J Arthropod-Borne Dis, 2012, 6(2): 112–118 M Limoee et al.: Toxicity of Pyrethroid and … 112 Original Article Toxicity of Pyrethroid and Organophosphorous Insecticides against Two Field Collected Strains of the German Cockroach Blattella germanica (Blattaria: Blattellidae) from Hospitals in Hamadan, Iran Mojtaba Limoee 1, *Behroz Davari 2,4, Seyed Hassan Moosa-Kazemi 3 1Depatment of Public Health, Nosocomial Infections Research Center, Kermanshah University of Medical Sciences, Kermanshah, Iran 2Department of Entomology, School of Medicine, Hamadan University of Medical Sciences, Hamadan, Iran 3Department of Medical Entomology and Vector Control, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran 4 Kurdistan University of Medical Sciences and Environment Health research Center, Kurdistan, Iran (Received 29 Feb 2012; accepted 26 Aug 2012) Abstract Background: The German cockroach, Blattella germanica is a major hygienic pest and mechanical vector for path- ogenic agents in hospitals and residential areas. The development of insecticide resistance is a serious problem in controlling of this pest. Toxicity of four commonly used insecticides (permethrin, cypermethrin, malathion and chlorpyrifos) against two hospital- collected strains of the German cockroach was investigated. Methods: Topical bioassay methods were carried out for detecting insecticide susceptibility of adult male cock- roaches. For each insecticide, four to six concentrations resulting in >0% and <100% mortality were used. Three to six replicates of 10 cockroaches per concentration were conducted. The differences between LD50 (µ g/g) values were considered statistically significant only when the 95% confidence intervals did not overlap. Results: Two hospital- collected strains of the German cockroach showed low to moderate levels of resistance to chlorpyrifos, permethrin, malathion and cypermethrin based on resistance ratios compared with susceptible strain. Conclusion: The low level chlorpyrifos resistance suggesting this insecticide may still provide adequate control of these strains. While the obsereved moderate levels of resistance to cypermethrin could imply developing resistance to this compound. Keywords: Insecticide resistance, Pyrethroid, Organophosphate, German cockroach Introduction The German cockroach, Blattella germanica (L) is a major hygienic pest in households, hospitals and residential areas. These insects have been recognized as mechanical vectors and reservoir for pathogenic agents (Pai et al. 2003, 2005, Kinfu and Erko 2008). In ad- dition to mechanical transmission of patho- gens, large indoor cockroach populations are also one of the causes inducing asthma (Roberts 1996, Miller and Koehler 2003, Kinfu and Erko 2008). Insecticide resistance is now a serious prob- lem, challenging the control of the German Insecticide resistance is now a serious prob- lem, challenging the control of the German - cockroach. In addition, selection with some insecticides could confer resistance to the new insecticides through cross-resistance (Wei et al. 2006). The development of re- sistance to different classes of insecticides in field-collected strains of the German cock- roach, Blattella germanica has been reported frequently (Cochran 1995, Lee et al. 1996, Pai et al. 2005). Pai et al. (2005) determined the resistance of the German cockroach from hospitals and *Corresponding author: Dr Behroz Davari, E-mail: davaribehroz@yahoo.com J Arthropod-Borne Dis, 2012, 6(2): 112–118 M Limoee et al.: Toxicity of Pyrethroid and … 113 households to propoxur, chlorpyriphos and cypermethrin in Taiwan. They concluded that the resistance patterns of propoxur> chlo- rpyriphos> cypermethrin in hospital strains and propoxur> cypermethrin> chlorpyriphos in household strains, might be due to the fre- quency of application of the insecticides. Chai and Lee (2010) determined the re- sistance of 22 field- collected strains of the German cockroach from various localities of Singapore to 6 classes of insecticides. Among the different classes of insecti- cides, the effectiveness and low mammalian toxicity of pyrethroids have resulted in these compounds being extensively used for cock- roach control. Nevertheless, due to frequent use of these compounds, control failure in some field populations have been reported resulting from the development of pyrethroid resistance (Walles and Yu, 1996, Dong 1998, Valles 1998, Valles et al. 2000, Wei et al. 2006). The insecticides from different chemical groups including organophosphorous, carba- mates and particularly pyrethroids have been used extensively to control the German cock- roach in Iran and as a result, insecticide re- sistance appears to have become a prevalent among its populations (Ladonni 1993, 1997, Ladonni and Sadegheyani 1998, Limoee et al. 2006, 2011). Earlier, we found pyrethroid resistance (in- cluding permethrin, cypermethrin and cyflu- thrin) and cross resistance to DDT in seven field-collected strains from Tehran, Iran (Li- moee et al. 2006). Synergistic studies and bi- ochemical assays revealed that the metabolic mechanisms were involved in resistance to per- methrin in those strains mentioned above (Li- moee et al. 2007). Nasirian (2010) summarized the susceptibility of different strains of the Ger- man cockroach to different classes of insec- ticides in Iran (Nasirian 2010). In a recent arti- cle, resistance to organophosphorous carbamate and pyrethroid insecticides, were reported in three hospital- collected strains from Kerman- shah, Iran. Permethrin resistance was suppressi- ble by synergist PBO, suggesting the oxidases involvement in resistance (Limoee et al. 2011). Although attempts were made to determine the insecticide resistance of the German cock- roach populations from different provinces of Iran, such as Tehran and Kermanshah (Ladonni 1993, 1997, Ladonni and Sadegheyani 1998, Limoee et al. 2006, 2011, Nasirian 2006), there is no information on insecticide resistance sta- tus in hospital strains of the German cockroach in Hamadan, a western province of the coun- try. Therefore, this study was undertaken to detect possible insecticide resistance in two hospital-collected strains of the German cock- roach using four commonly used insecticides from different classes including permethrin, cypermethrin, malathion and chlorpyrifos. Materials and Methods Cockroach Strains Three German cockroach strains were ex- amined in this study: SUS is the standard susceptible strain maintained since 1975 in the insectary at the School of Public Health, Tehran University of Medical Sciences with- out exposure to insecticides; two strains EcH and FH were collected in 2010 from two hospitals in Hamadan. Spraying with different insecticides in- cluding pyrethroids and organophosphates were not very effective in controlling of the German cockroach in those hospitals (per- sonal communication). All cockroaches were maintained in an in- sectary at 27±2 ºC, 60±10% RH, with a pho- toperiod of 12:12 h (L: D). Each strain was reared in the same size labeled glass jar. Cockroaches were provided with unlimited cat food and water. Tests were conducted on adult males of F2–F4 generations. Chemicals Chemicals used were permethrin, 93.7% (technical grade) cis:trans 60:40, cypermethrin, 97.5% (technical grade), (Zeneca, Haslemere, J Arthropod-Borne Dis, 2012, 6(2): 112–118 M Limoee et al.: Toxicity of Pyrethroid and … 114 UK), chlorpyripfos, 97% (technical grade) and malathion, 92% (technical grade), (Cyanamid Agro, India). CO2 was used as anesthetic and acetone as solvent. Bioassays methods Bioassay tests were performed by topical application of 1µl of a known concentration of insecticide solution to the first abdominal segment of the insects, using a hand micro- ap- plicator (Burkard, Scientific Ltd, UK) e- quipped with a 1.0 ml Hamilton glass syringe. Adult male cockroaches were anesthetized with carbon dioxide for 20–30 seconds before insecticide treatment (Valles 1994). For each insecticide, four to six concentrations resulting in >0% and <100% mortality were used. Three to six replications of 10 cockroaches per concentration were conducted. Control groups received acetone alone. Treated cock- roaches were kept in Pyrex glass jars pro- vided with food and water before scoring the mortality. Cockroaches were considered dead when they were unable to turn themselves to normal posture within one minute after being turned onto their dorsum. Data analysis Bioassay data were pooled and subjected to probit analysis (Finny 1972), using a personal computer. The differences between LD50 val- ues were considered statistically significant only when the 95% confidence intervals did not overlap. All LD50 values were converted from µg/cockroach to µg/g of cockroach body weight to avoid possible effect of weight dif- ferences on insecticide susceptibility. The Re- sistance Ratios (RRs) were calculated by di- viding the LD50 of the resistant strain by the LD50 of the susceptible strain. Results Toxicity of permethrin Both hospital-collected strains of the Ger- man cockroach showed the similar levels of resistance to permethrin based on resistance ratios (RRs) compared with SUS strain (RR values for FH and ICH strains were 3.36 and 3.15, respectively), (Table 1). Comparisons made between the 95% con- fidence intervals of the LD50 values of field strains with susceptible strain (SUS) indi- cated that both strains had significantly dif- ferent RRs (P< 0.05) (Table 1). Toxicity of cyprmethrin Different levels of resistance to cype- rmethrin were observed in two hospital-col- lected strains of German cockroach based on RRs compared with susceptible strain (SUS) (Table 1) showing resistance ratios, 3.23 and 6.18 at LD50 levels, for ICH and FH strains, respectively. Comparisons made between the 95% con- fidence intervals of the LD50 values of field strains with susceptible strain (SUS) indi- cated that both strains had significantly dif- ferent RRs (P< 0.05) (Table 1). Toxicity of malathion Two hospital-collected strains of German cockroach showed the similar levels of re- sistance to malathion with significant RRs based on overlap of 95% confidence inter- vals of the LD50 values, compared with sus- ceptible strain (SUS) (P< 0.05). Resistance ratio values for FH and ICH strains were 5.2 and 6.23, respectively (Table 2). Toxicity of chlorpyriphos Two hospital-collected strains of German cockroach had similar resistance levels to chlorpyriphos with significant RRs based on overlap of 95% confidence intervals of the LD50 values, compared with susceptible strain (SUS) (P< 0.05). Resistance ratio values for FH and ICH strains were 2.2 and 2.4, respec- tively (Table 2). J Arthropod-Borne Dis, 2012, 6(2): 112–118 M Limoee et al.: Toxicity of Pyrethroid and … 115 Table 1. Lethal dose values for permethrin and cypermethrin for insecticide susceptible and two hospital collected strains of German cockroach Strain n Y- intercept S lope(SE) X2 (df) LD50 (95% CL) µg/ga RRb SUSc Permethrin 201 0.99 3.89 (0.49) 3.621 (4) 10.70 (9.44–12.1) 1 Cypermethrin 169 3.11 3.3 (0.42) 3.57 (2) 3.74 (3.18–4.44) 1 FH Permethrin 169 0.24 3.06 (0.37) 1.03 (3) 35.97 (29.56–43.79) 3.36sig Cypermethrin 149 0.87 3.03 (0.40) 0.77 (3) 23.13 (18.96–28.46) 6.18sig ICH Permethrin Cypermethrin 149 190 0.81 2.83 2.74 (0.37) 2.01 (0.30) 1.30 (3) 2.63 (3) 33.70 (27.17–42.00) 12.07 (9.06–15.19) 3.15sig 3.23sig a Micrograms of insecticide/g of cockroach body weight bResistance Ratio cSusceptible strain sigSignificant Table 2. Lethal dose values for malathion and chlorpyriphos for insecticide susceptible and two hospital collected strains of German cockroach Strain n Y- intercept S lope(SE) X2 (df) LD50 (95% CL) µg/ga RRb SUSc malathion 157 1.39 2.71 (0.38) 1.39 (2) 21.47(17.48–26.11) 1 chlorpyriphos 183 1.31 4.91 (0.64) 6.96 (3) 5.66(5.11–6.33) 1 FH malathion 120 - 2.02 3.43 (0.52) 5.68 (2) 111.66(92.23–134.29) 5.20sig chlorpyriphos 160 - 0.05 4.61 (0.61) 5.42 (2) 12.46(10.91–14.00) 2.20sig ICH malathion 120 - 2.32 3.44 (0.56) 0.05 (2) 133.75(111.38–163.17) 6.23sig chlorpyriphos 160 - 0.28 4.68 (0.66) 0.71 (2) 13.47(11.91–15.20) 2.40sig aMicrograms of insecticide/g of cockroach body weight bResistance Ratio cSusceptible strain sigSignificant Discussion In contrast to our previous study (Limoee et al. 2006, 2011), the present study showed the low to moderate resistance levels of col- lected strains to pyrethroid, organophosphorous and carbamate insecticides. Hence, it could be suggested that the resistance to those group of insecticides are developing. Several researchers previously concluded that the lethal dose (LD) method especially by topical application might be appropriate for determining susceptibility levels of the German cockroach strains to different insecticides because the amount applied could be precisely measured (Milio 1987, Scott and Cochran 1990, Choo et al. 2000, Ladonni 2001). Thus, this method was used throughout this study. According to Reierson et al. (1998) 10 fold resistance measured by topical applica- tion is the critical point above which opera- tional control failures are likely to occur while, resistance ratio at 5x and below may still achieve a good control of the German cockroach population. Although we observed the high level of resistance to permethrin and cypermethrin in different field collected strains of the Ger- man cockroach in our previous studies (Limoee et al. 2006, 2011), being consistent with an- other study on pyrethroid resistance in some populations of this insect (Ladonni and J Arthropod-Borne Dis, 2012, 6(2): 112–118 M Limoee et al.: Toxicity of Pyrethroid and … 116 Sadegheyani 1998), in present study, the low to moderate resistance levels of collected strains from hospitals of Hamadan to pyrethroid in- secticides compared with the susceptible strain were detected. Our findings probably sug- gested that these strains had not been se- lected especially by permethrin for several generations. Nevertheless, cypermethrin re- sistance seemed to be higher (RR: 6.18) than resistance to permethrin. Continued use of pyrethroids may lead to a relatively high selection pressure. As re- ported by Zhai and Robinson (1991), the use of cypermethrin against the German cock- roach about 4 years resulted in high level of re- sistance and control failure (Zhai and Robinson 1991), while, the pyrethroid insecticides have been recently replaced with some carbamates or organophosphates such as bendiocarb and chlorpyriphos in controlling of German cock- roach populations of hospitals in Hamadan, Iran (Personal communication). We can conclude that according to Reirson et al. (1998), the resistance ratio of these two strains to chlorpyriphos compared with the susceptible strain probably reflect a relative- ly low levels of resistance to this compound because these strains have not been selected by chlorpyrifos for several generations. Hence chlorpyrifos may still provide adequate con- trol of these strains. These results are con- sistent with our previous studies on chlor- pyrifos resistance in some populations of the German cockroach from Kermanshah, Iran which resistance ratios ranged from 1.20 to 2.18 for chlorpyrifos (Limoee et al. 2011). Lee et al. (1996), reported low levels of re- sistance to propoxure due to low frequency of insecticide applications (bi yearly). Similarly, the two strains in this study were subjected to a low frequency of malathion application and then replaced by chlorpyrifos (Personal com- munication). These strains exhibited moder- ate levels of resistance to malathion (>5x) probably because of low frequency of appli- cation and short term usage of malathion. Indeed, based on our findings in this study, it can be suggested that chlor-pyrifos could still be a potent toxic organo-phosphate in control- ling of German cock-roach populations in the hospitals of Hamadan. On the other hand, a mixture of carbamate and cypermethrin has recently been used against hospital strains of German cockroach in Kermanshah (Limoee et al. 2011). The majority of toxicological and bio- chemical studies suggest that insecticide re- sistance in the German cockroach can be me- diated by a number of mechanisms, the most common being enhanced enzymatic metab- olism (Walles and Yu 1996, Valles et al. 2000, Wei et al. 2001). The synergistic studies sup- ported by biochemical assays implicated that p450 monooxygenases hydrolases were in- volved in permethrin resistance in some strains of this insect in Iran (Limoee et al. 2007). Finally, we concluded that the low level chlorpyrifos resistance suggesting this insec- ticide may still provide adequate control of these strains. While the observed moderate levels of resistance to cypermethrin could im- ply developing resistance to this compound. Thus, we propose further studies on changes insecticide resistance levels by using the dif- ferent synergists following insecticide selec- tion in these strains for providing in vivo evidences about the possible mechanisms of insecticide resistance. Acknowledgements The authors would like to express their sincere gratitude to authorities of the Deputy for Research and School of Public Health of Ker- manshah University of Medical Sciences for providing the research grant and administrative assistance. We also thank Dr Sedegeh Tava- ssoli and the entire staff of the Fatemieh and Ecbatan hospitals in Hamadan who helped us for collecting of cockroaches. The authors de- clare that there is no conflict of interest. J Arthropod-Borne Dis, 2012, 6(2): 112–118 M Limoee et al.: Toxicity of Pyrethroid and … 117 References Chai RY, Lee CY (2010) Insecticide re- sistance profiles and synergism in field population of the German cockroach (Dictyoptera: Blattellidae) from Sin- gapore. 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