## RESISTANCE OF INSECTS TO INSECTICIDES—
A NATIONAL PROBLEM

by C. H. HOFFMANN

Entomology Research Branch
Agricultural Research Service
U. S. Department of Agriculture
Beltsville, Md.

Purchased by U. S. Department of
Agriculture for Official use

The resistance of insects to insecticides is one of the most serious and baffling problems facing entomologists in this country today. Long ago entomologists recognized that some insects were adept at adjusting themselves to meet man's attempts to control them through the use of chemicals.

Resistance to insecticides was first observed in agricultural insects, particularly fruit insects. A number of cases of such resistance are cited in the critical review of the literature made by Babers (1949) and Babers and Pratt (1951). Apparently the first case was recorded by J. B. Smith in 1897. However, it remained for Melander—who in 1908 observed the failure of lime-sulfur to kill all San Jose scales *Aspidiotus perniciosus* (Comst.) on trees—to call attention in 1914—to the possibility that resistance in insects might become a serious problem. In 1910 Quayle observed that the California red scale *Aonidiella aurantii* (Mask.), and the black scale, *Saissetia oleae* (Bern.), in certain areas were more difficult to kill with hydrocyanic acid fumigation. In 1925 he called attention to the unsatisfactory control of the citricola scale, *Coccus pseudomagnoliarum* (Kuw.), with hydrocyanic acid in a small area in California. In 1928 Hough found a strain of the codling moth, *Carpocapsa pomonella* (L.), that could enter apples sprayed with arsenicals more readily than could the normal insect. In the same year Boyce developed strains of a fruit fly, *Drosophila melanogaster* (Meig.), that were resistant to hydrocyanic acid. In 1942 Boyce and Persing found resistance to tartar emetic sprays by a strain of citrus trips *Scirrothrips citri* (Moul.).

This brief history reminds us that the resistance problem was recognized before DDT came into widespread use. Let us now consider this problem in relation to our task of safeguarding agricultural crops from destructive pests and protecting man from annoying and disease-carrying pests.

*Fruit and Nut Insects*.—Several studies have confirmed the resistance of California red scale to HCN fumigation. The most resistant scales were about 25 times as difficult to kill as the nonresistant scales, and required an HCN concentration that was far beyond tree tolerance. Fortunately resistance to HCN was not linked with resistance to oil or parathion sprays, or mixtures of the two, which have been substituted for HCN fumigation and have enabled growers to produce crops. However, with the omission of oil sprays from some schedules the citrus red mite, *Metatetranychus citri* (Mcg.), became a problem.

Resistance to lead arsenate by the codling moth resulted in the decline of several fine apple-growing areas. However, with the substitution of DDT for lead arsenate about 1945 the resistance problem seemed to be solved. Control of the codling moth with DDT has not always been satisfactory in certain orchards since 1952. To determine if resistance to DDT is present, Hamilton studied progeny from 16 strains of codling moth larvae obtained from midwestern and western states during the last three years. Two strains from Washington and one from southern Ohio were definitely resistant. Growers who have difficulty in controlling the codling moth with DDT are increasing the number of applications and the quantity used. This procedure is questionable, for soon they will be exceeding the tolerance established for DDT, and there is no dependable method for removing much of the DDT without injuring the fruit. To avoid residues, DDT applications must not be made the last 40 days before harvest, and methoxychlor and TDE cannot be used in late season following a full early-season DDT schedule. Methoxychlor does not appear to be effective against DDT-resistant strains, and so cannot be substituted on a full schedule that would meet residue tolerance requirements. Parathion and mixtures of parathion and DDT are in wide use. EPN and malathion, either alone or with DDT, and

ryania, the latter of which is exempted from a tolerance, show much promise in controlling the codling moth.

Mites continued to be the number-one orchard-pest problem during 1955. Their increasing resistance to parathion, EPN, and malathion led to the testing of nonphosphorus compounds. Most promising, in addition to Aramite, which is widely used, were Rohm & Haas FW-293, dichlorophenyl benzenesulfonate (Genite), ovex (Ovotran), DMC, p-chlorobenzyl p-chlorophenyl sulfide (Mitox), and Chlorobenzilate. Promising phosphorus compounds include demeton and Diazinon. TEPP and Sulphenone are still used in mite control. In some areas the citrus red mite appears to have developed resistance to ovex.

Laboratory tests were conducted in Hawaii to determine if the oriental fruit fly, *Dacus orientalis* (Hendel), could become resistant to insecticides. Strains were developed that required at least 30 times as much DDT for a given mortality as did the original strains. The resistance increased gradually at first, but had become marked by the 30th generation. It increased much more rapidly when both larvae and adults were exposed to DDT than when only the adults were exposed.

In 1953 observations were made in Indiana on an outbreak of *Erythroneura lawsoniana* (Baker), an apple leafhopper, against which DDT appeared to be ineffective although in previous years the grower and others nearby had experienced little difficulty in controlling presumably the same species with DDT. This situation is suggestive of resistance. Reports from California cover a similar situation observed with the grape leafhopper, *E. comes* (Say).

In 1953 Michelbacher and Davis encountered a strain of the walnut aphid, *Chromaphis juglaudicola* (Kltb.), from the San Jose area in California which in the laboratory showed resistance to parathion seven times that of a nonresistant strain, and field experiments confirmed this resistance.

*Vegetable and Sugar-Beet Insects*.—Since 1952, commercial growers in the Charleston, S. C., area have had difficulty in controlling the cabbage looper, *Trichoplusia ni* (Hbn.), on cabbage and cauliflower with dosages of DDT that were formerly effective. A report from the Phoenix, Ariz., area stated that DDT was no longer effective there against the cabbage looper on lettuce. Additional reports suggested apparent resistance of this looper to DDT in localized areas of New York (pronounced in 1955), North Carolina (began about 1947), Ohio, Illinois, Wisconsin, and Minnesota. To meet this problem researchers have turned to other materials, and now toxaphene and endrin are recommended for the control of this insect on cabbage, and toxaphene on lettuce, where resistance to DDT has been encountered.

Apparent resistance of the important cabbageworm, *Pieris rapae* (L.), to DDT has been reported in Ohio, Illinois, Wisconsin, Minnesota, and New York. Oddly, there has yet been no difficulty with control of this insect in the Southeast.

Kring of Connecticut and Gould and McCrosky of Indiana record the difficulty experienced in these States beginning in 1951 to control the potato flea beetle, *Epitrix cucumeris* (Harr.), with dosages of DDT that formerly were effective. In Connecticut this failure occurred about five years after the insecticide came into general use, or after only five or six generations had been exposed to it. Kring found that chlordane, dieldrin, Dilan, endrin, and heptachlor, either alone or in certain combinations, gave control in fields where DDT was no longer effective.

Workers in New York and North Dakota have reported failure to control the Colorado potato beetle, *Leptinotarsa decemlineata* (Say), with DDT, and the necessity to revert to arsenicals and rotenone in those areas.

Reprinted from the 42nd Mid-Year Meeting Proceedings of the Chemical Specialties Manufacturers Association, Inc., May 1956

In Connecticut rotenone in some areas does not control the Mexican bean beetle, *Epilachna varivestis* (Muls.), as well as it did a few years ago, and increased dosages and more frequent application are needed. In 1954 Brubaker and Brett found that rotenone dusts also failed to control this insect in the foothills of western North Carolina. Preliminary studies indicated malathion or a mixture of malathion and methoxychlor might be substituted for rotenone.

Since 1952 Landis, Schopp, and others have found resistance of the green peach aphid, *Myzus persicae* (Sulz.), on potatoes in parts of Washington and Oregon to parathion and DDT, which previously gave good control. At dosages four or five times higher than those used successfully in 1950 and 1951, parathion applied with ground equipment seemed to be effective, but aircraft applications were unsatisfactory. In Maine potato growers have been able to control aphids with parathion following earlier applications of DDT.

From 1946 to 1951 growers of sugar-beet seed in Arizona obtained excellent control of lygus bugs with one application of DDT. By 1953 Hills found lygus populations in commercial fields increased so much after an application that redusting was needed within 2 or 3 weeks. In experiments with toxaphene instead of DDT no redusting was needed. Laboratory studies have indicated that several chlorinated hydrocarbon and organic phosphorus insecticides may be superior to the DDT dust that was formerly so effective.

*Greenhouse Pests.*—Resistance of the two-spotted spider mite, *Tetranychus telarius* (L.), was recognized in 1947 following their sensational control with aerosols containing HETP, TEPP, or parathion. In 1948 the resistant mites were prevalent in the East, being spread through the sale of rose plants from an infested greenhouse. Floyd Smith at Beltsville, Md., has found that these spider mites have become resistant to many of the new insecticides after repeated application and one to three years' use (Table 1). Of considerable interest is the record of a colony from a single rose plant in an otherwise clean house that was still highly resistant to parathion after eight years (1948-56) with no miticide treatment. Taylor and Smith have concluded that the resistance trait in this mite is inherited, and that it can be transmitted by either the male or the female parent.

Table 1. Resistance of Two-spotted Spider Mites to Insecticides (Aerosols)

<table><thead><tr><th>Insecticide</th><th>Year put into commercial use</th><th>Year first showing resistance</th></tr></thead><tbody><tr><td>HETP, TEPP, parathion</td><td>1947</td><td>1947</td></tr><tr><td>Sulfotepp</td><td>1948</td><td>1953</td></tr><tr><td>Ovex</td><td>1949</td><td>1953</td></tr><tr><td>Schradan, demeton</td><td>1950</td><td>1953</td></tr><tr><td>Aramite</td><td>1950</td><td>1953</td></tr><tr><td>Aramite plus schradan</td><td>1953</td><td>1954</td></tr><tr><td>Mitox</td><td>1953<sup>1</sup></td><td>1954</td></tr><tr><td>Chlorobenzilate</td><td>1953<sup>1</sup></td><td>1954</td></tr><tr><td>Rohm & Haas FW-293 (spray)</td><td>1954<sup>1</sup></td><td>1955</td></tr></tbody></table>

¹ Resistance developed before recommended for commercial use.

*Cotton Insects.*—In 1953 toxaphene at usual dosages failed to give satisfactory control of the cotton leafworm, *Alabama ar Gillacea* (Hbn.), in parts of Texas. Laboratory tests confirmed that this insect was about ten times as resistant to toxaphene as a few years previously.

Also in 1953 farmers were not obtaining usual control of the cotton aphid, *Aphis gossypii* (Glov.), with BHC, and though resistance was suspected it was not then confirmed. The failure in 1955 to obtain adequate control in different parts of the Cotton Belt led to renewed suspicions of resistance, and limited laboratory and field tests indicate that they were well founded.

The salt-marsh caterpillar, *Estigmene acrea* (Drury), is resistant to toxaphene and DDT-toxaphene combinations in parts

of Arizona and southern California. Experiments show that Dilan gives satisfactory control.

Of considerable import in 1955 was the laboratory work of Roussel and Clower, showing that in certain parts of Louisiana the boll weevil, *Anthonomus grandis* (Boh.), had become resistant to BHC, dieldrin, endrin, heptachlor, and toxaphene. Moreover, farmers in these areas failed to control weevils with these insecticides in spite of increased dosages and more frequent applications. Resistance was also suspected in localized areas of Mississippi and in southeastern Arkansas, where growers have had difficulty in controlling this pest. However, weather conditions have favored the weevil and made executing a regular spray program difficult. Under these circumstances it is difficult to evaluate the true significance of the boll weevil resistance problem at this time.

Resistance is also suspected in the following cotton insects: The cotton leaf perforator, *Bucculatrix thurberiella* (Busk), to chlorinated hydrocarbon insecticides in parts of the Imperial Valley of California; the cabbage looper to DDT, the southern garden leafhopper, *Empoasca solana* (DeL.), to DDT in southern California; and lygus bugs to toxaphene-DDT insecticides in the Buckeye area of Arizona. Resistance of spider mites to parathion has been suspected but has not been positively demonstrated in the field.

*Pests of Man and Livestock.*—You are all familiar with the phenomenal success with DDT for the control of the house fly in 1945, and how within five years this insect became so resistant that DDT had little value in many localities throughout the country. For a time it appeared that DDT-synergist combinations might solve the house fly problem, but the flies became resistant to the combination. Subsequently the house fly became resistant to other chlorinated hydrocarbon insecticides, and a shift was made to organic phosphorus compounds, including malathion, Bayer L 13/59, Diazinon, and TEPP. An outstanding development has been the use of these materials in baits. Recently Diazinon and malathion have been recommended as residual barn sprays.

In 1949 DDT failed to control the salt-marsh mosquitoes *Aedes taeniorhynchus* (Wied.) and *sollicitans* (Wlkr.) in several areas in Florida. By the summer of 1955 these mosquitoes had also developed resistance to BHC and dieldrin to such a degree in the intensively treated areas that control with these materials was also unsatisfactory. Research has shown that malathion for the control of adults and parathion for larvae are highly effective substitute materials for use on these resistant species.

Studies by Gjullin and Peters in the San Joaquin Valley in California in 1952 showed a high degree of resistance to DDT in larvae of *Aedes nigromaculis* (Lud.) and *Culex tarsalis* (Coq.). In 1953 and 1954 Gjullin and coworkers demonstrated that EPN was highly effective against resistant mosquito larvae in California, and that malathion gave good results when used in sprays and fogs against resistant adults.

Of great concern is the recent report by the U. S. Public Health Service of resistance of the common malaria mosquito, *Anopheles quadrimaculatus* (Say), to BHC, dieldrin, and chlordane in Bolivar County, Miss. Some entomologists believe that this resistance has resulted from continuous exposure to the agricultural chemicals that have been so widely used in that part of the country. Fortunately DDT is still an effective adulticide against this species.

Mention should be made that the northern house mosquito, *Culex pipiens* (L.), as well as *Psorophora confinnis* and *P. discolor*, are resistant to DDT. The last two species have also been found resistant to dieldrin in Mississippi.

*Household Insects.*—The widespread occurrence of resistance in house flies and certain mosquitoes to DDT treatments that were formerly outstanding has already been discussed.

In 1952 Heal and Babers found a strain of the German cockroach, *Blattella germanica* (L.), from Texas that was about 200 times as resistant to chlordane as normal roaches, and also highly resistant to DDT. It is believed that this resistance is fairly widespread. Strains of this species have also been reported to be resistant to chlordane, lindane, dieldrin, pyrethrum, and to pyrethrum plus piperonyl butoxide. Where these materials fail

to give adequate control, malathion and Diazinon may be used as spot treatments.

An increasing number of reports indicate difficulty in control-
ling fleas, particularly the cat flea, *Ctenocephalides felis*
(Bouche), with DDT and other insecticides. Though chlordane,
DDT, methoxychlor, and pyrethrum are recommended for use
in homes, they do not always furnish good control. The newest
material for control of fleas is malathion, which is used as a dip
or dust on animals, in kennels, and for spot treatment in homes.

DDT-resistance in the bed bug, *Cimex lectularius* (L.), is becoming fairly widespread. Lindane sprays have been recommended for the control of resistant strains. Because of reports of failure to obtain satisfactory control with lindane, it is suspected that resistance to this material is developing. Malathion is highly effective against resistant bed bugs, and eventually may be recommended for treatment of baseboards and other hiding places.

In New Jersey the American dog tick, *Dermacentor variabilis*
(Say), was reported by Hansen to have become resistant to
DDT in 1955.

In the laboratory Eddy and coworkers found that the human
body louse, *Pediculus humanus humanus* (L.), could develop a
high resistance in 25 generations when maintained on cloth im-
pregnated with 0.001 per cent of DDT. Mortality was very low
after initial exposures, but resistance developed. Resistant lice in
Korea and other parts of the world are being controlled effec-
tively with 1 per cent lindane and pyrethrum powders.

In Illinois in 1950, Bruce and Decker found that the larvae of
filter flies, *Psychoda alternata* (Say), became resistant to DDT,
which had previously given excellent control. Initial chlordane
treatments were effective against resistant larvae, but later ones
failed.

*Trends in Resistance*.—The foregoing review of examples of insects that have become resistant to insecticides leads to several important conclusions. It is clearly evident that many of our most important insect pests are resistant not only to DDT but also to other chlorinated hydrocarbons, all of which were so outstandingly effective at first. This failure of the chlorinated hydrocarbon insecticides has caused entomologists to turn to the highly effective organic phosphorus materials. In general they have been quite satisfactory, but some pests, particularly the mites, have become resistant to them. Resistant species are dispersed among several orders of arthropods—Hemiptera, Diptera, Coleoptera, Lepidoptera, Siphonaptera, Anoplura, and Acarina. Many species are presently under suspect, and there is every reason to believe that additional insect and mite pests will become resistant as the use of insecticides is continued year after year.

*Origin and Development of Resistance*.—The general occur-
rence of such an important phenomenon as insect resistance
naturally causes one to be curious as to how it originates and
what are the developmental steps. Insect resistance to insecti-
cides may originate in several ways:

(1) Selection by the chemical of the most resistant individuals in a population. The resistant individuals interbreed and transmit this characteristic to their offspring. The chance of such inter-breeding is increased when the insecticide is widely used and most of the susceptible individuals are killed. The continued exposure of the resistant progeny to the same insecticide results in progressive strengthening of the resistant characteristic in the genetic makeup of the entire population.

(2) Action of the insecticide in selecting those naturally oc-
curring mutants possessing factors conferring some degree of
resistance. The resistant progeny eventually become dominant.

(3) Production of resistant mutations arising from the action of repeated sublethal dosages of insecticide on the insect.

Resistance to an insecticide is usually far advanced before in-
vestigations can be carried out; therefore, much reliance is
placed on laboratory studies even though the factors of selection
may be different from those that occur naturally. Metcalf (1955),
in discussing the development of resistance to a toxicant by the
house fly under continuous selection, refers to two stages—(1)
an initial period of selection during which a five- to ten-fold in-
crease in resistance may be attained in 20 to 30 generations of

adult selection, or 5 to 10 generations of combined larval and
adult selection, and (2) a period of very rapidly intensified re-
sistance which may increase several times with each succeeding
generation and attain levels of 100 to 1000 times normal within
6 to 20 further generations. Of outstanding significance is the
fact that the development of tolerance to a given insecticide seems
to predispose an insect to the subsequent rapid development of
resistance to other insecticides, particularly those closely related
structurally. This situation has hindered efforts to control re-
istant pests with available alternate materials.

*Biochemistry of Resistance*.—Much work has been done on the biochemistry of resistance in house flies (Metcalf 1955). Early workers envisioned rather simple explanations, including the idea that a thick cuticle of the pulvilli and intersegmental membranes of the feet inhibited the rate of DDT absorption. This hypothesis was disproved, however, when the injection of DDT into the body cavity of resistant flies did not alter their resistance. Other workers have reported on minor differences in morphology, length of life cycle, biotic potential, and preferen- tial resting habits, but apparently they are not positively corre- lated with specific insecticide resistance.

Some of the most fruitful studies determining the nature of DDT resistance have been on its metabolism. Sternburg and co-workers first showed that resistant flies were able to degrade DDT to its nontoxic metabolite, DDE, whereas there was little or no change in susceptible flies. It has been demonstrated that surviving flies always contain more DDE than the dead flies, and that the more resistant the strain of flies the more readily do they accomplish the detoxification. Perry and Hoskins are of the opinion that the DDT-resistant fly is protected by three mecha- nisms—detoxification to DDE, detoxification to one or more unidentified compounds, and storage of unchanged DDT in parts of the insect where it has no appreciable harmful effect. Stern- burg and Kearns have shown that an enzyme is involved in the dehydrochlorination of DDT to DDE in DDT-resistant flies. It has also been shown that the same type of enzyme action is not involved in the resistance of certain other species to DDT. Al- though much good work has been done and a great deal of in- tensive research is under way, no one has yet been able to explain completely the mechanism of insect resistance to in- secticides.

*Impact of Resistance Problems*.—An ever-increasing number of agricultural, household, and other insects are becoming resist- ant to insecticides in localized areas throughout the country. As resistance becomes more widespread, it is predicted that the following serious problems will arise:

(1) The first reaction of a user not obtaining the usual degree of control with an insecticide is to increase the dosage or the number of applications or both. This response wastes his time and money.

(2) Increased amounts of insecticides may leave residues on the marketable crop that exceed the tolerances established by the Food and Drug Administration in fulfilling the requirements of Public Law 518. If these tolerances, which have been estab- lished on the basis of need and safety to the consumer of foods, are exceeded, the crop is subject to seizure.

(3) Failure to obtain good control with insecticides causes growers, pest control operators, and others to lose confidence in entomologists, county agents and consultants who provide them with insect control recommendations.

(4) In desperation some users may seek new materials that have not been thoroughly tested or recommended for a particular crop and run into various difficulties.

(5) Not being able to anticipate a reasonable period of usage of new insecticides on account of changes in recommendations to meet resistance problems, manufacturers are faced with a fi- nancial risk that may thwart research in this field.

(6) The stockpiling of insecticides in different parts of the country to meet control demands that cannot be determined too well in advance is likely to be curtailed to such a degree that emergencies cannot be met. Distributors cannot afford to stock- pile materials that are likely to be superseded by new ones.

(7) The development of resistance to an insecticide is dis-
couraging and demoralizing to research workers. In addition, it
increases the costs of research to meet urgent control needs.

(2-3)

*Overcoming Resistance Problems*.—There are several avenues open to help meet this national emergency:

(1) Fortunately, resistance to insecticides generally shows up first in small areas, so that there is time to make adjustments in control programs. In recent years a number of highly effective materials have been available for the control of each pest. The use of alternate materials to meet several serious resistance problems has permitted time for the development of better control measures.

(2) Although a given insecticide may prove ineffective against a certain pest on account of resistance, it still may be very effective against many others and usable without creating an additional hazard. In other words, insects vary greatly in their capacity to become resistant to certain insecticides.

(3) Many new insecticides are being produced, and doubtless some of them will, at least temporarily, help meet resistance problems.

(4) To lessen the opportunity for the development of insect resistance, insecticide applications should be carefully timed, and as few applications made as are consistent with good control.

(5) Cultural control and sanitation practices should be utilized to the maximum extent possible in reducing destructive insect populations.

(6) Advantage should also be taken of the various biological control agents. When insecticides are used, their choice and timing should be such that they will be least harmful to parasites and predators.

(7) Utilization of varieties of crop plants that are resistant to insect attack.

*Future Needs in Research*.—More research is needed on the mode of action of insecticides and on the mechanisms involved in the resistance of insects to different insecticides. Such information would be of benefit in seeking ways to avoid or overcome resistance. The possibilities of synergists or other materials to nullify resistance should be determined. In order to meet urgent control problems a constant study must be made of new materials for the control of insects and mites. Perhaps some insecticides or miticides will be developed to which they will not become resistant. Consideration should also be given to research on the use of two or more insecticides having entirely different modes of action, applied either in combination or as alternate treatments. To assist in recognizing the development of resistance in a pest, entomologists should conduct studies to establish mortality curves with insecticides in current use or promising for future use in widely separated areas to serve as a reference standard in the future. Of considerable benefit too would be more efficient methods of distributing insecticides. Of more than passing interest is the report in Canada of a parasite of the flying moth that is resistant to DDT. This suggests a new phase of research which might well receive considerable attention. There is also need for further evaluation of cultural control prac-

tices and biological control agents—parasites, predators, and diseases of insects. A combination of several control measures could cut down the number of insecticide applications required and perhaps slow up the development of resistance. For some crops the ultimate answer may lie in the development of plant varieties that are immune to insect attack. Certainly a broad research program is required if we are to meet this challenge of the insect world.

*Summary*.—The resistance of insects to insecticides is one of the most serious problems facing entomologists today. A review of the current status of resistance to the new insecticides points up the significance of this phenomenon among insects of agricultural, household, and medical importance. Each year additional pests are becoming resistant to DDT and other chlorinated hydrocarbon insecticides that heretofore provided outstanding control. The development of tolerance to a given insecticide seems to predispose an insect to rapid development of resistance to other insecticides. This failure of the chlorinated hydrocarbon insecticides has caused entomologists to turn to the organic phosphorus materials. In general they have been quite satisfactory, but some pests, particularly the mites, have become resistant to them.

Many problems arise when users of insecticides meet up with resistant insects. Higher dosages or more applications are used, which result in waste of time and money, and perhaps residues on marketable crops that exceed established tolerances. The insects may cause damage before effective substitutes can be applied. Growers and others lose confidence in those making control recommendations, and in desperation may use unapproved materials. Manufacturers face greater financial risk in developing insecticides, not knowing how long they may be used until resistance appears. Since distributors cannot afford to stockpile materials that are likely to be superseded by new ones, growers may not be able to meet emergency insect outbreaks.

When an insect becomes resistant to an insecticide, alternate materials should be used until better control measures can be worked out. It is unlikely that any one material will suddenly become ineffective against a large number of pests. Many new materials are being produced that will help us meet resistance problems. Whenever possible, cultural and biological methods should be used to reduce destructive insect populations. A broad research program is required if we are to meet this challenge of the insect world.

## REFERENCES

Babers, F. H. 1949. Development of insect resistance to insecticides. U. S. Bur. Ent. and Plant Quar. E-776, 31 pp.

Babers, F. H., and J. J. Pratt. 1951. Development of insect resistance to insecticides—II. A critical review of the literature up to 1951. U. S. Bur. Ent. and Plant Quar. E-818, 45 pp.

Metcalf, R. L. 1955. Organic Insecticides. Their chemistry and mode of action. 392 pp. Interscience Publishers, New York, London.