyl butoxide plus
ITrrethrins, 2.5 times for toxaphene, 9.9 times for DDT plus methoxy-
chlor, and 1.5 times for paraoxon diethyll p-nitrophenyl phosphate).
Here too then, some resistance for nonrelated insecticides seems
apparent. In a third paper Bruce and Decker (33) state "The high
resistance of the DDT strain I to DDT ...... contributes resistance
to other insecticides. DDT strains II and III with lower DDT re-
sistance, show but little resistance to other insecticides."
Earlier in the same paper, they state that DDT strain II was as
resistant as DDT strain I. They state further "In general, the
DDT-resistant strains exhibit a significant amount of tolerance
for methoxychlor. On the other hand, the highly resistant methoxy-
chlor strain shows little tolerance for DDT." Barber and Schmitt
(13) also found no resistance to DDT in a strain of flies resistant
to methoxychlor. Bruce and Decker continue, "The lindane-resistant
flies show some tolerance for chlordane, dieldrin, and toxaphene
but no significant resistance to DDT. This may indicate some simi-
larity in the physical or chemical structure in lindane, chlordane,
dieldrin, and toxaphene molecules." The chemical structure and
physical properties of the above compounds show no readily recog-
nized similarities except for the presence of chlorine. The basis
for this suggestion then is not clear to the writers. Sufficient
research has not yet been done to determine whether or not these
compounds show any similarity of physiological action.

D'Alessandro e a4i. (42) suggest that resistance to "other
insecticides" ma. be due to inability to penetrate the chitinous
covering rather than a resistance to the insecticide itself."
They intimate that there is possibly a difference in reaction be-
tween mists and residues. The reason for this suggestion is not
clear, since the senior author and Scerrino (h43) had shown that
resistant flies are resistant to both sprays and deposits. Numerous
other authors have, of course, shown that resistance is independent
of the method of application. The authors at that time di- not have
sufficient data to state -hether or not their super-resistant strain
;vas also resistant to chlordane.

According to Brown and Hiogers (28) the Jllenville ?'-re ista-nt
strain was markedly resistant to l,l-(1 i..isy? eopeniat:-., a c:-'ouind
in which three methyl _r'i,. : are substituted for Vh 2 three chlorine
atoms in methoxychlor. PL_.' and 1..j (iL) collected a field
.Str.Arin that was seven times as resis _.t a5 their normal .rai on
an LD-5C basis to DDT. ',:-re was '-,ssibly a Ji.-l. resist"ice to
methoxychlor, hut none to ,_-idr.r, .. a .:ene hexachloride,
aldrin, chlordane, or emulsifiable :'?"-eie piperon.:l buto:.xi,-
p.T thrins mixture).

- 11 -

The strain of wild flies collected in Denmark by Keiding and
Van Duers (71) was resistant to JDT and its analogs. No resistance to
benzene hexachloride, chlordane, or toxaphene was evident. King and
Gahan (74) reported that the LT-70 (time of exposure in minutes re-
quired Tor 70 percent mortality) for females of a wild strain (Lee)
was 11 times that of females of the normal strain for DDT, 2.6 for
methoxychlor, 1 for chlordane, and 0.6 for benzene hexachloride.

March and Metcalf (99) state "The resistance of the Bellflower
strain to gamma benzene hexachloride, toxaphene, chlordan-type com-
pounds, parathion, and pyrethlrins is small or inappreciable." Later
(102) they find that "There are now two types of resistant fly strains
in the field in southern California. The Bellflower type is resistant
only to DDT and related compounds, and the Pollard type is resistant to
other chlorinated hydrocarbons such as lindane and dieldrin as well as
to DDT and related compounds." The data presented, however, indicate
that for the Bellflower strain for which specific resistance to DDT and
its analogs is claimed, the LD-50 is 8 times that of the normal lab-
oratory flies for benzene hexachloride, 2.8 times for toxaphene, 1.9
times for heptachlor, 2.2 times for aldrin, 1.6 times for dieldrin,
1.3 times for parathion, and 2.3 times for allethrin. Dr. Mletcalf, in
a personal communication, explains that for "practical" purposes, they
have not considered any insects as resistant until such resistance was
10 times that of the normal insects. These authors, therefore, evalu-
ate resistance in terms of effectiveness of control rather than as a
physiological difference between strains.

Knipling (78) reviewed some of the work that has been done on flies
resistant to insecticides. He stated that tests have been made at
Orlando to determine whether DDT-resistant flies were resistant to
other insecticides, and the tests showed the flies had considerable
resistance to methoxychlor, some slight resistance to chlordane, but
little if any to benzene hexachloride. Apparently he referred only to
wild resistant strains, since Wilson and Gahan (170), as indicated above,
considered the "Orlando resistant strain" resistant to all other insec-
ticides tested.

Pimentel et al. (130) state T-ien DDT-resistant flies were subjected
to lindane, dieldrin, or parathion pressure and not DDT, the DDT re-
sistance did not decrease, and in some cases even increased. If this
occurs in the field, then the control of flies with residual materials
is not a bright prospect." That such events do occur in the field is
indicated by the work reviewed by Knlplirig (79), who, _--in referring to
tests made at Orlando on a Ztrain, of flies coilcted from a local dairy,
stated that the approximate magnitude of resistance compared to nornal
flies was as follows assumingg a level of 1 for the normal strain):
"DDT 8, methoxychllor 4, chlordane 4, toxaphene 8, lin l:ne 8, dieldrLn
greater than 16, and aldrin greater than 16. The flies exhibited no
resistance to parathion, pyrethrins plus piperonyl butoxide, and alleth-
rin plus piperonyl butoxide." The dairy from wlich these flies were

- 12 -

collected had been treated with DDT from 1945 through 1947. In 1948
one application of methoxychlor was made. In the spring of 1949 two
treatments of pyrethrins plus piperonyl butoxide were made at 1-week
intervals. Dieldrin emulsion was then used during the remainder of
the 1949 season. In 1950 fixed aerosol equipment was installed for
the daily application of lindane aerosols. A pyrethrum-piperonyl
butoxide aerosol was then substituted, but results were unsatisfactory
as most of the flies left the building before knock-down occurred.

Physiological and morphological differences between resistant and
normal strains

Wiesmann (168) reported that the "Arnas" strain of resistant house
flies had thicker pulvilli and articular membranes of the joints than
did his normal laboratory strain, thus indicating that resistance was
due to failure of a lethal quantity of DDT to reach a site in which
physiological action would occur. Bettini (18) and March and Metcalf
(99) agreed that when DDT solutions were injected directly into the
hemocoele of the flies, the same relative degree of resistance was
noted, thus demonstrating that resistance was not due to lack of pene-
tration alone.

Hookum was quoted (7) as being unable to detect any structural
differences between normal and resistant flies. D'Alessandro and his
coworkers (42) measured the diameter of the pulvilli of the pretarsi
of normal and resistant flies and found that on the average, the diameter
of the resistant insect's pulvilli was less than that of the susceptible
flies. It was clear, however, that the great difference in suscepti-
bility was not proportional to the size of the pulvilli. March and
Lewallen (98) compared normal and resistant (Bellflower) strains and
found no gross morphological differences. In general the tarsi of the
Bellflower strain tended to be shorter than those of the normal flies.
The tarsal widths were variable. No significant differences in the
thickness of the cuticula from the first tarsal segment were found.
Neither was the slight difference in life cycle noted significant.
The average life cycle of the Bellflower strain was 18.1 days and that
of the laboratory strain 17.9 days. Over a period of 7 months, no dif-
ference in the weight of female flies was observed. Both strains were
similarly immobilized by heat and cold, an indication that there was
no significant difference in the general vigor of the two strains. The
authors concluded then that neither thickness of the cuticula nor general
vigor are concerned in the increased resistance of the Bellflower strain.
"The differences in the dimensions of the tarsal segments of the two
strains are not uniform enough or large enough to indicate they contribute
to the resistance."

Neri (123) observed that when DDT was applied to different portions
of the body of the fly, the resulting mortality varied greatly. However,
when resistant flies were used, except for applications to the antennae,
no differences were noted. Applications of DDT to the antennae of re-
sistant insects were in no cases lethal. Wilson (169) also found

- 13 -

variation in the mortalities obtained from applying pyrethrum solutions
to varying parts of the bodies of normal flies but did not use resis-
tant insects.

Bruce (29) reported that the total development time from egg to
adult of his resistant strains appears to be 1 to 2 days longer than
that of the normal laboratory strain. He concludes that under field
conditions the resistant flies would then tend to disappear when in
competition with flies of a shorter life cycle. Bruce did not elab-
orate on the reasoning by which this conclusion was reached.

Pimentel et al. (131) also report that the life cycle of several
strains of resistant flies was longer than that of non-resistant strains.
The difference was most noticeable during the larval period and the
length of this period seemed to be definitely related to the degree of
resistance. "DDT-resistant flies from larvae which pupated the first
h8 hours were less resistant to DDT than flies from larvae which pupated
the last h8 hours in a given larval population for the strains studied.
No significant differences between a highly DDT-resistant strain and a
non-resistant strain were found in the number of eggs laid, length of
egg stages, percent hatching, length of pupal stage, pupal weights,
sex ratio, preoviposition period or length of adult stage."

Babers and Pratt (10) found that the cholinesterase activity of
the heads of resistant Orlando-Beltsville) adult flies was consistently
lower than that of the normal flies for the first 5 days after emergence.
The enzymatic activity of both strains with respect to acetyl choline
was inhibited by excess substrate. Differences due to strain were not
determined sinceonly 1 resistant and 1 normal strain were studied.

Sacktor (144) determined the cytochrome oxidase activity of a
strain of resistant flies derived from the Ellenville strain of Barber
and Schmitt (1h) and the normal laboratory strain of the Army Chemical
Center. The enzyme activity of the brei from whole flies increased
rapidly immediately after emergence. A peak was usually reached about
the second day and a gradual decrease followed until about the fifth
day. The changes after the first day were much more apparent in the
resistant strain. The resistant strain consistently had a greater
cytochrome oxidase activity than did the normal insects. In the normal
strain, the male and female insects on a per fly basis had similar
enzyme activities. However, on a weight basis, the male normal flies
showed more oxidase activity than did the females. In the resistant
strain, the females had about 20 percent more activity than did the males.

Sternberg, Kearns, and Bruce (15_4) applied ethanol solutions of DDT
externally to the pronotum of both normal and resistant flies (Bruce and
Decker (33) multi-strain I) and after a specified time the remaining
external DDT was removed by washing. The flies were then crushed, ex-
tracted with ether, and the ether analyzed for D"1 and the suspected meta-
bolic products DDE (2,2-bis-(p-chloropLenyl)-l ,1 dichioroethylene) and
DDA (2,2-bis(p-chlorophenyl) acetic acid). The rate cf Pbscrpti'n of Pr'"

- 14 -

was :hest the first hour attur aT.plic.tion, possibly due to the effects
of, the alcohol used as solvent. The normal, or susceptible, strain
absorbed I'[ at a fairly constant rate for 4 hours. The rate then
decreased sharply, coinciding with the appearance of paral:'sis. Unlike
the susceptible flies, resistant flies continued to absorb DDT at a
steadily diminishing rate so that by 28 hours after treatment, no 7DT
could be recovered from outside the body when the original dosage was
0.5 microgram per fly.

In the susceptible flies, analysis showed that there was a very
slow metabolism of DDT but that neither DDE nor DDA were metabolic
products. However, in the resistant strain, the absorbed DDT was
rapidly metabolized, essentially to DDE but some DDA was also found.
The evidence for the identification of the metabolic products was based
mainly on spectrophotometric data and cannot be considered as conclusive.
After 54 hours a group of 20 flies that had metabolized over 50 micro-
grams of DDT had only excreted 5 micrograms of DDE. The method of
metabolism was not determined. In vitro tests for metabolism were in-
conclusive. It seemed possible that the resistant flies were able to
metabolize DDT before it reached its site of physiological action.

More recently Sternberg and Kearns (153) have expanded their work
on the metabolism of DDT to a considerable degree. In an attempt to
determine the site of degradation of the DDT, tissues from several
portions of house fly anatomy were analyzed. After the external appli-
cation of DDT to resistant flies, the degradation product DDE was found
in most areas of the body but was most abundant in cuticle from the
regions of the body to which the dosage was applied, thus suggesting
the possibility that the DDT was degraded to DDE during the time it
was being absorbed through the cuticle-hypoderm.

If the mouth parts, legs, and wings of resistant flies were removed
before the DDT was applied, neither DDT nor DDE was found in any of
the internal tissues of the flies. DDE was found only in the entire
head and thoracic and abdominal cuticle. Further experiments showed
that if DDT was fed the flies in milk (2 mg. of DDT per milliliter), it
was degraded in the digestive tract. The DDE produced was transported
to all parts of the body, but non-metabolized DDT was retained in the
digestive tract and did not accumulate in other parts of the body. The
ability of resistant flies to degrade oral doses of DDT before any has
reached a vital site thus appears to be a further factor in resistance
of DDT. Whether or not susceptible adult flies were able to degrade
DDT in their digestive tract was not determined. In view of the work
with larvae mentioned later, this question is of considerable interest.

If DDT was applied to susceptible flies following removal of the
mouth parts, only 70 percent of the 0.5 microgram dose applied was
recovered; 42 percent of the recovered DDT was found in the thoracic
cuticle, legs, and wings, and 28 percent was still on the external por-
tions of the fly. Neither DDT nor any metabolite that responds to the

- 15 -

Schechter-Haller test was,-e from the internal tissue of the
fly. The authors were unable to confirm the res-lts of T .'uger et al.
(84) and Bot (25), who, using bioassay methods, recovered a material
toxic to other flies from the thoracic ganglia, 2iapighian tubules, and
gut of DDT-poisoned flies. The material was presumed to be DDT.
Lindquist et al. (93), as reported below, found that both susceptible
and resistant flies rapidly metabolized DDT to a product nontoxic to
mosquito larvae. Sternberg and Kearns (153) found that both susceptible
and resistant flies absorbed DEE at comparable rates and neither strain
was able to further metabolize this compound. Both strains were also
unable to further metabolize DDA. Degradation of DDTf to DDE in vitro
by the cuticle-hypoderm of resistant flies was demonstrated, but the
conversion was below 20 percent. No other tissues tested were able to
degrade the DDT. It was also found that resistant larvae could metabo-
lize externally applied DDT to DDE and store it in considerable quantity.
Susceptible larvae on the other hand metabolized a considerable portion
of the DDT but the metabolite, as in the case of the adult, was unknown.
It was not, however, DDE or DDA.. Ilhen 10 micrograms of DDT were applied
to each of 10 susceptible and 10 resistant larvae, the data presented
indicate that the susceptible larvae absorbed the DDT at a very much
greater rate than did the resistant larvae. The authors did not comment
on this difference. When larvae were placed in medium containing DDT
for varying periods and then analyzed for DDT and DDE, it was found that
both DDE and DDT were present on the external portions of the resistant
larvae. Only DDE was found internally in these larvae. On the external
portion of susceptible larvae, only DDT was found, but internally both
DDT and DEE were found. If the DDT-fed larvae were allowed to pupate
and the pupae analyzed, similar results were obtained. The resistant
adults emerged normally and contained DDE both externally and internally.
The few susceptible adults that emerged contained DDE internally only.
If DDT was applied topically to resistant pupae, they also were able to
absorb and degrade it to DDE. Susceptible pupae were also able to de-
grade DDT to DDE to some extent.

In many of the above experiments, the quantities of DDT and DDE
determined were considerably below those for which the analytical methods
available are usually considered accurate. The authors, however, appar-
ently encountered no difficulty in this respect. Perry and Hoskins
(128) found that DDT absorbed by both normal and resistant flies was
metabolized to what was first thought to be DDA (5) but later presumed
to be DDE (128). This was contradictory to the results of Sternberg
et al. (154lU Who found that only a resistant strain could metabolize DDT
to DEE but agreed in general with the results reported by March and
Metcalf (103). From the normal strain in one experiment Perry and
Hoskins recovered 30 percent of the amount of DDT applied externally
as internal DDT and 66 percent as DDE. With the resistant strain the
applied DDT recovered as DDT was 7.5 percent and DDE 33.1 percent.
These authors conclude "It is obvious that increased ability to convert
absorbed DDT into DDE is characteristic of the resistant strain.!' The
authors analyzed both survivors and dead insects and found "In a given
experiment, the survivors on the average always had converted more DDT

- 16 -

than those that had diea, and hence ability to r.-':e this co-.nverion is
a major factor in variation -in resistance .within individuals of a -iven
strain." 'rom dead resistant insects treated with DDT plus :iperonyl
cyclonene, only 3.4 percent of the allied -',i .as recovered as con-
ra.ted Ajith the 17.6 percent without the synercist. fiiey conclude then
that in flies treated with BD2-synergist mi.tur: the conversion to .L'-
van1 largely lj r:cveited. They state also that the total recovered L 92 from
the resistant strains ..s at least a third less than that applied. 1.arch
and Letcalf (102) tested the compounds commonly used as synergists for
-: thrum to determine the effect on DDT-resistant flies. Little or no
effective synergism for DJT was observed. I cey (103) also studied the
metaboli-in of DDT and found that the super-Bellflower strain of resistant
flies ._-s able to metabolize larger quantities of LD at a greater rite
than -,ere normal flies. DTE was the principle degradation product and
there was no evidence of the formation of DDA. The end products as yet
have not been completely defined.

Lindqlist and his associates (93), using radioactive -'DT and bio-
assay 7irthods, have found that DDT applied externally to lies is sl:wiy
absorbed. Once absorbed, hov':v,;-r, it is rapidly metabolized by both
normal and resistant (Orlando) insects to a material nontoxic to mosquito

In the writers' laboratory (unpublished results) little difference
in mortality was obtained when dosages varying from %0 microgr rs of DJT
per fly to 300 mi-crograms were applied externally to female resistant
(Orlando-Beltsville) flies. The volume of solution applied was l.14
microliters per fly. From the first dosa-e the mortality at 24 hours
was 18 percent and from the highest dose 16 percent. Only a small per-
centage of the DDT absorbed was accounted for by analysis of extracts of
the acetone-washed flies. rour, such extracts small amounts of DDT and
a compound presumed to be DDE were recovered. In no case was [?D- found.
Results obtained from the analysis of normal flies have been inconclusive
to date. In some cases small amounts of DDT and D-'E have been recovered
and in other cases neither material -,as found. The Schechter-Haller
method of analysis which -,as used is not well adapted to the small quan-
tities of DDT involved when normal flies are used, and some difficulty
with interfering materials was encountered.

Transmission of resistance

Bruce (29) states "Further study with the resistant flies has shown
Several reciprocal crosses of susceptible and resistant flies that
resistAnce is neither dominant nor recessive, but rather than biting a
gene characteristic is probably a cytol.:, U. characteristic." o data
[iven to support this statement. Bruce (29) and !ruce and Decker
(33) interpret data obtained by crossing resi-sTant insects ':ith normal
insects to i,,.- n that both !TWin and female flies carry the resistant
character. "Tolerance mi 'lit sir.-ly be described as a multiple-:ene
character ..ih causes indifferent rhaps :orpholicil
ch'i,,-', s." ter in th s, .Ae i(e I.,i,.r they state ",i.ere is reason to believe

- 17 -

that resistance to toxicants by house flies may be attributed to indif-
ferent genetical changes, ephemeral acquired tolerance, morphological
alterations or even changes in habit." The genetical data presented is
limited to the LD-50 of the male and female progeny of the Fl, F2, and
Fl generations resulting from the reciprocal crossing of a resistant
and normal strain of flies. Bruce, in a discussion of Knipling's paper
(78), commented that information on crossing strains shows not so much
a Lendelian inheritance as a dilution or cytoplasmic inheritance. Bruce
(29), Decker (6), and Bruce and Decker (33) observed that "fly resis-
tance to DIDT is not all chemical in nature" and that in certain insec-
ticide-treated barns, house flies no longer rested on the treated surfaces
but preferred to rest on the floor or on the animals. No data were

D'Alessandro and his coworkers (h2) started a strain of susceptible
flies from a single pair of flies captured in a locality which had not
been treated with DDT. This strain (partinicensis) showed a relatively,
homogeneous sensitivit to DDT and all individuals were killed after a
35-minute exposure to DDT-treated cages. In a strain (tiberina) secured
from Lissiroli's laboratory, the knock-down time varied between 20 min-
utes and 2 hours and thus contained both susceptible and resistant ind-
viduals. After repeated selection to eliminate the susceptible individu-
als, a strain (supertiberina) v;.as obtained capable of living many days
in DDT-coated cages. On the other hand, the attempt to select a resis-
tant strain from partinicensis was unsuccessful. Apparently the strain
had been deprived of resistant characters. The authors were unable to
increase resistance in individual flies of the strain partinicensis by
repeatedly exposing them to nonlethal doses of DDT. The toxicity of DDT
was not accumiulative after such series of exposure, an indication that
the flies were able to detoxify or metabolize the material. i'ie authors
define as susceptible flies those unable to survive 35 minutes in a
DDT-treated cage, as medium-resistant those able to survive 1 to 2 hours,
and as super-resistant those able to survive 24 hours in the cage. In
a study of the transmission of the resistance, sufficient data was not
obtained to establish a strict definition of a phenomena within the 1....
of heredity. However, the strain of susceptible flies remained homo-
geneously susceptible. The strain w-ith individuals of varying resistanc-
so remained, and the super-resistant strain remained homogenouslr re-
sistant. By pairing a super-resistant female and a sensitive i,,Lje,
177 individuals were obtained, 5.5 percent srisitive, 0O percent medilui-
resistant, and L.5 percent super-resistant. 1i'ie mixed generation thus
produce was allowed to intei'_r-:d promiscuously and the F3 generation
consisted of 2J1.2 percent susceptible, 73.1 percentt medium-resist:ant,
and 2.7 percent super-resistant. All the super-resistant flies were
females. The arrival then of super-resist_- t female flies in a sensi-
tive population would lead to a diffusion of the resistant ch u cter.
The authors concluded that the Fhonriomenon of DDT resistance does not
represent a definite q'iar.titative characteristic but one that is ex-
tremely varied in degree. Also that -, a series of selective isola-
tions beginning with moderately resistant individuals, it is pr-Lible to

obtain -:_r'rrations that are highly resistant to the poison. The data
presented in the manuscript are very :..e-,er and few of the te::.r.i '
used are described. According to the authors, sporting data will be
published elsewhere. Until such information is available the work
cannot be properly evaluated.

In the writers' laboratory, after about 92 generations of inten-
sive selection by exposure of each enrieration of house flies to DDT
spr,.s or residues, the population is still by no means homogeneous
(unpublished results). Pupae to start this line of flies, now designated
Orlando-BelP iville resistant, were from the 55th generation of the
Orlando resistant colony. For about the last 20 generations selection
has been made by holding adult flies in cages coated with continuous
deposits of DDT (about 2 grains per square foot) for several days after
their emergence. The colony is maintained from the survivors. Although
the percentage of survivors has increased, many flies are still killed
by the treatment.

Control of resistant flies

That the extent of development of resistance to insecticides by
the house fly was not quickly realized by all entomologists is indicated
by Hatfield's (66) statement in 1949: "Personally, I think DDT produced
in 191:8 or 1949 applied like the 1946 and 1947 DDT was applied, will
give results comparable to the 1946 or 1947 results." He implied that
if DDT v-as applied at the rate of 200 mg. of DDT per square foot it
would give excellent control of resistant flies. The many laboratory
reports of testing resistant strains against this and even higher dosages
shows the error of his supposition.

Following the failure of JDTf to control flies in certain areas,
recoiPnendations were often made to change insecticides. Ayars (8) stated
"Fort' -tely flies resistant to DDT are relatively susceptible to lin-
dane, chlordane, toxaph.:e, and dieldrin. ... Entomologists expect
lin3Jie to be effective a-irInst house flies for about 2 ye.ars. By
about 1952 or 1953 perhaps a new insecticide aill be needed." c-srna
(11,i 116, 117), Patrissi, Barbieri, and Bessler (127), Bettini and
Bara-ch--j. (20), and others (1, 2, 0, !i, 57, 141) Fcrp-- thht the '"'-
resistant flies could be effectively controile7Tvith chlor lane, lin iane,
or toxaphene. Decker (6) proposed alternate use of, chlor.lan,
and dieldrin. Instead of the 2 years' effectiveness predicted by Ay.ars
for lindane, March and Metcalf (10C, 101, 102) found that in the field
several DDT-resisbant strains had in a few months become resistant to
lindane and dieldrin as well as to _''T. They state (101) "These new
developments, though as yet only on a limited scale, indicate that
-'.vD!ibually standard fly control procedures ::a,;, have to be re-evaluated
and revised, and that emphas's may have to be placed on sanita-, :measures,
repellent materials, and space spr--ys, rather than on residual appli-
cation of insecticides." Fjelddaln (52) discussed DDf resistance and
the -3se of methoxychlor as a control chvir:ical for the resistant flies.

- 18 -

- 19 -

Gahan and Weir (55) used gamrrma benzene hexachloride for the control
of liusca domestic vicina Lacq. in Egypt. It was not stated whether or
not other insecticides had ever been used in the area. For the first
10 months, fresh applications were made once a month with very effective
results. After 1 year, the intervals between treatments were shortened
to 1 week and even then effective control ',ras not obtained. Laboratory
tests confirmed the field tests and showed that resistance had developed.
No other insecticides were tested. The authors conclude that "the re-
sults obtained ... indicate that any benefit derived from a change to
this insecticide (in order to control DDT-resistant flies) might be only

Bruce (30) stated that the substitution of methoxychlor for use
against DDT-resistant flies gave unsatisfactory results due to the rapid
acquisition of tolerance for the compound. Bruce (6) suggested +that
"extensive use of one insecticide ;,dll produce a tolerant strain more
quickly," and later he (31) stated "Probably the development of insecti-
cide tolerance can be avoided by not relying upon a single highly effec-
tive material for the control of any particular pest. Use a chemical
to supplement other kinds of control methods and change chemicals as
soon as there is any evidence of resistance developing to an insecti-
cide." To date little evidence has appeared which indicates that chang-
ing residual type insecticides under natural conditions will in any
way prevent the appearance of resistant strains.

Catalano and lariani (39) report that the addition of fats or oils
to DDT solvents remarkably increases the todcity of the material and
that "flies that are most resistant to residuui.'s of the usual solutions
of DDT behave like sensitive flies on being exposed to residuums obtained
from solvents containing olio. Davidovici et al. (h4f) also report that
the addition of lanoline to DDT-kerosene solutions enhances the toxicity
of the residue to resistant flies and mosquitoes. Ir;e increased toxici'-;,
was lost after about 3 weeks when crystals of U, began to appear in the
residues. D'Alessandro et al. (!_2) report that gaaij.a of 7DI in a fat
solution is lethal to super-resistant flies that, according to 3ettini
(18), are able to survive $ gamma in olive oil when injected directly
into the thorax.

Perry and Hoskins (128) found that piperonyl cyclonene (for chem-
ical composition see 162T--ncreased the c:xiity of :I' for two sFt 2
of resistant flies. Thus 5 micrograms of DDT alone per fly gave 42.1
percent mortality and 5 microgr-tms of DDT plus 25 microg-i :s of piper-
onyl cyclonene gave 87.9 percent. If the synergist was increased to
50 micrograms, the mortality dropped to 76.8 percent. J'ith the normal
strain, no synergism was observed. This work has not yet been confirmed.

Because DDT failed to control flies due to the develo!.er.nt of
resistance and partly because of recommendations that DDT be avoided
in dairy barns or on dairy animals, Hansens (62) invest ted several
other insecticides. Lindane or r:.etho:,-chlor was recommended as replace-
ment for DDT in these circumstances.

- 20 -

According to Upholt (16o), "the often expressed opinion that the
situation could be solved by rotating insecticides is ... without
fo,'j-i.tion." He believes that resistant strains whose degree of re-
sist: '.ce is low will lose resistance if kept out of contact ;with in-
secticides. Practically, "it is a vain hope to expect under field
conditions to maintain any population of flies out of contact with
_. or ,ith any other insecticide, which is used to such an extent
as to become a factor in resistance." A strain of flies from an area
in __ih resistance was pronounced, quickly lost their resistance in
the laboratory when not exposed to DDT. In the field, however, after
mnoe than a :-eir after stoppi'e of the -DT program, the resistance was
not dmuninished. The exploration of course was that the stoppage of the
use of En' for fly control did not stop the use of DDT in the area for
o A.[cr C. -rIcultural pests and did not remove the DDT from any areas in
which l1-gc : entitiess had been applied. In Upholt's opinion, the
developme:n-, of resistant insects had been greatly accelerated by the
substitution of DDT for adequate sanitation as a fly control measure.

Other Species

In Illinois, filter flies (Psychoda alternate SEy) were no lonpper
controlled by treatment of the larval habitat with DDT, according to
'r ice (30) and Bruce and Decker (33), after 2 years of excellent con-
trol. The first chlordane treatment was highly successful, the second
only moderately so, and the third and fourth were near failures.
Sh'aerffenhcrg (1l45) studied the absorption of DDT and theresistance
to the compound in certain May beetles.

Ontriry to results in the United States, Raucourt (136) reported
that lead arsenate was still very effective in France for the control
of the codling moth. According to him, the failure of lead arsenate to
control the insect in the United States had not, in published reports,
been attributed to resistance. This statement is in error, since
several publications have appeared, as noted in the earlier review (9),
on the subject of such resistance. Lore recently Palm (126) states
that the entire Hudson Vall;- orchard :-ea has developed a codling moth
problem that lead arsenate will not han le. He quotes S. W. Haman as
finding in 1945 that only the very high :-ices received for the fruit
at th-it time enabled grc.vrrs in western New York to make a profit
because of the failure of arsenate to control the insect. Up to
19L9 it was the opinion of Ecv,-ral entomologists famril-ir with the subject
.) t.. t no case of codling moth resistance to I'LF or of other D'?i'-
esistant fruit insects had be"-;I provc i. more recent references
.ave been noted.

Pielou (129) states that "Invest t-Lions ire in propress to de-
terine the possibility of breeding st .L.s of enec:cil insecCs
resistant to rC'' a(nd other materials." se..seatterts hve not r ro-
:seO far enough to predict their outcome.

- 21 -

That resistance to lead arsenate by the peach twig borer (Anarsia
lineatella Zell.) had developed in a small area in California was sus-
pected by Bailey (11). Laboratory experiments by Summiers (15) tended
to confirm these suspicions. No tests have been conducted with other
insecticides, nor is it clear whether economic control of the insect
with lead arsenate is still indicated.

Shepard (Cl6, p. 327) quotes U. Peters as finding that an exposure
to a small quantity of hydrocyanic acid causes a stupefying effect on
the granary weevil (Sitophilus granarius (L.)) that results in cessa-
tion of respiration. The cyanide can thus only enter the insect by
diffusion and hence the high resistance of the species.

Kono (82) found that prefumigation of the rice weevil (Sitophilus
oryza (L.)) with low doses of carbon disulfide for 2h4 hours lowers sus-
ceptibility. There was no significant difference in oxygen consumption.
Berim and Edelman (16) studied the resistance of several insects to
benzene hexachloride and 1DT and found that it varied greatly with time
of day and the season.

Ricci (118) studied the action of DDT on the oriental cockroach
(latta orientalis L.) and found that strains obtained from areas pre-
viouslytreated with DDT showed increased resistance to the insecticide.

In a colony of body lice (Pediculus hiumanus corporis Deg.), after
the exposure of 8 generations to DDT, no increased resistance to the
insecticide was noted (72).

According to D'Alessandro et al. (42), Kriani found that after
two years of treatment with DDT the dictyospermum scale (Chrysomphalus
dictyospermi (L:org.)) had become resistant to a considerable degree.
After the firsttreatment with DDT only 2 to 5 percent survived, but
after 2 years of treatment 25 to 30 percent survived equivalent dosages.

Johnson and Hill (69) report that in Hawaii bed bugs (Cimex
lectularius L.) have become resistant to DDT.

Stanley W. Bromley (personal communication), from the results of
field tests, has concluded that the bark beetle (Scolytus multistriatus
(Marsh.)), the carrier of Dutch elm disease, has developed a considerable
degree of resistance to DDT and to a lesser extent to methoxychlor.
The beetles were from an area that had been treated with DDT for
several seasons.

According to Jayewickreme (68) field-collected larvae of Anopheles
subpictus Grassi showed resistance in laboratory tests to films of
mineral oil (Shell >alariol). The resistance was attributed to in-
creased vigor caused by favorable environmental conditions.

A strain of the cockroach (Blattella gLcnianica (L.)) resistant to
DDT has been developed by J. L. (;r i-T 5T (personal communication).

o 7 rions of selective exposure to J, .3 times as mcr-
- ,s .ircd for .n LD-' as ,v necessary in .,e unselected '_-.sects.
o-lo'.inL 7 ene '-tions of selective exposure to benzene hexachloride.
only a slight increase in tolerance w.;as noted. In the -'. 2-resisrant
Insects, no difference in reproductive performance '.as noted but ".e
* CTTiee number of n-' n er e<- case was lower in the -D-2-resistant
sLrdn than in the normal strain.

o species of bug, :-recies not -iven out a- 'ently the lar-4 mil-
eed cu, (rncopeltas *asciatus (Dall.), has oeen rer';. ..- ur.ner
2D as developing resistance to nicotine in the laboratory.


:_eiander (l0) is generallyy credited with the first ic-tion on
insects snowing resistance to insecticides. According to 3ruce (2),
Piper in 1913 found the San Jose scale to be resistant to lime-s'mLur.
The reviewers have searched the usual reference sources but have been
un.abLe to locate a reference to this work by Piper. However, it a :-cars
that John 3. (152) observed resistance some years previously.
Smith in l3u7 stated "Insects which succum.b readily to kerosene in the
Atlantic States defy it absolutely in Colorado, while ;,e are just as
li easily destroy the San Jose scale in California are ridiculously ineffec-
tive in the Atiantic States. 2his very scale is chn.rging its life
hi-, orvand habits in the Last materially in several directions. I
will venture the prediction that in half a dozen years it will not be
considered a first class pest in jew Jersei-, though i would not Like to
e.-tend this proph ecy to localiuies with which I an less familiar."
.onro (111) has discussed the subject of resistant insects ar.d
speculates to some extent on various aspects of the proble-..

'ariations within insect species was s,. tested in 1o:4 by '.-lsh
(163). Certain wood-boring and plant-feeding beetles could to some ex-
tent be divided into groups by Food .r.. ference. .-i.ey occurred in the
sa2ne locality and nad definite differences in bioloZr, but often sh:.;-ed
no or at the most very minor structural differences. I'horpe (158) a-.d
later Smith (151) reviewed the occurrence of biological races in "_isects.
In some insect species, the European fruit lecanium (Lecanium cor:-.i -uche),
for cx.'.le, the external appearance varies considerably de'endiin, on
the host plant. On the elm this scale has a large iruinose form and on
the arbutus it is small and siLnv.. Ebeling (49), although unsuccessful
in the field, .as able to transfer the insect from its normal host to
other plants in the laboratory. Succeeding "ene rations confor .ed to
the apu':'r"mce of those insects regulrirly found on the :-.ew host plant.

4o report concerning the development of resistance to insecticides
by the hessian fly (h.'tophat destructor (Say,)) has been observed.
Some of the work with this insect, however, si.-e:s to warrant co;mient in
this review, because of the possible bearing on the resistance problem.
Data presented by Painter (125) in 1930 tended to show that the hessian
fly population in one locality is a mixture of two or more genetically
distinct strains that seem to differ only in their ability to infest dif-
fei ent varieties of wheat. Cartwriight and ':oble (J3.) further explored

- 22 -

- 23 -

this observation and found that the Dawsor. variety of wheat .'Ts resis-
tant to the hessian fly in California and Kansas but was susceptible
to the fly in Indiana. Thus the California and Indiana strains of
flies seem to be physiologically different races. No reference h -.s
been noted indicating the effect of testing the California strain on
Dawson wheat in Indiana or that of the Indiana strain on Dawson wheat
in California.

There have been a number of reports of insects, particularly house
flies, having specific resistance to one insecticide or its analogs.
Other reports indicate that high resistance to one insecticide confers
some resistance to most other insecticides, regardless of structure.
The two apparently divergent views would be reconciled to a considerable
degree if a standard definition of the term resistance and uniform
methods of testing were adopted. Quayle (135) proposed that an insect
species be termed resistant if there was a decreased effectiveness of
an insecticide compared with that of former years. This definition
seems to the writers to infer at least a direct relationship of resis-
tance to economic control. Tests under field conditions have many
variables that determine to a considerable degree the effectiveness of
an insecticide. Under laboratory conditions several of the more important
variables are eliminated, temperature, humidity and nutrition of the
insects being relatively constant. The writers have considered that
two strains of insects vary in resistance if, when tested under similar
conditions, there is a statistically significant difference in their
response to a given quantity of insecticide. Physiologically, a slight
variation in response may be of considerable importance but at the same
time have no appreciable bearing on the effectiveness of the material
as an economic poison, since complete control may still be obtained with
readily usable quantities.

In some cases the dosage of insecticide used has been such that all
insects of the strains tested were killed and the conclusion reached
that there was no difference in resistance between the two strains. It
is obvious that such data cannot be considered a valid test for resis-

Small differences in rearing methods might often account for some
differences in results between laboratories. Swingle (156), Larkos and
Campbell (10h), and licGovran and Gersdorff (107) have pointed out the
effect of variations in susceptibility resulting in changes of diet. It
is also necessary of course that temperatures to which the insects are
exposed during the tests be relatively constant and also comparable to
those used in other laboratories. Lindquist and his associates (92, 95)
have shown the great effect of temperature on the knock-down and mor-
tality obtained from given doses of DDT.

IMany factors other than resistance often cause the failure of an
insecticide to effect control. The term resistance then should not
indicate in any way failure to obtain economic control. When house fly

Lreistaice to was first observed, the failure to reconir i as
danger signals the sr.all increases i-n resistance to other insecticides
led to ;n'y such statements as "DDT-resistant flies are readily con-
trolled by", etc. Such statements, while perfectly correct at the
tirme, in only a few instances carried v'arinis that such control would
probably be only temporary. Indeed, if the data of ,,ilson and GO..n
(170) showing that the Orlando resistant strain was also resistant to
several other unrelated insecticides had not been largely ignored, re-
searchers concerned with the field control of flies might have recog-
nized that substitution of other highly toxic residual insecticides
would result only in short term effectiveness.

At present, however, it seems that almost any positive statement
concerning resistance will probably have to be rescinded or modified.
The thought has been expressed in several publications that resistance
follows repeated exposure to insecticides. Resistance of the red scale
to hydrocyanic acid appeared in several isolated areas in California
in 1916. Effective control of these resistant insects was obtained
with oil sprays and the use of hydrocyanic acid was gradually supplanted
by them. One would have expected that the resistance to hydrocyanic
acid would disappear after the continued use of oil. However, the
scales in these areas today are still resistant to hydrocyanic acid
and the areas are appreciably larger than they were in 1916. Oil sprays
are still used on them, and no resistance to the oil has apparently
developed. Also after 25 to 30 years use of oil sprays against the
San Jose scale, there is no evidence that lime-sulfur resistant scales
have developed resistance to oil sprays. However, as Upholt (160)
points out, the scale insect as well as the codling moth are hTgly
specialized species restricted to a limited number of host plants.
Especially with the scale insect, the species is not particularly
adaptable. On the other hand, the house fly hes adapted itself to
all conditions to which man has adapted himself. Thus the house fly
might be expected to develop resistance. whereas rith the scale insects
the development of resistance to hydroc- anic acid might be considered
an accident of nature. Such a specialized insect would not be expected
to rapidly adapt itself to changes in environment. "The house fly has
a very high biotic potential, and when a population seers essentially
to be wiped out, a very few survivors are able to restore the popula-
tion rapidly to its normal size." "Adaptability is associated with a
high frequency of gene mutation: and when these adaptations do become
inherited through mutations, we have a situation which may be similar
to the observed resistance in flies." Citrus thrips became highly
resistant to tartar emetic in a comparatively short time but have not
bec-eiie resistant to other insecticides used before or since the intro-
duction of tartar emetic.

Another anomaly is the arsenic-resistant tick. Apparently only
one species is so far involved. Yet the infested cattle i.ave been
heavily infested with several other species of ticks that, although
ye rcat in the same manner, are still readily susceptible to arsenic.

- 25 -

Mhy, too, should the resistance largely disappear when the tick-infested
cattle were moved inland? Similar anomalies occur in the case of the
house fly. Strains that maintained resistance on the West coast for
many generations lost the resistance rather rapidly when pupae were
sent East to start a colony. Other cases have occurred in which appar-
ently contradictory results were obtained when pupae or eggs from re-
sistant strains of house flies were sent from one laboratory to another.
Errors in sampling and even misidentification of strains have been sus-
pected. To the writers, it seems more logical to assume that the
results as obtained are correct and that more attention should be
given to the indications and interpretations of such data.

The great difference in response to insecticides by the various
populations of flies is emphasized by Upholt (160). He reports that
in two areas in which dieldrin applications were made late in 1949 or
early 1950, a high degree of resistance was soon developed and essen-
tially eliminated the practicability of residual chemical control of
flies in those areas with known insecticides. However, there are areas
other than the above two in which dieldrin has been used for a longer
period and with more frequent applications but no resistance is appar-
ent. All populations therefore do not develop resistance to dieldrin
at the same rate, but it seems probable that within a year from the time
of its first use resistance can be expected.

At times positive statements have been widely disseminated that,
from the results published in the literature, seem to have little or no
data to support them. In a recent editorial (4) it is stated "l.ow the
latest reports hint that after 20 or so generations of immunity (to
insecticides) by flies, DDT and other chlorinated insecticides again
become effective." There has been no evidence published that in the
slightest degree seems to indicate such a trend. It may be that the
author intended to say "20 or so generations without exposure to in-
secticides" but that is not what was published.

Plackett and Hewlett (132) and Hewlett and Plackett (67) statis-
tically treated data obtained following the application of
mixtures of two insecticides to insects, and discussed the effects on
resistant insects of such mixtures if the tao toxicants acted independ-
ently. According to their reasoning, if twvo insecticides having in-
dependent physiological action were applied jointly to insect popula-
tions, the development of strains of insects highly resistant to either
insecticide would be prevented because the insects .most resistant to
each toxicant would be killed: only those insects with intermediate
resistance would survive and any survivors could be expected to give
rise to 9escendarts also of intermediate resistance. -,o error in the
statistical treatment is at once apparent but it is difficult to -LJ. r-
stand how a dose of insecticide v;iich kills the tougher individuals of
a population would fail to kill the weakerr ones first.

For r-iany years insecticides have for convenience been classified
according to their generally accepted mode of entry into the insect as

- 26 -

stomach ooisons, contact poisons, or fumigants. The implication has
sometimes been made erroneously that the mode of entry really consti-
tutes mode of action. There seems to be widespread misconception that
changing the mode of entry of the insecticide into resistant flies
would drastically change or even overcome resistance. Since it has
been shown that resistance is maintained when the insecticide is in-
jected, physical factors such as thickness of integument or lack of
penetration are not the cause of resistance in house flies and once
an insecticide reaches its site of action, no evidence has ever been
presented to indicate that mode of action varies with mode of entry.

The role that the mixing of various strains of insects in the
field plays in the development of resistance has yet to be determined.
However, because Bishopp and Laake (21) reported that marked house
flies were recovered as far as 13 to 14 miles from the point of re-
lease, considerable effect must be expected. In other species of
insects even greater dispersions are found.

No adequate genetical study of resistance in the house fly has
yet been reported in the literature. Bruce (29), Bruce i Knipling
(78), and Bruce and Decker (33) presented a hypothesis for the method
of transmission of inheritance. To date, however, their published data
consist only in the determination of dosage-mortalities for the Fl,
F2, and FI5 generations resulting front the reciprocal crosses of a strain
of normal with a resistant strain of flies. whetherer such crosses were
made with individuals or whether they were mass crossings is not clear.
':either is it clear how the neacer data presented justify the conclus-
ions made.

According to D'Alessandro and his associates (42) DDT sensitivity
and DDT resistance are hereditarily transmitted to the progeny and
apparent modifications are not met with in the course of many genera-
tions. Here again, the data so far presented do not seem sufficient
to justify the drawing of definite conclusions.

The difficulties involved in the interpretation of uata obtained
from mass crossings are considerable. Several questions that rust be
considered in mass crossing are: j.Tiat percentage of the population is
resistant? What is the percentage of susceptible individuals? These
questions apply equally to the normal strain, for it is well knovn that
all insects of a given population do not respond in the same way to the
same dose of insecticide. Are the results obtained from mass crossings
with subsequent determiination of the LD-50 of the F1 population com-
parable to results from individual crossings with determination of the
resistance of each individual progeny? How can the resistance of an
individual insect be determined?

It has been generally accepted that antigens, the substances that
disturb protein synthesis in the organism in such a way that antibodies
are proJuced, are iLacromnclecules and are generally proteins or protein

- 27 -

complexes. However, Loiseleur (96, 97) has reported that antibodies are
formed in the serum against such simple substances as ethyl alcohol,
ethylamine, sodium acetate, xylose, and arginine. The presence of the
antibodies was demonstrated, except in the case of sodium acetate and
ethylamine, by an increased viscosity of the serum after addition of
the injected substance. When sodium acetate and ethylamine were used,
flocculation was observed. Viscosity tests are generally not accepted
as conclusive proof of the formation of antibodies. In the event that
Loiseleur is correct in the interpretation of his results, his findings
may elucidate to a considerable degree the phenomena of the development
of resistance to various materials by vertebrates and possibly inver-
tebrates as well. Such an explanation might also explain certain cases
of cross tolerance, since the injection of a uniform antigen solution
leads to the formation of multiple antibodies directed against the dif-
ferent determinant groups of the antigen molecule.

In the recent literature there are numerous references to "DDT and
other chlorinated hydrocarbon insecticides." Often it is obvious that
the author considers that "chlorinated hydrocarbon insecticides" are
closely related chemically and should be expected to behave in a similar
manner physiologically. Possibly this reasoning is based on the assump-
tion of the correctness of the hypothesis of LYartin and -.V'in (1C5) con-
cerning the mode of action of DIDT. According to this hypothesis, the
chlorophenyl rings confer lipoid solubility on the molecule whereas the
remainder of the molecule liberates hydrochloric acid at vital centers
and is responsible for toxicity. Although such a possibility must be
considered, little experimental data have appeared in its support and
much of the data suggest that the theory is not correct. Dieldrin and
aldrin, for example, are excellent chlorinated hydrocarbon insecticides
but are little affected by strong alkali in vitro. Such stability in
vitro does not preclude breakdown in vivo but the probability is strong.

Admitting for a moment the possibility that the toxicity of DDT
was due to the liberation of hydrochloric acid, it would seem probable
that DDT-resistant flies would be generally better buffered than the
susceptible insects. The authors and N. Beroza of the Division of
Insecticide Investigations (unpublished results) therefore ran titra-
tion curves on the brei obtained by grinding whole flies in distilled
water under deobase. No difference was noted between the curves ob-
tained from the brei of normal (Beltsville normal) and resistant flies
(Orlando-Beltsville). Further doubt on the validity of the theory
arises when one considers the isomers of DDT.

It is well known that a shift of the chlorine atom on one of the
benzene rings of the DDT molecule to any other position r.arkedly lowers
the insecticidal value of the compound but these isomers readily de-
hydrochlorinate. Several hundred analogs of DDT have been prepared
and in none of them has the insecticidal value of DDT been approached.
Methoxychlor and the fluorine analog of DDT are also often included in
lists of such "chlorinated hydrocarbons." To assume that all chlori-
nated hydrocarbons should act pysiologically in a similar manner is

- 28 -

^out as logical as assuming that all plant material should act on in-
sects in a ,i._rier similar to pyrethrum. The situation is further
confused by the several reports that methoxyclor-resistnrt flies are
not resistant to DDT, but that DDT-resistant flies usually show high
cross tolerance for the methoxy compound.

Haarer (60), speaking of the developLent of resistance to benzene
hexachloride by the already arsenic-resistant blue tick, states: "It
seemed that news such as this quietly trickLn through scientific
circles, and appearing in small sections of the overseas. press, is
enough to make headlines as big as those concerning the new atomic bomb,
if only the significance of the matter were properly understood."

In the United States flies, mosquitoes, cockroaches, and bark beetles
have been definitely reported as developing resistance to the organic
insecticides currently used in large quantities for the control of in-
sects by contact with residual deposits. The development of resistance
to benzene hexachloride by the blue tick in South Africa, of Drosophila
to DFDT, and of bed bugs to DDT, proves that other species can and have
developed resistance, and indicates further that the development of
similar resistance by more species can be expected to occur in nature
at any time.

It should also be pointed out that all work so far reported on the
development of resistance in insects has been done on populations; never,
apparently, has resistance been developed in an individual insect.

In conclusion it may be pointed out that the phenomenon of resist-
ance is not much better understood now than it was at the time of the
first report of its occurrence. The method of development or of trans-
mission is not clear, and no adequate suggestion has been made as to
what should be done to overcome the resistance. An extensive and coorli-
nated long-range program seer s to be indicated, but currently most of the
research efforts seem to be directed to,'ard te:;po rary expedients rather
than toward the solution of the problem. If one thing in the picture is
clear, it may be summed up in Decker's suggestion (46)--":Lemer.,ber what
has happened to DDT and proceed cautiously in the use of other insecti-

It is obvious from the various reports reviewed that there is var-
iation between the response of the various strains, both normal and re-
sistant, to the several insecticides. It is not possible, apparently
to make any stat'merit that will cover all cases but the following gen-
eralities begin to aprear:

1. A very high level of resistance to one insecticide confers a
certain aiourit of cross tolerance for other insecticides of widely dis-
similar structure.

2. '.'een the level of resistance is low, the resistance is almost

- 29 -

specific and is limited to other insecticides of closely related

3. Strains of house flies resistant to all insecticides for which
the attempt was made have been developed in the laboratory. This in-
cludes materials of plant origin as well as synthetic organic materials.

4. The resistance level of wild strains of house flies is by no
means as high as that of laboratory-developed strains.

5. The degree of resistance in house flies varies considerably
from generation to generation.

6. :ost strains of resistant house flies, either laboratory or
field, tend to lose their resistance if exposure to the chemical ceases,
but some strains maintain resistance for many generations.

7. Adequate evidence for the formulation of a theory concerning
mode of transmission of resistance from parent to offspring has not been

8. Once resistance to any insecticide is developed by house flies,
the development of resistance to other insecticides proceeds at an
accelerated rate.

9. Resistance by other species of economic importance will probably
be developed.

10. The several resistant strains of house flies do not behave in
an analogous manner, and progeny from the same parent strain often have
different responses to insecticides when tested in other laboratories.
It is almost impossible, therefore, to compare directly results between

11. The resistance of house flies is not due to the failure of the
insecticide to penetrate the cuticle since resistance is evident when
the insecticide is injected directly into the body cavity. Resistance
is also apparent regardless of whether the insecticide is applied as a
mist, dust, or residue.

- 30 -

Literature Cited

(1) Anonymous.
1949. Methoxychlor, a manual of technical data. E. I. du Pont
de Nemours & Co., July 1.

1949. Controlling flies with BHC. Agr. Chem. 4(4): 30-32, 73,

(3) ____
19h9. Are insects rallying against DDT. Amery. Fruit Growers
69(2): 17, 39.

(h) _____
1950. How's your supply of chemicals? Do you have enough? Pest
Control 18(9): 7.

(5) ______
1950. Pepper extract lets DDT kill super-flies. Farm Jour.,
March: 17.

(6) _
1950. Switch chemicals in 1950 Dr. Decker says at Purdue con-
ference. Pest Control 18(3): 12, l, 16, 18, 20.

(7) ______
1050. Says tolerant fly structures same as others. Pest Control
18(4): 22.

(8) Ayars, James S.
1950. Flies thrive on DDT? Kill them with lindane. Successful
Fearing 48(7): 18, 104.

(9) Babers, Frank H.
1949. Development of insect resistance to insecticides. U.S. Bnr.
Ent. and Plant (uar. E-776, 31 pp. [Processed.]

(10) ______ and Pratt, Jr., John J.
1950. Studies on the resistance of insects to insecticides.
I. Cholinesterase in house flies (Musca domestic L.)
resistant to DDT. Physiol. Zool. 23: 58-63.

(11) Bailey, Stanley F.
1948. The peach twig borer. Calif. Agr. Expt. Sta. Bul. 7C8,
56 pp.

(12) Barber, George H., and Schmitt, John B.
1948. House flies resistant to DDT' residual sprays. N. J. Agr.
Expt. Sta. mul. 742, 6 pp.

- 31 -

(13) Barber, George H., and Schmitt, John B.
19149. A line of house flies resistant to methoxychlor. Jour.
Econ. Ent. 42: 844h-6h45.

(14) __ and Schmitt, John B.
1949. Further studies on resistance to DDT in the house fly.
Jour. Econ. Ent. h2: 287-292.

(15) ______ Starnes, Ordway, and Starnes, Eleanor B.
1948. Resistance of houseflies to insecticides. Soap and Sanit.
Chem. 24(11): 12C, 121, 143.

(16) Berim, N. G., and Edelman, N. IM.
1949. On some physiological factors which determine the resis-
tance of insects to dichlorodiphenyl trichloroethane
(DDT) and hexachlorocyclohexane. Dokl. Akad. NauV. SSSR
67: 585-588 [English translation by R. Ericson.]

(17) Bertholf, J. H.
1950. DDT resistant mosquitoes in Broward County, Florida.
Fla. Anti-losquito Assoc. Ann. Rpt. 21:
80-83. [Processed.]

(18) Bettini, Sergio.
1948. Contributo allo studio della resistenza all' azione del
DDT nelle mosche domestiche. Riv. di Parassitol. 9:
137-142; 1st Super di Sanit. Rend. 11: 1131-1136.

(19) _
1948. I nuovi insetticidi. 1st. Super di Sanit. Rend. 11: 821-

(20) ______ and Barachini, Benivieni.
1948. Primi resultati della lotta con 1' octa-klor ed il
Gamnaesano control le mosche domestiche resistenti al
DDT. 1st. Super di Sanit. Rend. 11: 8l41-848.

(21) Bishopp, F. C., and Laake, E. W.
1921. Dispersion of flies by flight. Jour. Agr. Res. 21: 729-

(22) Blauvelt, qi. E., and Hofilan, Julius R.
1948. Parathion aerosol for greenhouse pest control. H. Y.
State Flower Growers Bul. 29, 6 pp.

(23) Blickle, Robert L., Capelle, Asher, and Lorse, I. J.
1948. Insecticide resistant houseflies. Soap and Sanit. Cien..
24(8): 139, 141, lh9.

- 32 -

(24) Bohart, R. M., and Murray, W. D.
1950. DDT resistance in Aedes nigromaculis larvae. CaLif.
Mosquito Control Assoc. Ann. Oonf., Proc. and Papers:
18, 20-21. [Processed.]

(25) Bot, Jan.
1949. Bydrae tot die verklaring van die working van die insek-
tisides DDT, hexachlor, siklohexan, chlordane en
toxaphene. Luctor et Emergo, Leiden. 13$ pp.

(26) Bradley, George H., and Lyman, F. Earle.
1950. Discussion of five years use of DDT residuals against
Anopheles quadrimaculatus. Jour. Natl. Malaria Soc.
9: 113-118.

(27) ______ and Lyman, F. Earle.
1950. Remarks on the present status of the malaria eradication
program. Fla. Anti-MIiosquito Assoc. Ann. pt. 21:
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