Development of insect resistance to insecticides


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Development of insect resistance to insecticides
Physical Description:
31 p. : ; 27 cm.
Babers, Frank H
United States -- Bureau of Entomology and Plant Quarantine
United States Department of Agriculture, Agricultural Research Administration, Bureau of Entomology and Plant Quarantine
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Washington, D.C
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Insecticide resistance   ( lcsh )
Insecticides   ( lcsh )
federal government publication   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )


Includes bibliographical references (p. 22-31).
Statement of Responsibility:
by Frank H. Babers.
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Caption title.
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General Note:
"May 1949."

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University of Florida
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Full Text

United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine


By Frank H. Babers,
Division of Insects Affecting Man and Animals

The use of the term "resistant insects" to denote those tolerant
to insecticides is probably open to criticism. However, for lack of a
better term it is used herein to designate strains of insects that for
some reason are able to withstand a larger dose of an insecticide than
are other apparently normal insects of the same species. The resistant
strains in nature often occur within a few miles of nonresistant strains,
and differences in susceptibility become very noticeable.

The phenomenon came into prominence when Melander (_4), speaking of
the San Jose scale, raised his now famous question "Can insects become
resistant to sprays?" In 1902 Piper (7Y) had obtained excellent control
of the scale with lime-sulfur. Melander in the same area about 11 years
later found 74 percent of the scales alive although the dose of insecti-
cide had been increased 10 times.

Since Melander's observation the list of resistant insects described
in the literature has grown to sizable proportions. Quayle (6) noted the
increased resistance to hydrocyanic acid fumigation of the California red
scale, A di aurti (Mask.)./ (ChrsomhAlus arantii Mask.), and
the black scale, Saissetia ola (Bern). As usually practiced, fumigation
was expected to effect such a high mortality of scale insects that re-
treatments would be needed only about every 3 years. The results of fumi-
gation with hydrocyanic acid had in 1916 become so unsatisfactory that
often trees did not remain free from scales for a single year. Hough ()
found a strain of the codling moth, Carpocasa omonella (L.), that could
enter apples sprayed with arsenicals more readily than could the normal
insect. The same year Boyce ( ) actually developed strains of Drosophila
melanogaster Meig. and Aphis ospi Glov. in the laboratory that were
resistant to hydrocyanic acid fumigation. Resistance to hydrocyanic acid
fumigation by the citricola scale, Coccus seudomanollarum (Kuw.), was

I/ This work was conducted under funds allotted by the National
Military Establishment to the Bureau of Entomology and Plant Quarantine.

2/ The scientific names used in the literature sometimes differ from
the name currently accepted by the Association of Economic Entomologists
(). The current names are used, followed in parentheses by those em-
ployed by the author of the citation.


described by Quayle (78). The progeny of confused flour beetles,
yri: ima 22auL Duv., resistant to hydrocyanic acid fumigation were
also resistant, according to Gough (2,). In South Africa an arsenic-
--sistant tick, philu decoorats (Koch), was found by Du Toit and
others (22). Boyce and Persing (7) found resistance to tartar emetic
sprays among the citrus thrips, ScirtothriDs (Moult.), and
Knipling (_4) found that larvae of the screw-worm, Callitroza americlaa
C. and P. (Cochliomyia americana), could acquire resistance to pheno-
tthiazino. Mosna (67) described a variety of the mosquito, Culex pipiens
. ito & resistant to DDT, and Missiroli (6) told of the failure of
DDT to control flies in Italy in 1945-1946 due to their resistance.
Lindquist and Wilson (56) and Wilson and Gahan (27) have developed a
laboratory strain of the house fly, Musca domestica L., that is resistant
to DDT and other insecticides. There seems little doubt therefore that
both in the field and in the laboratory there have been developed insects
that differ in physiological response to insecticides from other insects
that are, as far as can now be shown with one exception, the same morpho-

It is !he purpose of this publication to review the development
of resi7+.+o&ce to insecticides by the several insects in an effort to
det&crcdne whether sufficient evidence has been presented to warrant
a theory as to the cause of this resistance.

San Jose Scale

In his original report on resistant San Jose scale, Melander (Q)
stated that he had observed failure of lime-sulfur to kill all the
insects first in 1908 and again in 1910. However, it was not until
1913 ti-. he attempted extensive experiments. Where all the nonresist-
ant scales were dead within 6 weeks after spraying, from 4 to 17 percent
of the injects from the resistant area were normal. Trees sprayed with
oil emulsion, however, were free of scale in both areas. He noted that
cany of the surviving resistant insects were males. In the usual field
procedure about every 10th generation of scales is sprayed. An acquired
immunity .-hrefore seemed unlikely to belander. He did believe, however,
that the rjiJtanoe was Inherited. He gave additional evidence in 1915
(6) to show that the resistance was not due to weather, faulty applica-
on, iprcpy ly mixed sprays, the condition of the trees, or apparently
to ay car-biration of extrinsic factors. In his third paper Melander
(W) abridged and sumnarized the data accumulated since 1908. In
atteapts to start a laboratory colony of San Jose scale, he brought
"literrlly millions" of scales from infested areas. Scaly apples were
tied on trees, scaly scions were grafted, and scaly limbs entwined in
young treos. Every attempt to acclimatize the scale to the Pullman,
Wash., area failed, and yet this was the insect that was able to sur-
vive very heavy doses of lime-sulfur. He found great individual varia-
tion in the tolerance to lime-sulfur, soda-sulfur, and barium-sulfur
sprays. The tolerance varied from locality to locality and from year
to year, and winter mortality varied with the severity of the weather.


A temperature of -30* F. destroyed more scales than the usual spraying,
whereas in mild winters 98 percent of the young scales survived. There
was no evidence that the San Jose scale was becoming more and more
resistant to polysulfide sprays.

Flint (24) supported Melander's stand that in certain areas the
San Jose scale, Aspidiotus perniciogus Comst., was resistant. He stated
(concerning orchard owners)& "A number of these men are university
graduates and are thoroughly familiar with the theoretical as well as
practical side of spraying. Not one in 50 is satisfied with lime-
sulfur for controlling the San Jose scale. -Whatever the entomologists
may think, it is practically useless to advise these men to continue
spraying with lime-sulfur." Apparently Melander's work had aroused
great controversy,although I find no rebuttal published. Flint, as did
Melander, obtained control of the insect idth oil emulsions.

California Red Scale

Two years after Melander's original observation, Quayle (76) gave
evidence to show that both the red and black scales in certain areas
were much more difficult to kill with hydrocyanic acid than in former
years. Additional data were given in Quayle's next paper (77). He stated
that he had some evidence that survivors of a fumigation are more resistant
to a second fumigation than are individuals that have not been previously
fumigated; also that the greatest resistance is shown by scales on trees
that have been fumigated regularly once or even twice a year. He found
considerable difference in susceptibility among insects from various parts
of the same tree, and the susceptibility seemed to be related to the food
supply of the scale. During the molting period resistance wes at its
highest. Results of field experiments varied because changes in the
humidity of the air affected the dryness of the canvas tent used in fumi-
gation and therefore its ability to retard diffusion of the gas. The dos-
age of hydrocyanic acid required for control oQf the resistant strains was
so high that it was unsafe for the tree except under the most favorable

Woglum (98) supported the observation of Quayle that both red and
black scales were, in certain areas, resistant to fumigation with hydro-
cyanic acid. Gray and Kirkpatxick (26,27, Z8) also agreed that there
were resistant and nonresistant strains. They showed that mortality
among both strains was greatly reduced if the insects were first exposed
to a low dose of hydrocyanic acid before exposure to the killing dose.
The resistant strains, however, could still withstand much the heavier
doses. This protective mechanism was called protective stupefaction.
These authors recommended a fumigation technique that would obtain a high
initial concentration and thus avoid the protective stupefaction.

Pratt, Swain, and Eldred (75) also studied this protective stupefac-
tion for red, black, and citrus scales, and confirmed Gray and Kirk-
patrick's findings on both resistant and nonresistant strains. According
to Quayle (79, p. 198), A. F. Swain in 1928 and 1931 conducted tests on


the comparative resistance of red scale to fumigation under form trees
and in a fumatorium. He found significant differences between resistant
and nonresistant strains. These data apparently have not been published.
Also according to Quayle, A. F. Kirkpatrick carried on extensive tests in
1936 on comparative resistance of the scales from various localities and
found significant differences. Quayle gave no literature reference to
this work.

Ebeling's (2) data indicated that "in any given grove, the trees
in the higher portions of the grove usually have the greatest infestation
of scales." He stated: "It is a common observation that the red scale
can be more easily controlled by the usual control measures in the less
elevated portions of groves than on the higher elevations." In a grove
where the highest elevation was 40 feet above the lowest, the difference
in temperature was about 4.73 F. Because the foothill districts were
warmer, they were generally planted to lemons, and it is in these areas
that the so-called resistant scales were found. Ebeling states: "It is
not known to what extent 'resistance' as applied to red scale in the foot-
hill districts is due to the higher temperature prevailing in these
districts. Nor is it known to what extent the apparent resistance in
these districts may be accounted for by the fact that the lemon is the
preferred host of the red scale." He also suggested that, when the
population density was so great that the insects overlapped at their
margins, mortality from fumigation might be reduced.

Knight (Q2) pointed out that the tolerance of the red scale to fumi-
gation with hydrocyanic acid was in inverse ratio to its activity. This
was true whether the inactivity was induced by natural or artificial
means. Insects in the pupal stage and during molting were least suscep-
tible to hydrocyanic acid. These observations, of course, do not explain
the differential resistance of the two strains. In 1932 Knight (Q) found
that oil sprays gave poorer kills on heavily infested fruit than on lightly
infested fruit. The kill on lemons infested with 1-100 scales was 95.97
percent, with 100-200 was 83.58 percent, and with 200-300 only 72.90 per-
cent. He suggested that a similar reduction might occur when heavily in-
fested fruit was fumigated.

Moore (6) stated that "under favorable conditions, there is no sig-
nificant difference between the kills of 'resistant' and #nonresistant'
red scale." Density of scale population had no effect on the kill ob-
tained by fumigation with hydrocyanic acid, nor did the concentration,
provided there was no faulty distribution. Using insects from the area
where resistance was first found, he determined the time-concentration
constant (Knight _, Brinley and Baker 2). His results were irregular
and indicated '"that there was some uncontrolled factor influencing the
success of a fumigation which overshadowed the time-concentration constant.
In other words, if conditions are unfavorable, increasing the dosage or
the time will not insure a perfect kill." Exposure to a low concentration
prior to the regular fumigation did not always reduce the kill. Several
field fumigations were made to determine whether the number of late-second-
molt and early-gray adults, commonly considered the least susceptible


stages, had any effect. He stated "clearly the stage of development is not
the controlling factor," and in the same paragraph that "It would appear as
if conditions at the time of the fumigation decide its success or failure
and the stage of development of the insect decides whether it will or will
not be a survivor of an unsuccessful fumigation."

Temperature and humidity had an effect on the kills obtained by fumiga-
tion. Moore (6_) listed as 'unfavorable" conditions high temperature, low
relative humidity, pro-exposure to low concentrations, exposure to low rela-
tive humidity preceding the fumigation or low temperature following the
fumigation. Under these conditions he stated that "with the 'resistant'
strain the kill may fall very low* and that "under the same conditions the
nonresistant red scale reacts but slightly." Data are not given to support
these statements. In experiments where resistant and nonresistant insects
are directly compared, the effects of fumigating warm and cold lemons, of
fumigating at varying temperatures and humidities under varying environmental
conditions, and of varying the temperature before and after fumigation were
determined. Many of the kills were higher than 90 percent and therefore not
too acceptable from a statistical standpoint. In every case, however, the
kill of resistant insects was considerably lower than that of the nonresistant.
It is not clear, therefore, how Moore's conclusion that "under favorable
conditions, there is no significant difference between the kills of 'resistant'
and 'nonresistant' red scale" was reached.

Moore (65) concluded that the difference in kill between the two "strains"
was due to climatic conditions, method of application, or variations in the
concentration of hydrocyanic acid, since good and poor kills were obtained
the same night. In 1936 he (66) studied the relationship of concentration (C)
and exposure time (1) and concluded that lethal constants for resistant and
nonresistant strains were CT .5 and CO."8, respectively, apparently reversing
his stand that there was no difference in tolerance to hydrocyanic acid. He
concluded that the main differences between the resistant and nonresistant red
scales in California were their reactions to concentration, exposure, and the
temperature at which the fumigation is conducted, and these differences were
not acquired from previous fumigations. Such conclusions of course do not
explain the phenomenon.

Cunningham (15) stated that, "when one considers the complexity of the
problems involved in tent fumigations, it becomes obvious that many points
must be investigated before the theory of 'resistant' scales can be accepted."
Haas (&) attempted to relate resistance to hydrocyanic acid fumigation to the
chemical content of several scale insects. Because Clayton (2) increased
the tolerance of tomato plants to hydrocyanic acid by the injection of glucose,
and because Keeser (2) found resistance of rabbits to orally administered
sodium cyanide was increased when the iron content of the tissues was increased,
Haas suspected that the glucose and iron content of the scale insects might also
be related to resistance. He determined ash, iron, phosphorus, manganese,


nitrogen, sulfur, copper, glucose, wax, alcohol-soluble material, and
crude chitin content of the insects. No relation was established
between resistance and iron or phosphorus content. A reduced copper
content and an increase in the amount of wax and reducing substances
present might be related. Quayle (78) reviewed the entire subject of
resistance to hydrocyanic acid fumigation by the scale insects anad
presented additional data. He stated (p. 206) that Knight, several
years before and then a colleague of his, had concluded that the resist-
ant red scale was more resistant to oil sprays than the nonresistant
strain. Chamberlin, also a colleague, concluded that there nas a
difference between the two strains with respect to desiccation. Quayle
did not consider these data sufficient to justify their claims, and
their published paper (De Ong, Knight, and Chamberlinj7) did not
contain statements to that effect. Quayle concludes that mne resistance
spread frcn an original focus. He also reported that the resistance
differential occurred when methyl bromide and ethylene oxide were used
as fumigants. Carbon disulfide was tried but injured the fruit, and the
results therefore were not reported.

Dobzhansky (21, p. 190) concluded that the resistance was brought
about through natural selection. It first appeared either through muta-
tions or because a mixture of the resistant and nonresistant strains was
present in the original infestations. In his opinion the exact origin
would not be determined. He stated (p. 191)t "However that may be, the
emergence of the resistant race of the red scale is clearly due to the
differential survival of the two genotypes in fumigated orchards.4
Lindgren (49) confirmed the existence of the two strains and also studied
protective stupefaction. He used laboratory strains of the resistant
and nonresistsat insects. No difference in results was noted between
these laboratory-reared insects and those collected from areas in which
varying resistance was observed.

Dickson (18) concluded that the resistance is inherited. It
depends on a single gene or group of closely linked genes in the X-
chromosome and is therefore sex-linked. Crosses between the resistant
and nonresistant strains gave female offspring intermediate between the
two. The F1 males inherited their mother's resistance. Lindgren (a)
studied the effect of time of exposure, high peak concentration, tempera-
ture, and other factors on the fumigation result. He used laboratory-
reared insects of both resistant and nonresistant strains. Differences
in susceptibility to fumigation occurred in insects of all stages.

Lindgren and Dickson (51) found that resistant and nonresistant
red scales showed no difference in susceptibility to oil sprays. Cressman
(1) studied the relative susceptibility of the two strains to sprays
containing oil, solvent, and cube resins. With the oil and solvent alone
the mortality of the resistant strain was 25.7 percent and of the non-
resistant strain 29.7 percent. This difference was not significant.
With 0.006, 0.008, and 0.013 percent of cube resins, the respective
mortalities were 77.4-81.6, 88.2-89.9, and 94.1-96.8 percent. (The first
figure of each pair is for the resistant strain and the second for the
nonresistant.) The differences in mortality between the two strains were

not great but were claimed by the authors to be highly significant
statistically. The method of statistical analysis used was not given.

The two strains were found by Hardman and Craig (3p) to be different
in their phys logical response to low doses of hydrocyanic acid. In
both strains the spiracles close 3 to 5 minutes after hydrocyanic acid
has been admitted to the fumigation chamber. In the nonresistant strain
they remain closed only for 1 minute, then open. and death follows. In
the resistant strain the spiracles remain closed at *- .. 30 minutes,
and the scales can survive lethal concentrations for that time. Quayle
(a) disag3e-es with Hardman and Craig's conclusion that the ability to
keep the spi.racles closed explains the resistance. as reasoning is
based on ti!: behavior of the two strains toward protective stupefaction.
This phenomenon occurs then exposure to cyanide is so short that spiracles
do not close. Lindgren and Sinclair (,U) further confirmed the existence
of the resistant and nonresistant strains of red scale. They found that
after fumigation with hydrocyanic acid gas more hydrocyanic acid was
recovered from the nonresistant than from the resistant insects.

lust, Nelson, and Busbey (1W6) found that the protective stupefaction
of the red scale occurred with both the resistant and nonresistant insects.
At 59* F. the protection for nonresistant females persisted less than I
hour, but at 77* it lasted at least 2 hours. The amount of protection was
not affected by repeated exposure to low dosages, but the duration of
protection was. hst and Busbey (1__) compared the susceptibility of
the two strains to methyl bromide. The resistant mature females were
more resistant than the nonresistant insects, but in the early gray adult
stage, a stage in which the difference in susceptibility to hydrocyanic
acid is marked, the nonresistant insects were somewhat more resistant to
methyl bromide at 40-minute exposures. At longer exposures, 120 to 180
minutes, the difference was in the same direction but was not considered
significant. In concentration-mortality tests the median lethal concen-
tration for methyl bromide for early gray adults of the resistant strain
was 68 mg. per liter and for the nonresistant insects 73 mg. per liter.

Using the resistant strain only, Yust, Busbey, and Howard (j0)
investigated the effect of mixtures of methyl bromide and hydrocyanic
acid. With methyl bromide alone a concentration that gave only 16.4
percent kill at 50* F. gave 99.5 percent at 77%. When a concentration
of methyl bromide that alone gave no kill was used, a mixture of methyl
bromide and hydrocyanic acid gave a better kill of insects in all stages
than was obtained with the separate gases. Methyl bromide used alone
seemed to protect the scales somewhat in the late second-molt and early
gray-adult stages from a later hydrocyanic acid fumigation. Protection
of the scales against hydrocyanic acid by other gases was previously
observed in their laboratory, but no references or data were given. In
another experiment a mixture of methyl bromide and hydrocyanic acid was
used in which the concentrations were much that approximately equal kills
would be obtained if the gases were used separately. Under these
conditions there was a synergistic action on the gray adult stage and
an antagonistic action on the mature females. TYust, Busbey, and Nelson
(=0) determined the reaction of laboratory-reared insects of the two


strains to various factors, such as temperatures before, during, and
after fumigation, exposure to sublethal dosages, and certain combina-
tions of these factors. When the scales were preconditioned and post-
conditioned for 4 hours at the treatment temperature, the kill of
resistant second-molt and mature-female stages and nonresistant second-
molt insects was higher with the lowering of fumigation temperature but
the kill of nonresistant mature females was not influenced. The kill
of resistant scales was influenced more by the temperature after fumiga-
tion than was the kill of the nonresistant strain. With second-molt
insects the kills of both strains were decreased by prefumigation with
sublethal dosages, but the differences in kill were more marked with
the resistant strain.

Yust and Howard (105) studied the factors that influenced the
results when red scale was fumigated in the laboratory with hydrocyanic
acid. Using insects from the laboratory stock of the resistant strain,
they determined the effects of condition of host, isolation of the insect
on the fruit, difference of age within stages, and temperature variation.
Better kills were obtained on lemons that were slightly soft, and scales
that were fused together were slightly more difficult to kill than were
isolated insects. Difference in age within stages also affected the
kill, as did a rise in temperature following fumigation. Still using
laboratory-reared resistant insects, Yust and Busbey (O10) determined
the effect of fumigating wet and dusty fruit. Protective stupefaction
occurred on both wet and dry fruit, but the kill was significantly
better on wet fruit whether or not protective stupefaction occurred.
The kill was slightly better on clean fruit than on dusty fruit.

That the resistance to hydrocyanic acid was inherited was demon-
strated by Yust, Nelson, and Busbey (107). Laboratory colonies were
started in 1935 by collecting resistant and nonresistant insects in the
field. Throughout a period of more than 6 years both the differential
and degree of resistance were maintained. No differential in growth
rates was observed, and the permeability of the wax covering of the insect
was not a factor. As would be expected, the resistance of the female off-
spring of crosses of the two strains was intermediate between the two.
The FI males resembled their mother in resistance. The resistance of
the resistant strain, the nonresistant strain, and crosses of the two
could be increased by exposure to repeated fumigations. The increased
resistance was apparent after only two or three fumigations in the resist-
ant stock and crosses but was apparent after only one fumigation of the
nonresistant stock. The authors concluded, therefore, that there were
some nonresistant insects in the resistant stock and some resistant
insects in the nonresistant colony. Yust (99) found the maximum number
of progeny from one female scale of the resistant strain in natural
conditions to be 300.

The influence of repeated fumigations was further investigated by
Yust, Nelson, and Busbey (108). When laboratory-reared insects from the
resistant strain were fumigated with such concentration that complete
kill was not obtained, the resistance of the offspring of the survivors
was greatly increased. When nonresistant insects were treated similarly,


after the fourth fumigation the resistance was greater than that of th..
ordinary laboratory-resistant strain. The authors had previously been
of the opinion that the laboratory colony of resistant insects contained
a few nonresistant scales which would be eliminated after a few fumiga-
tions. However, they now conclude; "In view of the high mortality of the
non-resistant strain in these tests, it is difficult to attribute increased
resistance entirely to the elimination of nonresistant individuals in the
original stocks." In field studies on the influence of concentration on
fumigation results with resistant scales, Yust, Busbey, and Nelson (104)
found that a high average concentration of gas was essential but that a
high initial concentration was not necessary.

lust, Nelson, and Busbey (12) iuzigated scales collected from a
number of neighboring groves, six within a 2-mile-square area and a
seventh about 10 miles away. The survival ranged froa 84 to 12 percent,
and all degrees of resistance were encountered. Protective stupefaction
was further studied by Yust, Nelson, and Busbey ( ,Q) using the resistant
strain only and making no comparison with nonresistant insects. A higher
concentration of prefumigation gas was required to produce stupefaction
at 77* F. than at 50*. In some cases death occurred before the insect
was protectively stupefied. The stage of development affected the length
of time stupefaction persisted. In general, the duration was longer with
mature females than with scales in the second molt. YusB, Nelson, Busbey,
and Fulton (111) investigated the influence of various e.:. sure-concentra-
tion combinations on fumigation results. Here again, only the resistant
strain was used and no comparison was made with nonresistE.-Lt insects.
In general, the kills were proportional to the ca centration when the
product of exposure multiplied by time was constant. The relationship
did not hold with protectively stupefied insects.

Lindgren and Dickson (5), using strains that had been reared 65 to
70 generations in the laboratory without fumigation, also found the
differential resistance to be maintained. This number of gen. --l ions
occurs in about 25 to 30 years in the natural state. From a g -,. tics
standpoint they conclude: "The mcre nearly the population becomes one
of purely resistant scales, the greater is the likelihood that tL.. genes
for non-resistance present will be contained in semi-resistant females
and hence the slower the population change." Dickson and Lindgr--. (10)
determined the number of generations of red scale in the field to be from
2 to 3.2 per year. In summarizing their experiments over 10 years -
determine whether there are strains resistant to hydrocyanic ac'i -a-
tlon, they concluded that the resistant strain then formed a larn.
of the scale population. Dickson and Lindgren (20) found that the E2
of generations of red scale varied from 2 per year along the con
in the interior. Insects belonging to both resistant a..J nonresie --tr.
strains were present in all areas examined.

Lindgren and Gerhardt (54) found no difference between the strain r.
in susceptibility to fumigation with ethylene dibromide. When they s,-
7*7 ug. of this fumigant per liter of air, 90 percent of the nonresistant
and 92 percent of the resistant insects were killed. When 0.1 mg. of
hydrocyanic acid plus 7.7 mg. of ethylene dibromide was used, the kil

- 10 -

dropped to 70 percent of the nonresistant and 32 percent of the resist-
ant strain. With 0.2 mg. of cyanide plus 23.1 mg. of ethylene dibromide
per liter the kill was 98 percent of nonresistant and 48 percent of
resistant scales. When 0.2 mg. of hydrocyanic acid was used alone, the
kills were 95 and 50 percent, respectively.

Munger (E9) found no difference in reproductive ability and normal
mortality between the two strains at varying conditions of temperature.
He also (personal communication Yust to King 1948) found no difference
in the developmental time of the two strains.

Black Scale

In his first paper on resistant scale insects, Quayle (76) included
the black scale, Saissetia oleae (Bern.). His attention had been called
in 1915 to the difficulty in killing this insect near a small town in
California. His experiments showed considerable variation in susceptibil-
itj to hydrocyanic acid fumigation between insects from this locality and the
so-called normal or nonresistant strain. His 1922 paper (72) added con-
firming evidence. Woglum (98) first noted resistant black scales in 1912
in the area described by Quayle (76.). His findings were not published
at that time. He agreed that resistance in that area had increased.
Gray and Kirkpatrick (26, 2, 2_), on the basis of laboratory experiments,
decided that the resistant strain of black scale actually is resistant
to fumigation with hydrocyanic acid but not immune. However, a dose
high enough to give effective control is d dangerous to the tree. As with
red scale, protective stupefaction occurred.

Pratt, Swain, and Eldred (75) confirmed the presence of a resistant
strain of black scale and also agreed that protective stupefaction occurred
when exposure was first made to a low dose of hydrocyanic acid before the
application of the killing dose. Swain and Buckner (42) found no difference
in the susceptibility of egps from resistant and nonresistant black scales
to fumigation with hydrocyanic acid. Lindgren and Dickson (5), using
laboratory-reared offspring of collected insects from areas in which
strains occurred, found the same variation in resistance. Lindgren and
Sinclair (a), following fumigation, recovered more hydrocyanic acid from
nonresistant insects than from resistant insects.

Codling Moth

The codling moth, Carpocapsa pomonella (L.), first appeared in Colorado
about 1891, and general spraying with arsenicals was begun 3 or 4 years
later. Hough (3A, .) found the ability of larvae from a Colorado strain of
this moth to enter apples sprayed with lead arsenate to differ from that of
larvae from the strain common in Virginia. Haseman and Burk (32) fed measured
doses of arsenic to codling moth larvae of both Colorado and Missouri strains.
As they observed no difference in the killing dose, they concluded from
limited experiments that no difference in resistance existed. Haseman and

Meffert (2.), using sodium arsenite and lead arsenate, could find no
difference in susceptibility between Virginia, Colorado, and Missouri
strains when the toxicants were injected into the hemocoele, or through
the mouth into the digestive tract. They agreed with Haseman and Burk
that no resistance to arsenic had been developed by the Colorado strain.
They admitted, however, that in the field this strain was more difficult
to control with arsenical sprays than was the Virginia strain. Webster
(94) reported that both lead arsenate and oil-lead arsenate sprays were
becanming less effective each year for the control of the codling moth.

Hough (36) found the Colorado strain more able to enter apples
sprayed with cryolite, barium fluosilicate, and rotenone, as well as
lead arsenate, than was the Virginia strain. 3y rearing successive
generations of larvae from both strains on fruit sprayed with lead
arsenate, he found that both strains, now called Colorado-K and Virginia-
K, had greatly increased their ability to enter sprayed fruit. Crosses
between "normal" Colorado and "normal" Virginia strains produced larvae
intermediate between the two in ability to enter sprayed fruit. This
intermediate position was maintained throughout the 10 generations
tested. Colorado larvae endured starvation better than did the Virginia
strain. No difference in tolerance to potassium cyanide fumigation was
noted between the eggs from the four strains when the eggs were less than
24 hours old. However, eggs containing embryos showed a similar variation
to that exhibited by mature larvae. Hough (32) in his introduction

"The history of codling moth control in commercial apple-
growing districts of Virginia during the past 25 years in-
cludes the following pertinent items: the number of sprays
has doubled, and in many orchards it has trebled; the volume
of sprays applied per tree of bearing age and similar size
has more than doubled; spraying equipment has greatly in-
creased in mechanical efficiency; the number of sprayers pe
unit of acreage has increased; the minimum dosage of 2 pouri:Is
of lead arsenate in 100 gallons of water has increased to 3
pounds; effectiveness of lead arsenate has been improved by
refinements in its manufacture; lead arsenate has been
fortified by the use of oil as an ovicide and as a sticker;
other materials have been added to the sprays to increase
efficiency and orchard sanitation adopted to supplement
chemical control; but the codling moth is still the most
important apple insect and the percentage of injury is at
least as high as it was 20 to 25 years ago."

From larvae collected from a number of sources he wps able derelo
strains with increased ability to enter fruit sprayed with various insecti-

Steiner, Arnold, and Summerland (88), working in Vincernies, Ind.,
found that codling moths from two orchards in the area differed greatly
in their ability to enter apples sprayed with lead arsenate. The two
orchards bad been sprayed in a similar manner until 6 years before the

- 12 -

:-.xci'-I' !:r l when the spraying of the orchard containing the nonresistant
strain v;& discontinued. The authors therefore assume that the differen-
tial had developed since that time although no experimental evidence is
giver. to justify the assumption. No difference in ability to enter un-
sprayed fruit was observed.

Drosophila and Cotton Aphids

Bcy e (5), by artificial selection, produced strains of the vinegar
fly, Di ;c->. ls melanogaster Meig., and the cotton aphid, Ahis gossypii
Glov., that showed a slightly increased resistance to fumigation after
seven generations. Selection was made by exposing the population to a
concentration of hydrocyanic acid that would effect a high percentage of
kill. The eggs from the survivors were collected and allowed to hatch,
and the process was repeated. Boyce's report was described by him as a
prog-.'..s report only, but no subsequent publications have been noted.
Boyce was also using the granary weevil, Sitophilus granarius (L.), the
:o*-;a fly beetle, Tribolium confusum Duv., and the saw-toothed
: .r beetle, Orysaephilus surinamiensis (L.), but no report of his re-
sults will these insects was made. Quayle (78) mentioned that Droso-
onils. was then being tested in his laboratory to determine the possibil-
ity of m]c ng it in resistance studies.

L Heritier j-id Teissier (5) reported that a strain (ebony) of
Dro. :. *^i in their laboratory reacted in a very abnormal manner to
carbon Lioxide. Normally, Drosophila can be anesthesized readily with
the -. and maintained in that state for a considerable time. The
ins t then recovers completely and without any apparent residual effect.
The si-- -'Lible strain, however, under certain conditions of temperature
and -s :--ncsntration, did not recover. In further work these workers
(J) oL .: that, although the susceptible factor was inherited, it was
tran,..t`i.d independent of the chromosomes and could therefore not be
consiK' red as a Mendelian mechanism. Later they (47) postulated that
the susceptibility to carbon dioxide is subordinated by the presence in
t1e cyI, o.rsirSn of a diffusible substance which they call "sigma."
L' Herii- k.- and de Scoeux (48) transplanted ovaries from wild resistant
flies into the susceptible strain and then mated them with resistant
ale t,.. eggs were laid, the females with transplanted ovaries were
still susr -r bible to carbon dioxide. From the eggs both ebony and
-:.;-, clly wild-type flies were obtained. All the ebony strain were
^c -ti le and also some of the wild t-pe. When the F1 flies were
T. ~t 1is did not transmit the susceptibility but all the females
did. Sonneborn (87) considers the phnomenon to be similar to that
occurin in Paramecium aurelia when certain strains develop a substance
that is toxic to other "normal" strains. This trait is inherited and
is apparently due to a cytoplasmic rather than to a genic difference.
B, R, Bartlett (personal communication) collected 16 strains of
Drosop-iila, representing diversified climatic and food-habit features,
from laboratory and field sources. These strains he isolated as family
lines and tested as to their susceptibility to DDT, tartar emetic, and
hydrc-?-an-Lic acid. Differential resistance between strains was demonstrated

And shown to be an inherited factor rather than nutritional. Successive
selective insecticidal treatments resulted in increased resistance in
some strains but not in others, and when it did occur the increase was
very slow. Resistance to DDT and hydrocyanic acid was determined for
adults only, but resistance to tartar emetic was demonstrated for both
larvae and adults. A number of strains were resistant to tartar emetic,
and the resistance was not specific. The most hydrocyanic acid-resistant
strain was thought to have high specific resistance to this fumigant but
the author also states: "Adult flies of this strain were shown, however,
to be resistant to diethyl ether and to subfreezing cold exposure as well
as anesthesia induced from either of these two treatments." Since only
a summary of Bartlett's work is available to the reviewer, this seeming
discrepancy as to specificity cannot be clarified.

The strain showing greatest resistance to DDT was also resistant to
the fluorine analog, moderately resistant to hydrocyanic acid and tartar
emetic, and slightly if at all resistant to isoborrl thiocyanoacetate.

Citricola Scale

The story of resistance in the citrioola scale, Coccus pseudo-
magnoliaru (Kuw.), is somewhat the same as for the other insects. Quayle
(7) states that his attention was called to unsatisfactory hydrocyanic
acid fumigation results against this insect in a very small area in
California in 1925. Prior to that time satisfactory control had been
obtained. The dosage recommended was sharply increased, but results were
still poor. The area in which tolerance was manifested extended rapidly
until 1933-1934, when for some unknown reason the citricola scale dis-
appeared so completely that, except in a very few cases, treatment for
the pest was not renewed. The scale began to reappear in 1936. Test
fumigations indicated that the resistance continued. In the meantime
laboratory studies on the resistant strain' s survival of hydrocyanic
acid fumigation agreed with the results observed in the field.

Flour Beetles

Using the confused flour beetle, Gough () developed a strain
resistant to fumigation with hydrocyanic acid by selecting the survivors
from a series of fumigations in which the dose was so regulated that all
insects were not killed. This resistance was shown to be inherited. The
female insects were much more susceptible than the males in one generation,
but in others little difference was shown. Pupae were least susceptible,
followed by the adult, larvae, and eggs. Gough was unable to detect any
morphological difference between the two strains. He could find no
correlation between length of life cycle or body size and resistance.
There did seem to be some relation to oxygen uptake. The resistant
strain used 2.803 cu. m. of oxygen per hour per milligram of live weight,
and the nonresistant only used 2.163 cu. mm. Gough noted that under
certain conditions adult beetles emitted a substance that was toxic to
the beetles.

- 13 -


Knipling (4) found that phenothiazine showed considerable varia-
tion in toxicity to larvae of lacilia sericata (Meig.) and L. curina
Mied. He stated that K. C. Bushland had noted (unpublished) a similar
fact concerning the screw-worm. Knipling developed a resistant strain
of screw-worms oy rearing the larvae through successive generations
exposed to a sublethal amount of phenothiazine in the breeding medium.
After 11 such generations the number of the resistant screw-worms sur-
viving toxic concentrations of phenothiazine was 18 times that of the
normal stock. The fifth generation showed no differential resistance
to diphenylamine or diphenylamine oxide. The susceptibility of later
generations to these chemicals was not determined.

Blue Tick

Early in 1940, according to DuToit, Graf, and Bekker (22), reports
were received that the single-host blue tick, Boophilus decoloratus (Koch),
was not being controlled by the sodium arsenite dips that formerly had
proved effective. Only a small area (30 by 15 miles) in South Africa was
then affected. Field studies by the authors confirmed the reports. The
usual procedure of checking the water used, the quality of chemicals,
dipping methods, etc., was followed, but the ticks appeared to be def-
initely resistant. Other species, Rhipicephalus evertsi, R. appendic-
ulatus, R. simus. R. capensis and Amblomma hebraeum, in the area did
not display this resistance and were effectively controlled by arsenic.
Omer-Cooper and -Whitnall (71) collected both resistant and nonresistant
strains of these ticks and dipped them in arsenical solutions in the
laboratory. A marked variation in susceptibility between the two strains
was noted. Concentrations of the toxicant were used that would be
dangerous to use on cattle, but the ticks were still resistant. Whitnall
and Bradford (95) found that the resistant ticks were 100 percent con-
trolled with 0.0029 percent of gamma benzene hexachloride, while 1 percent of
DDT only gave 60 percent control. Comparison was not made with non-
resistant insects.

Citrus Thrips

A strain of citrus thrips, Scirtothrips citri (Moult.), that was
tolerant to tartar emetic was described by Boyce and Persing. (),
Persing et al. (72), and Boyce, Persing, and Barnhart (8). In 1939
tartar emetic-sucrose solution had been recommended for the control of
citrus thrips (Boyce and Persing (69), and in 1941 resistance was noted
in certain lemon groves in the San Fernando Valley, Calif. In laboratory
tests 1,695 nonresistant insects were killed, but 19.3 percent of the
resistant strain survived out of 1,365 sprayed. Four times the usual
recommended dosage (12 pounds each of tartar emetic and sugar per acre)
still gave unsatisfactory control. Smith (85) suggests that resistant
strains of this thrips may be present in certain areas in Transvaal,
South Africa.

- 15 -

McGregor (a) collected in the field thrips that were resistant and
nonresistant to tartar emetic. After seven generations in the laboratory,
the differential in resistance was still apparent. No survivors of spray-
ing or their progeny were used in the tests. The resistance was not due
to insects avoiding feeding on the tartar emetic.


Mosna (6_) found that Culex pipiens autogenicus taken from the
Pontine marshes near Rome withstood the action of DDT for 32-48 hours
whereas his laboratory strain exposed to similar doses died in 3-5 hours.
The existence of resistant strains of mosquitoes was also acknowledged
by Missiroli (1_).

House Flies

Missiroli (6_) also emphasized that the house fly in Naples, Italy,
had become resistant to DDT. McGovran et al. (8) had described a
temporary resistance in house flies that had received a knock-down dose
of pyrethrins before being exposed to a higher "killing"1 dose. This
phenomenon was apparently similar to the so-called "protective stupefac-
tion" of Gray and Kirkpatrick (26).

Wiesmann (96) tested flies collected from Arnas, about 1,000 kilo-
meters north of Stockholm, Sweden. The control of house flies in this
area in 1946 with DDT had been much poorer than expected. Absorbed
through the tarsi, the lethal dose of DET for the Arnas strain was 2.5-5
gamma,whereas for his normal strain it was only 0.025 gamma. The Arnas
strain was also more resistant to temperature than the Basel laboratory
strain. In addition to physiological differences Wiesmann found certain
morphological differences between the two strains. The extremities of
the resistant, or Arnas, strain were notably more pigmented. The tarsal
hair tuft was much stiffer, the tarsal joints were larger, and the
pulvilli and articular membranes of the joints were about one-third
thicker. The two strains could therefore be considered as two races.

Sacca (83) reported the presence of resistant house flies in certain
areas in Italy. In his opinion they are a different species and he
suggested the name Musca domestic var. tiberina. Lindquist and Wilson
(6) developed a strain of house flies in the laboratory that was much
more resistant to DDT than the so-called normal laboratory stock. A
large population of flies from the laboratory colony was sprayed with such
concentration of DDT that about 90 percent kill was obtained. Succeeding
generations were each sprayed with DDT. As the exact quantity required
could not be estimated in each case the 90 percent mortality finally was
obtained by exposing the sprayed insects to either heat to decrease
mortality or cold as required (personal communication from H. G. Wilson).
After 3 generations of such treatment a difference in susceptibility to
DDT was noted. After 14 generations it was marked, only 29 percent of
the special stock being killed by a dosage of DDT that killed 68 percent

- 16 -

of the regular stock. The same year V'ilson and Gahan (9_) found that
the resistant flies were al- more resistant to chlordane, pyrethrins
plus piperonyl cyclonene, chlorinated camphene, rotenone, and Thanite.
By selective breeding, therefore, a race of resistant flies had been

The laboratory of Blickle, Capelle, and Morse (4) was accidentally
contaminated with benzene hexachloride. Their house fly colony became
reduced to a few individuals and their present colony has been developed
from those survivors. The flies have been exposed to benzene hexachloride
from the time they emerged from the puparia until they were used in tests.
This situation has continued for over 3 years. In the first generation
39.9 percent were killed. oy exposure to 0.01 percent of the gamma isomer.
In the 28th generation only 6 percent were killed following exposure to
0.03 percent. A differential resistance to DDT, pyrethrirns, and Lethane
384 Special aliphaticc thiocyanate) also was present, but not to the
extent of that with benzene hexachloride.

From a resort hotel at Ellenville, N. Y., Barber and Schmitt (l)
collected flies in which resistance to DDT had apparently developed.
From these flies a colony was established and tested in the laboratory.
A definite differential existed when the resistant flies were compared
with the ordinary laboratory strain in resistance to DDT (technical or
pure), methoxychlor, and TDE. The authors also state that these flies
"showed no resistance at all to residues of toxaphene, chlordane, parathion,
the gamma isomer of benzene hexachloride, and tetraethyl pyrophosphate.
Seemingly, therefore, flies of the Ellenville line have acquired a specific
resistance to DDT in its various forms but none to certain other compounds.t"
To the reviewer, from the data included, this statement seems justified
only for toxaphene. With chlordane 15 minutes' exposure to 14.4. mg. per
square foot gave mortalities of 95.6 percent for the laboratory strain and
60 percent for the wild strain. With parathion the same exposure gave 96.2
and 77.8 percent mortalities, respectively. With the gamma isomer all
flies were killed at all exposures and concentrations tested. With
tetraethyl pyrophosphate long exposure to a low concentration had no effect
and short exposure to the high concentration used gave complete kill. With
chlordane and parathion, from casual inspection of these figures, a dif-
ferential resistance seems definite, but no conclusions can be drawn from
the data for the gamma isomer or tetraethyl pyrophosphate. Therefore,
unless considerable data other than those published in this paper are avail-
able, the claim that the resistance is specific for DDT and its analogs must
be questioned. It is possible that the authors have in mind, instead of a
physiological differential resistance between the two strains, a resistance
of such degree that economic control of the insects by insecticides other
than DDT would be prevented. From their data a statement that satisfactory
control could still probably be obtained with insecticides other than analogs
of DDT would seem justified but not that a specific resistance was indicated.

- 17 -

Bettini and Barachini (.) report failure to control house flies with
5.3 grams of DDT per square meter in an area whbre excellent control had
previously been obtained. Complete control up to 6 months was obtained
following treatment with a 1.5-percent kerosene solution of Octa-Klor
(chlordane) applied at the rate of 2.35 grams per square meter. Although
the control was attributed to the Octa-Klor alone, it should be noted
that owii* to the scarcity of kerosene, in some cases the material had
been added to kerosene already containing 5 percent of DDT; so a combined
effect is possible. With Gammexane alone (percentage of gamma isomer not
given) dosages up to 3.57 grams per square meter gave unsatisfactory re-
sults. When the Gammexane was dissolved in the kerosene containing 5
percent of DDT, excellent control of flies was obtained. However, the
workmen and supervisors complained of nausea and headache.

The development of strains of house flies resistant to DDT, pyrethrins,
and an unnamed botanical extract was claimed by Barber, Starnes, and
Starnes (&). Measured doses (apparently LDcO) of each insecticide in
acetone were applied to the thorax of adult flies. alf the treated flies
were retained in petri dishes and half in oviposition cages. Succeeding
generations were treated in a like manner. After 1 generation and con-
tinuing through 10 generations, fewer treated flies died among those held
in petri dishes than among untreated checks. The figures for the DDT-
treated flies held in the oviposition cages were inconclusive, but those
for flies treated with pyrethrum and the botanical extract were considered
to be significantly different. The authors conclude: "The evidence appears
to show that the flies of the treated lines had become measurably resistant
to each of the insecticides by the second generation and maintained such
resistance throughout the experiments.' The average weight of puparia from
the 30 pneraticms tested was 16 mg. for the checks and 18 mg. for the
treated flies. The authors suggest that this difference may have been due
to killing off of the weaker flies, which may have been the smallest. They
report the average weight of the puparia of their sero generation as 20 mg.
and do not comment on the seeming discrepancy. More puparia failed to
emerge from treated lines than from the checks. The authors postulate that
small amounts of the active principles of the insecticides might be trans-
mitted from the treated flies through the eggs and larvae to the puparia.
To the reviewer this postulation seems unjustified and, indeed, until the
differences in survival between the "petri-dish" and 'oviposition-cage"
flies is explained, the authors' conclusion as to resistance seems ques-

Wild strains of house flies were collected by King and Gahan (.) from
areas in Teuas, Georgia, North Carolina, California, and Florida where
failure to obtain control with DDT had been reported. Such failure has
probably been due in part to inadequate sanitary practices and faulty ap-
plication of the insecticide (personal communication from E. F. Knipling).
The resistance of several of these strains when tested under carefully
controlled conditions was definitely higher than that of the laboratory
strain with which they were compared, but was much lower than that exhibited
by the resistant strain of Wilson and Gahan. The resistance was apparently
general, but the differential for other insecticides was Uot so great as for


Evidence was presented that some of the treated surfaces in barns
were repellent to flies. It was not indicated whether adequate controls
were run to distinguish between the repellency due to the insecticide
and the possible effect of the other ingredients in the mixtures used.
Chlordane wettable powders seemed somewhat attractive to the flies.

psy Moth

According to Melander (60), R. W. Glaser had informed him that by
feeding increased doses of lead arsenate to gypsy moths a strain had
been reared that fed on heavy doses of the chemical apparently without
toxic effects. On the other hand, Campbell (1l) states that "Glaser
himself, however, was never convinced that he had proved or disproved
the development of immunity or of tolerance to arsenic, and consequently
the results of his experiments were never publishedt."


From the foregoing reports the use of either term "tolerant" or
"resistant" insects seems applicable. Certainly the phenomenon is
distinct from that connoted by the term "immune." Huff (g) lists
many insects in which true immunity has been demonstrated. Newell (20),
speaking of colonies of honey bees that are resistant to American foul-
brood, states that the characteristic is resistance, not immunity. At
least part of the resistance to foulbrood is due to the resistant bees'
habit of cleaning out dead diseased larvae from the comb and not allowing
the disease to reach the spore stage. The nonresistant strains do not do
this. In other words, the resistant bees are the better housekeepers.
The resistance described is also distinct from the resistance or
weakening induced by diet (Phillips and Swingle 7, Karkos and Campbell
57) and the susceptibility encountered due to the age of the insect.
The variation in susceptibility to a toxicant between various stages of
insects is well known (Campbell 10, Simanton and Miller U). The same
variation was found in many of the insects mentioned above. Suscepti-
bility also varies with respiratory rate (Cotton =.

Ripper (82) believes that a combination of chemical and biological
control may be necessary. However, the delicate balance between concen-
tration of insecticide, time of spraying, and number of parasites re-
quired would seem to preclude the general use of this method of control.

The theory underlying the variations in the number of individuals
of two animal species living together where one species is a parasite of
the other is discussed by Volterra (9.)*

Reviews of the general subject of resistant insects have been made
by Thorpe (21), Quayle (78 81,), and Smith (86).

It is difficult to arrive at definite conclusions as to whether the


resistant insects are generally tougher insects or whether some of the
resistance is specific for the chemical at hand. Tust (personal
communication to W. V. King) considers the resistance of red scale
specific for hydrocyanic acid. The results of Knight, Quayle, Chamberlin,
and Cressman, however, do not entirely support this conclusion. That the
resistant strain of red scale and the codling moth give rise to resistant
progeny seems conclusive; with other insects the evidence varies. There
are many points that require clarification. Both resistant and non-
resistant strains of red scale have been reared for years under laboratory
conditions without being subject to fumigation. The original stocks in
each case were obtained from field collections. It seems very improbable
that pure strains of the two species were collected in each case; yet
after 10 years and longer without fumigation the variation in resistance
was still pronounced and of the same order of magnitude. With one colony
of resistant house flies (King &0) the resistance dropped rapidly when
spraying was discontinued, and was nearly normal in about 12 generations,-
an indication that it is an acquired resistance.

Melander's (60) observation that both strains were effectively con-
trolled by oil emulsions was based on field tests. No careful laboratory
work was done to determine whether any differential resistance existed.
Other results of field tests may have been misinterpreted owing to the
influence of other factors, Quayle (772), for instance, obtained about
5 1/2 percent greater kill of scale insects on thinly foliated lemon trees
than on those with large amounts of foliage. Insects were also much
more difficult to kill on fruit than on twigs or leaves and more difficult
to kill on vigorous shoots such as suckers than on less thrifty twigs and
leaves. Thompson (0) showed that fewer scale insects were found on
citrus trees deficient in magnesium than on "normal trees." Haseman (W1
found that the mineral content of the soil on which host plants were
grown affected to a considerable degree the number and hardihood of
insect parasites feeding on them. Lindgren and Dickson (A) and Cressman
() in laboratory tests, however, found no difference between the
strains of red scale's resistance to oil spray. Cressman found the
resistant strain more resistant to cube resins than the nonresistant insects,
although the differential was not so large as that obtained with hydro-
cyanic acid,. Indeed, in the three pairs of figures given, the differences
were only 4.2, 1.7, and 2.7 percent. These differences, although highly
significant statistically, without statistical analysis would usually
have been considered normal biological variation. Lindgren and Gerhardt
(54) found no difference in resistance to fumigation with ethylene di-
bromide. When mixtures of ethylene dibromide and hydrocyanic acid were
used, the usual differential occurred. However, the quantity of ethylene
bromide used apparently had little effect. Therefore, it would seem that
further work must be done before a definite conclusion can be drawn.

An arsenie-resistant cattle tick was easily controlled with gamma
benzene hexachloride, but no figures were given to compare the relative
resistance of normal ticks. The resistant house flies reported by
Wilson and Gahan (22), Blickle 1. (4), Barber, Starnes, and Starnes
(2), and King and Gahan (g) were resistant to all insecticides tested.
No report of testing against other materials than DIT was made by Wiesmann,

-20 -

but his resistant strain was also more resistant to high temperatures
than were his normal insects. His resistant insects, however, exhibited
certain morphological differences from the nonresistant strain. These
differences were the only morphological differences reported in the
papers covered by this review. Specific resistance to analogs of DDT
for a strain of house flies was claimed by Barber and Schmitt (&), but
their conclusions must be questioned.

The general subject of tolerance to drugs is covered in a short
discussion in Cushny's Pharmacology (16). The author states that
congenital tolerance to drugs is well known. The hedgehog apparently
is unaffected by many "poisons," and the rabbit is tolerant to very
large doses of atropine. The most familiar example of acquired
tolerance is that of the user of tobacco to nicotine. In a short time
nicotine is a normal constituent of all tissues. Some tissues of an
individual may acquire tolerance to a drug while others do not. The
brain, for instance, may be tolerant to large doses of morphine while
the bowels are not and the subject becomes very constipated. A chronic
drunkard may become less sensitive to other poisons acting on the ame
cells. In some cases the tissues destroy more of the poison than pre-
viously. Others excrete it mawe rapidly and still others absorb it less
readily. Tolerance is soon lost if the drug is discontinued for a shart
time. The arsenio-eating habits of natives of Styria and Tyrol are
often given as examples of arsenic tolerance. However, no tolerance to
arsenic in solution has ever been demonstrated, and until this is done,
the whole subject of arsenic tolerance must remain unsettled. In
Reichenstein miners exposed to arsenic-containing ores are short lived
and exhibit all signs of arsenic poisoning. Cmpbell (1) was Unable
to produce tolerance to arsenic in silkworm by feeding sublethal doees
to larvae.

The tolerance of the rabbit to atropine mentioned above can probably
be compared to the lack of susceptibility of certain insects to various
insecticides. DDT, for instance, readily controlled the Japanese beetle,
codling moth, house fly, and mosquito (22). Other insects, such as the
boll weevil, red spider, and Mexican bean beetle, seem to be little af-
fected. Since DDT acts as both a contact and stomach poison, these dif-
ferences become all the more interesting.

To summarize the present status of resistant insects, there is no
question that several insects have developed resistance in the field to
certain insecticides that formerly gave good kills. Similar resistance
has been developed in laboratory strains through breeding successive
generations of insects that are exposed to toxic but not 100-percent
lethal doses of insecticides. Many factors are involved and no single
type of resistance is apparent. The following conclusions may be drawn
from the literature reviewed in this paper:

1. That the differential resistance of strains of the California
red scale and the codling moth, at least, are inherited through Mendelian
laws, with genes playing their usually accepted role.

2. With Drosophila susceptibility to carbon dioxide is inherited,
but seems to be due to a cytoplasmic factor rather than to a gene.

3. In one strain of house flies morphological as well as physiological
differences have been reported.
4. The resistance may be either natural and inheritable or, theoret-
ically, acquired and not transmitted genetically.

5. No clear explanation of the phenomenon has been offered.

6. The evidence seems to be in favor of a generally increased
resistance rather than a specific resistance, although the evidence is
not conclusive.

7. The resistance is not to be confused with immunity.

8. Finally, sufficient evidence has not yet been accumulated to
permit postulation of a theory as to the cause of the phenomenon.


Literature Cited

(1) Barber, George H., and Schbmitt, John B.
1948. House flies resistant to DDT residual sprays. N. J. Agr.
Expt. Sta. Bul. 742, 8 pp.

(2) _____ Starnes, Ordway, and Starnes, Eleanor B.
1948. Resistance of houseflies to insecticides. Soap and Sanit.
Chem. 24(11)t 120, 121, 143.

(3) Bettini, S., and Barachini, B.
1948. Primi resultatidella lotta con L' octa-Klor ed il Gamaesano
control le mosche domestiche resistenti al DDE. Riv.
di Parassitol. 9(2)t 85-91.

(4) Blickle, Robert L., Capelle, Asher, and Morse, W. J.
1948. Insecticide resistant houseflies. Soap and Sanit. Chem.
24(8)t 139, 141, 149.

(5) Boyce, A. M.
1928. Studies on the resistance of certain Insects to bydro-
cyanic acid. Jour. Econ. Pat. 211 715-720.

(6) _____ and Persing, C. A.
1939. Tartar emetic in control of citrus thrips on lemons.
Jour. Econ. Ent. 32: 153.

(7) _____ and Persing, C. A.
1942. Resistance of citrus thrips to tartar emetic in the San
Fernando Valley and a tentative substitute control
program for emergency conditions. Calif. Agr. Ept.
Sta. News Letter 21, 2 pp.

(8) _____ Persing, C. A,. and Barnhart, C. S.
1942. The resistance of citrus thrips to tartar aetic-
sucrose treatment. Jour. Econ. Ent. 35s 790-791,

(9) Brinley, F. J., and Baker, R. H.
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