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Toxicity of three organophosphorus compounds and pyrethrins to malathion-resistant Tribolium castaneum (Herbst) (Coleoptera, Tenebrionidae)
3.
stored Prod. Res., 1969, Vol. 4, pp. 279-283.
Pergamon
Press.
Printed
in Great
Britain.
Toxicity of Three Organophosphorus Compounds and Pyrethrins to Malathion-resistant ~~~6~~~~~c~~~~~e~~ (Herbst) (Coleoptera, Tenebrionidae)* ROY
D. SPEIRS
and J, LARRY
ZETTLERf
Stored-Product Insects Research Branch, Market Quality Research Division, Agricultural Research Service, USDA, Savannah, Georgia, U.S.A. (First received 10 June,
1968, and in jnal form 30 September, 1968)
Abst~~t-Malathion-r~istant red flour beetles, ~~bo~ia~ castaneum (He&t), were tested for susceptibility to Bay 77488 (o,o-diethyl phosphorothioate o-ester with phenylglyoxylonitrile oxime), Bay 78182 (o,o-diethyl phosphorothioate o-ester with (o-chlorophenyl)glyoxylonitrile oxime), diazinon, and pyrethrins by topical applications. The malathion-resistant beetles showed 13 *3-fold resistance to pyrethrins, 19.7-fold resistance to Bay 78182, 7 *7- and 10. ‘I-fold resistance to diazinon, and only 4.5- and 9.5- fold resistance to Bay 77488. Only about one-quarter as much Bay 77488 as malathion was required for the LD,, in the resistant beetles. About one-third as much Bay 78182 as malathion was required. Diazinon and pyrethrins were less toxic than malathion to the resistant insects, INTRODUCTION
IN ORDER to have effective control methods available in the eventuality that resistance in insects increases to the extent that present chemical controls are no longer effective, it is important to test new materials and methods against resistant strains of insects. The need for prompt work on this problem is especially great in stored-product insect control because of the stringent limits on the materials that can be used with safety on stored products and the relatively long time required to obtain the information necessary to establish safe practices with new materials. BROWN (1961) and others have summarized the information on the development of resistance to insecticides by a wide range of insect species. BUSVINE * Company names are used in this publication solely to provide specific information. Mention of a company name does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not signify that the product is approved to the exclusion of other comparable products. t Research Entomologist and Agricultural Insects Research and Development Laboratory,
Research MQRD,
279
Technician, USDA.
respectively,
Stored-Product
ROYD. SPEIRSand J. LARRYZETTLER
280
(1962), LINDGREN and VINCENT (1966) and PARKIN (1965) reviewed insecticide resistance in stored-product insects. PARKIN and FORSTER (1963) reported a resistance in the red flour beetle, Tribolium castaneum (Herbst), collected from stored peanuts in Nigeria as high as 52-fold to malathion, one of the few insecticides commonly used for control of stored-product insects. A line from a Nigerian strain has developed resistance as high as 200-fold to malathion after selection pressures (DYTE and BLACKMAN, 1966). SPEIRS et al. ( 1967) reported lower levels of resistance to malathion in several strains of T. castaneum collected from stored products in Georgia and Florida. The present studies were conducted from 1965 to 1967 with two of the Georgia strains from peanuts. Bay 77488 (o,o-diethyl phosphorothioate o-ester with phenylglyoxylonitrile oxime), Bay 78182 (o,o-diethyl phosphorothioate o-ester with (o-chlorophenyl) glyoxylonitrile oxime), diazinon, and pyrethrins were used in these tests. MCDONALD and GILLENWATER (1967) reported Bay 77488 as a promising new compound against several species of stored-product insects. Bay 78182 is related chemically to Bay 77488 and was found to be highly toxic to the confused flour beetle, 1. confwum Jacquelin duVa1, in preliminary investigations (L. L. McDonald, 1966, personal communication). STRONG and SBUR ( 1961, 1965) found diazinon effective as a grain protectant. Formulations containing pyrethrins were extensively used before the use of malathion and still are used for storedproduct insect control. MATERIALS AND METHODS The Georgia peanut-l (GP-1) strain of malathion-resistant flour beetles used in these tests was collected November 16, 1962, from farmers’ stock peanuts in a Georgia warehouse in which malathion had been used for 4 years. The insects were reared to the F, generation without selection pressure before testing. Mortalities from malathion in various generations of laboratory rearing during which selection pressures were used are shown in Table 1. Topical applications of insecticides (pg/g) were by the method described below. Oil sprays were applied by the method described by SPEIRS (1962). In this method the insecticide was formulated in a 50-50 mixture (w/w) of deodorized kerosene and tetraTABLE1.
SUSCEPTIBILITY TO MALATHIONOF IN VARIOUS
Generation
F,-F, F, F, F
F:--% F 1s F 18 F 20 F 21
THE
GENERATIONS
Rate
GEORGIA
PEANUT-
OF LABORATORY
of application*
(No selection pressure) 4.8 pg/cm2 19.4 pg/cm2 19.4 fig/cm” (No selection pressura) 526 cLg/g (No selection pressure) 526 cLglg 789 I-lglg
1 STRAIN
OF
Tribolium
REARING
Mortality
(%)
46 46 96 86 56 72
* Insecticide applied as settling oil sprays for rates in pg/cm2 and topically as acetone solutions for rates in pg/g.
castaneum
Toxicity of Three Organophosphorus Compounds and Pyrethrins TABLE
2. SUSCEPTIBILITY
TO MALATHION IN
OF THE
GEORGIA
PEANUT-~
STRAIN OF
281
Tribolium castaneum
VARIOUSGENERATlONsOFLABORATORYREARING
Generation
Collected adults FI F, Fs-FI, F 1.3 F 14 F 1s F 16 F I, F 18
Topical application &g/g)
Mortality
476 762 1524 (No selection pressure) 238 476 381 2380 1905 952
60 70 65 62 63 55 92 84 64
(%I
chloroethylene. Various concentrations were sprayed into stainless steel towers and allowed to settle for 30 min onto the insects exposed in open Petri dishes on the tower floors. The insects were then transferred to clean dishes and held for mortality observations. The Georgia peanut-4 (GP-4) strain was collected March 4, 1964, from farmers’ stock peanuts in silos in Georgia in which malathion had been used for 3 years. Mortalities from malathion in various generations of laboratory rearing during which selection pressures were used are shown in Table 2. The insecticide was applied by the method described below. The insects were reared on a mixture of 47 -5% each of white flour and maizemeal and 5% of brewer’s yeast. During rearing, application of insecticides, and post-application holding, the temperature was 27 f 1.1% and the r.h. was 60 -& 5 per cent. The test insects were 7- to 14”day-old adults. The sexes were separated in the pupal stage for several of the tests. The Savannah laboratory strain (susceptible) was included as a standard for comparison. This strain had been maint~ned for at least 15 years without insecticide-selection pressure. The average weights of the insects in mg for each strain were as follows: GP-I-1.9, GP-4-2 . I, and standard laboratory-2 *2. Insecticides were applied topically to the insects to establish dosage-mortality regression lines, The insecticides were premium-grade malathion (95%) obtained from American Cyanamid Company; pyrethrins (20% extract) obtained from Fairfield Chemicals Division, FMC Corporation; diazinon (technical grade 92 - 1%) obtained from Geigy Chemical Corporation; and Bay 77488 (technical grade 98 - 3%) and Bay 78182 (technical grade 99%) obtained from Vero Beach Laboratories, Inc., Vero Beach, Florida. Bay 78182 and pyrethrins were not tested against the GP-I strain. The insecticides were dissolved in distilled acetone and diluted with distilled acetone to the desired concentrations of actual insecticide. Application was by a micrometer-driven syringe on an automatic micro-applicator. The insects were anesthetized by a continuous flow of carbon dioxide in a Btichner funnel, after which they were held individually with a small suction tube while 0.5 ~1 of the insecticide solution was applied to the pronotum. For each insecticide and insect strain a minimum of four dosages of the insecticide were used, and
ROY D. SPEIRSand J. LARRYZETTLER
282
at least 80 insects were treated at each dosage. Control insects received applications of acetone alone. Some of the insects were treated at each dosage on different days to allow for day-to-day variation. Following application the insects were held without food in Petri dishes with paper-lined bottoms (10 insects/dish). Mortality was determined 120 hr after application of the insecticide. Insects showing little or no response when probed lightly were considered dead or moribund. The Cornell Computing Center, Cornell University, performed the probit analyses of the mortality data, using the method described by SOKAL (1958). RESULTS The results of these tests are summarized in Table 3. Bay 77488 was the most toxic insecticide to the malathion-resistant strains of beetles. Only about onequarter to one-third as much Bay 77488 as malathion was required against the resistant strains at the LD~~ level. Bay 78182 was the most toxic insecticide to TABLE 3. DOSAGE-MORTALITY DATAFORTHREESTRAINS OF Tribolium castaneum TREATEDTOPICALLY WITHINSECTICIDES
Insecticide
Generation tested
Bay 77488 Malathion Diazinon
F 21 F 21 F 16
Bay 77488 Bay 78182 Malathion
F 1P F 18 F 10 F 18 F 11 F,
Diazinon Pyrethrins Bay 77488 Bay 78182 Malathion Diazinon Pyrethrins
-
Lnso kg/g)
Confidence limits for LDsO
Georgia peanut-l 190 125-288 628 492-802 1499 1402-1602 Georgia peanut-4 90 76-108 236 199-281 394 338-460 704 547-907 1076 992-l 168 5423 4268-6890 Standard laboratory 20 17- 23 12 lo13 36 3239 140 130- 151 408 270- 617
Slope
Resistance ratio
2.32 2.64 4.09
17.4 10.7
3.43 2.99 2.81 2.75 3.22 1.84
4.5 19.7 10.9 19.3 ?,7 13.3
4.17 6.99 4.21 4.34 2.65
-
9.5
-
the susceptible strain. The descending order of toxicity for the insecticides tested against the GP-4 strain was Bay 77488, Bay 78182, malathion, diazinon, and pyrethrins. Although the malathion-resistant insects showed resistance to all the insecticides tested, they were the most resistant to Bay 78182, moderately resistant to diazinon and pyrethrins, and least resistant to Bay 77488. The male and female beetles did not differ greatly in susceptibility to the insecticides. Mortalities in the acetone-treated controls did not exceed 2 per cent. In the analyses, corrections were made for control mortalities.
Toxicity of Three Organophosphorus Compounds and Pyrethrins
283
DISCUSSION
The resistance to pyrethrins in the malathion-resistant insects could have been acquired from the use of pyrethrins in the storage facilities before the changeover to malathion and from the occasional use of pyrethrins for killing moths, as a supplement to the malathion treatments. The resistances to Bay 77488, Bay 78182, and diazinon are more likely to be attributable to cross-resistance from malathion, since there is no history of the use of these insecticides for peanut insect control. Return to the previous control methods using pyrethrins appears untenable in view of the resistant strains, unless the resistance problem can be overcome. However, it is possible that some synergistic combination might be more effective against these insects. The degree of insect resistance to Bay 78182 and to diazinon makes these compounds less promising than they would be otherwise. Bay 78182’s high toxicity to insects and its low mammalian toxicity, LD&* greater than 1000 mg/kg (Walter M. Zeck, 1966, personal communication), make it worthy of further consideration. Bay 78182 was more toxic to the resistant beetles than was malathion. However, the cross-resistance from malathion and the relatively low slope for the malathion-resistant strain (GP-4) compared with that for the susceptible strain indicate that higher levels of resistance to this compound may develop if it is used in an extended control program. Bay 77488 was the most promising of the compounds tested. It was more toxic to the resistant insects than were any of the other compounds, and the insects showed low cross-resistance to it. In addition, the low mammalian toxicity of this compound, LD~~ about 8000 to 10,000 mg/kg (Walter M. Zeck, 1965, personal communication), makes it a most promising candidate insecticide for further testing. REFERENCES BROWN,A. W. A. (1961) The challenge of insecticide resistance. Bull. est. Sot. Am. 7, 6-19. BUSVINE,J. R. (1962) Insecticide resistance among pests of stored products. l-runs. 11th int. Congr. Ent., Vienna, 1960 3, pp. 226-221. DYTE, C. E. and BLACKMAN, D. G. (1966) The rust-red flour beetle and malathion. Pest Inzst. lies. 196!5,40. LINDGREN, D. L. and VINCENT,L. E. (1966) Development of resistance in stored-product insects to insecticides. Cereal Sci. Today 11, 12-14, 26. MCDONALD,L. L. and GILLENWATER,H. B. (1967) Relative toxicity of Bay 77488 and Dursban against stored-product insects. 3. econ. Ent. 60, 1195-1196. PARKIN,E. A. (1965) The onset of insecticide resistance among field populations of stored-product insects. 3. stored Prod. Res. 1, 3-8. PARKIN, E. A. and FORSTER,R. (1963) The rust-red flour beetle and malathion. Pest infest. Res. 1962, 38-39. SOKAL,R. R. (1958) Probit analysis on a digital computer. 3. econ. Ent. 51, 738-739. SPEIRS, R. D. (1962) Contact, residue, and vapor toxicity of new insecticides to stored-product insects. U.S. Department of Agriculture Mktg Res. Refit 546, 31 pp. SPEIRS, R. D., REDLINGER,L. M. and BOLES, H. P. (1967) Malathion resistance in the red flour beetle. 3. ecm. Ent. 60, 1373-1374. STRONG,R. G. and SBUR, D. E. (1961) Evaluation of insecticides as protectants against pests of stored grain and seeds. 3. econ. Ent. 54, 235-238. STRONO,R. G. and SBUR, D. E. (1965) Interrelation of moisture content, storage temperature, and dosage on the effectiveness of diazinon as a grain protectant against Sitophilus oryrae (L.). 3. econ. Ent. 58, 410-414.
stored Prod. Res., 1969, Vol. 4, pp. 279-283.
Pergamon
Press.
Printed
in Great
Britain.
Toxicity of Three Organophosphorus Compounds and Pyrethrins to Malathion-resistant ~~~6~~~~~c~~~~~e~~ (Herbst) (Coleoptera, Tenebrionidae)* ROY
D. SPEIRS
and J, LARRY
ZETTLERf
Stored-Product Insects Research Branch, Market Quality Research Division, Agricultural Research Service, USDA, Savannah, Georgia, U.S.A. (First received 10 June,
1968, and in jnal form 30 September, 1968)
Abst~~t-Malathion-r~istant red flour beetles, ~~bo~ia~ castaneum (He&t), were tested for susceptibility to Bay 77488 (o,o-diethyl phosphorothioate o-ester with phenylglyoxylonitrile oxime), Bay 78182 (o,o-diethyl phosphorothioate o-ester with (o-chlorophenyl)glyoxylonitrile oxime), diazinon, and pyrethrins by topical applications. The malathion-resistant beetles showed 13 *3-fold resistance to pyrethrins, 19.7-fold resistance to Bay 78182, 7 *7- and 10. ‘I-fold resistance to diazinon, and only 4.5- and 9.5- fold resistance to Bay 77488. Only about one-quarter as much Bay 77488 as malathion was required for the LD,, in the resistant beetles. About one-third as much Bay 78182 as malathion was required. Diazinon and pyrethrins were less toxic than malathion to the resistant insects, INTRODUCTION
IN ORDER to have effective control methods available in the eventuality that resistance in insects increases to the extent that present chemical controls are no longer effective, it is important to test new materials and methods against resistant strains of insects. The need for prompt work on this problem is especially great in stored-product insect control because of the stringent limits on the materials that can be used with safety on stored products and the relatively long time required to obtain the information necessary to establish safe practices with new materials. BROWN (1961) and others have summarized the information on the development of resistance to insecticides by a wide range of insect species. BUSVINE * Company names are used in this publication solely to provide specific information. Mention of a company name does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not signify that the product is approved to the exclusion of other comparable products. t Research Entomologist and Agricultural Insects Research and Development Laboratory,
Research MQRD,
279
Technician, USDA.
respectively,
Stored-Product
ROYD. SPEIRSand J. LARRYZETTLER
280
(1962), LINDGREN and VINCENT (1966) and PARKIN (1965) reviewed insecticide resistance in stored-product insects. PARKIN and FORSTER (1963) reported a resistance in the red flour beetle, Tribolium castaneum (Herbst), collected from stored peanuts in Nigeria as high as 52-fold to malathion, one of the few insecticides commonly used for control of stored-product insects. A line from a Nigerian strain has developed resistance as high as 200-fold to malathion after selection pressures (DYTE and BLACKMAN, 1966). SPEIRS et al. ( 1967) reported lower levels of resistance to malathion in several strains of T. castaneum collected from stored products in Georgia and Florida. The present studies were conducted from 1965 to 1967 with two of the Georgia strains from peanuts. Bay 77488 (o,o-diethyl phosphorothioate o-ester with phenylglyoxylonitrile oxime), Bay 78182 (o,o-diethyl phosphorothioate o-ester with (o-chlorophenyl) glyoxylonitrile oxime), diazinon, and pyrethrins were used in these tests. MCDONALD and GILLENWATER (1967) reported Bay 77488 as a promising new compound against several species of stored-product insects. Bay 78182 is related chemically to Bay 77488 and was found to be highly toxic to the confused flour beetle, 1. confwum Jacquelin duVa1, in preliminary investigations (L. L. McDonald, 1966, personal communication). STRONG and SBUR ( 1961, 1965) found diazinon effective as a grain protectant. Formulations containing pyrethrins were extensively used before the use of malathion and still are used for storedproduct insect control. MATERIALS AND METHODS The Georgia peanut-l (GP-1) strain of malathion-resistant flour beetles used in these tests was collected November 16, 1962, from farmers’ stock peanuts in a Georgia warehouse in which malathion had been used for 4 years. The insects were reared to the F, generation without selection pressure before testing. Mortalities from malathion in various generations of laboratory rearing during which selection pressures were used are shown in Table 1. Topical applications of insecticides (pg/g) were by the method described below. Oil sprays were applied by the method described by SPEIRS (1962). In this method the insecticide was formulated in a 50-50 mixture (w/w) of deodorized kerosene and tetraTABLE1.
SUSCEPTIBILITY TO MALATHIONOF IN VARIOUS
Generation
F,-F, F, F, F
F:--% F 1s F 18 F 20 F 21
THE
GENERATIONS
Rate
GEORGIA
PEANUT-
OF LABORATORY
of application*
(No selection pressure) 4.8 pg/cm2 19.4 pg/cm2 19.4 fig/cm” (No selection pressura) 526 cLg/g (No selection pressure) 526 cLglg 789 I-lglg
1 STRAIN
OF
Tribolium
REARING
Mortality
(%)
46 46 96 86 56 72
* Insecticide applied as settling oil sprays for rates in pg/cm2 and topically as acetone solutions for rates in pg/g.
castaneum
Toxicity of Three Organophosphorus Compounds and Pyrethrins TABLE
2. SUSCEPTIBILITY
TO MALATHION IN
OF THE
GEORGIA
PEANUT-~
STRAIN OF
281
Tribolium castaneum
VARIOUSGENERATlONsOFLABORATORYREARING
Generation
Collected adults FI F, Fs-FI, F 1.3 F 14 F 1s F 16 F I, F 18
Topical application &g/g)
Mortality
476 762 1524 (No selection pressure) 238 476 381 2380 1905 952
60 70 65 62 63 55 92 84 64
(%I
chloroethylene. Various concentrations were sprayed into stainless steel towers and allowed to settle for 30 min onto the insects exposed in open Petri dishes on the tower floors. The insects were then transferred to clean dishes and held for mortality observations. The Georgia peanut-4 (GP-4) strain was collected March 4, 1964, from farmers’ stock peanuts in silos in Georgia in which malathion had been used for 3 years. Mortalities from malathion in various generations of laboratory rearing during which selection pressures were used are shown in Table 2. The insecticide was applied by the method described below. The insects were reared on a mixture of 47 -5% each of white flour and maizemeal and 5% of brewer’s yeast. During rearing, application of insecticides, and post-application holding, the temperature was 27 f 1.1% and the r.h. was 60 -& 5 per cent. The test insects were 7- to 14”day-old adults. The sexes were separated in the pupal stage for several of the tests. The Savannah laboratory strain (susceptible) was included as a standard for comparison. This strain had been maint~ned for at least 15 years without insecticide-selection pressure. The average weights of the insects in mg for each strain were as follows: GP-I-1.9, GP-4-2 . I, and standard laboratory-2 *2. Insecticides were applied topically to the insects to establish dosage-mortality regression lines, The insecticides were premium-grade malathion (95%) obtained from American Cyanamid Company; pyrethrins (20% extract) obtained from Fairfield Chemicals Division, FMC Corporation; diazinon (technical grade 92 - 1%) obtained from Geigy Chemical Corporation; and Bay 77488 (technical grade 98 - 3%) and Bay 78182 (technical grade 99%) obtained from Vero Beach Laboratories, Inc., Vero Beach, Florida. Bay 78182 and pyrethrins were not tested against the GP-I strain. The insecticides were dissolved in distilled acetone and diluted with distilled acetone to the desired concentrations of actual insecticide. Application was by a micrometer-driven syringe on an automatic micro-applicator. The insects were anesthetized by a continuous flow of carbon dioxide in a Btichner funnel, after which they were held individually with a small suction tube while 0.5 ~1 of the insecticide solution was applied to the pronotum. For each insecticide and insect strain a minimum of four dosages of the insecticide were used, and
ROY D. SPEIRSand J. LARRYZETTLER
282
at least 80 insects were treated at each dosage. Control insects received applications of acetone alone. Some of the insects were treated at each dosage on different days to allow for day-to-day variation. Following application the insects were held without food in Petri dishes with paper-lined bottoms (10 insects/dish). Mortality was determined 120 hr after application of the insecticide. Insects showing little or no response when probed lightly were considered dead or moribund. The Cornell Computing Center, Cornell University, performed the probit analyses of the mortality data, using the method described by SOKAL (1958). RESULTS The results of these tests are summarized in Table 3. Bay 77488 was the most toxic insecticide to the malathion-resistant strains of beetles. Only about onequarter to one-third as much Bay 77488 as malathion was required against the resistant strains at the LD~~ level. Bay 78182 was the most toxic insecticide to TABLE 3. DOSAGE-MORTALITY DATAFORTHREESTRAINS OF Tribolium castaneum TREATEDTOPICALLY WITHINSECTICIDES
Insecticide
Generation tested
Bay 77488 Malathion Diazinon
F 21 F 21 F 16
Bay 77488 Bay 78182 Malathion
F 1P F 18 F 10 F 18 F 11 F,
Diazinon Pyrethrins Bay 77488 Bay 78182 Malathion Diazinon Pyrethrins
-
Lnso kg/g)
Confidence limits for LDsO
Georgia peanut-l 190 125-288 628 492-802 1499 1402-1602 Georgia peanut-4 90 76-108 236 199-281 394 338-460 704 547-907 1076 992-l 168 5423 4268-6890 Standard laboratory 20 17- 23 12 lo13 36 3239 140 130- 151 408 270- 617
Slope
Resistance ratio
2.32 2.64 4.09
17.4 10.7
3.43 2.99 2.81 2.75 3.22 1.84
4.5 19.7 10.9 19.3 ?,7 13.3
4.17 6.99 4.21 4.34 2.65
-
9.5
-
the susceptible strain. The descending order of toxicity for the insecticides tested against the GP-4 strain was Bay 77488, Bay 78182, malathion, diazinon, and pyrethrins. Although the malathion-resistant insects showed resistance to all the insecticides tested, they were the most resistant to Bay 78182, moderately resistant to diazinon and pyrethrins, and least resistant to Bay 77488. The male and female beetles did not differ greatly in susceptibility to the insecticides. Mortalities in the acetone-treated controls did not exceed 2 per cent. In the analyses, corrections were made for control mortalities.
Toxicity of Three Organophosphorus Compounds and Pyrethrins
283
DISCUSSION
The resistance to pyrethrins in the malathion-resistant insects could have been acquired from the use of pyrethrins in the storage facilities before the changeover to malathion and from the occasional use of pyrethrins for killing moths, as a supplement to the malathion treatments. The resistances to Bay 77488, Bay 78182, and diazinon are more likely to be attributable to cross-resistance from malathion, since there is no history of the use of these insecticides for peanut insect control. Return to the previous control methods using pyrethrins appears untenable in view of the resistant strains, unless the resistance problem can be overcome. However, it is possible that some synergistic combination might be more effective against these insects. The degree of insect resistance to Bay 78182 and to diazinon makes these compounds less promising than they would be otherwise. Bay 78182’s high toxicity to insects and its low mammalian toxicity, LD&* greater than 1000 mg/kg (Walter M. Zeck, 1966, personal communication), make it worthy of further consideration. Bay 78182 was more toxic to the resistant beetles than was malathion. However, the cross-resistance from malathion and the relatively low slope for the malathion-resistant strain (GP-4) compared with that for the susceptible strain indicate that higher levels of resistance to this compound may develop if it is used in an extended control program. Bay 77488 was the most promising of the compounds tested. It was more toxic to the resistant insects than were any of the other compounds, and the insects showed low cross-resistance to it. In addition, the low mammalian toxicity of this compound, LD~~ about 8000 to 10,000 mg/kg (Walter M. Zeck, 1965, personal communication), makes it a most promising candidate insecticide for further testing. REFERENCES BROWN,A. W. A. (1961) The challenge of insecticide resistance. Bull. est. Sot. Am. 7, 6-19. BUSVINE,J. R. (1962) Insecticide resistance among pests of stored products. l-runs. 11th int. Congr. Ent., Vienna, 1960 3, pp. 226-221. DYTE, C. E. and BLACKMAN, D. G. (1966) The rust-red flour beetle and malathion. Pest Inzst. lies. 196!5,40. LINDGREN, D. L. and VINCENT,L. E. (1966) Development of resistance in stored-product insects to insecticides. Cereal Sci. Today 11, 12-14, 26. MCDONALD,L. L. and GILLENWATER,H. B. (1967) Relative toxicity of Bay 77488 and Dursban against stored-product insects. 3. econ. Ent. 60, 1195-1196. PARKIN,E. A. (1965) The onset of insecticide resistance among field populations of stored-product insects. 3. stored Prod. Res. 1, 3-8. PARKIN, E. A. and FORSTER,R. (1963) The rust-red flour beetle and malathion. Pest infest. Res. 1962, 38-39. SOKAL,R. R. (1958) Probit analysis on a digital computer. 3. econ. Ent. 51, 738-739. SPEIRS, R. D. (1962) Contact, residue, and vapor toxicity of new insecticides to stored-product insects. U.S. Department of Agriculture Mktg Res. Refit 546, 31 pp. SPEIRS, R. D., REDLINGER,L. M. and BOLES, H. P. (1967) Malathion resistance in the red flour beetle. 3. ecm. Ent. 60, 1373-1374. STRONG,R. G. and SBUR, D. E. (1961) Evaluation of insecticides as protectants against pests of stored grain and seeds. 3. econ. Ent. 54, 235-238. STRONO,R. G. and SBUR, D. E. (1965) Interrelation of moisture content, storage temperature, and dosage on the effectiveness of diazinon as a grain protectant against Sitophilus oryrae (L.). 3. econ. Ent. 58, 410-414.