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1 January 2013 Insecticide Susceptibility of Field-Collected Populations of Culex tritaeniorhynchus in the Republic of Korea
Dae-Hyun Yoo, E-Hyun Shin, Dong-Kyu Lee, Young Joon Ahn, Kyu-Sik Chang, Hyun-Kyung Kim, Seong-Yoon Kim, Chan Park
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Abstract

The toxicities of 10 insecticides were examined against late third instar Culex tritaeniorhynchus Giles (Diptera: Culicidae) using the direct-contact mortality bioassay. Six geospatially-distant field mosquitoes were collected from Chuncheon-si, Hwaseong, Seosan. Jeonju, Daegu, and Busan in the Republic of Korea. Marked regional variations of insecticide susceptibility were observed. Field populations of Seosan, Jeonju, and Daegu from agricultural areas showed higher to extremely higher insecticide susceptibility to pyrethroids than those of Chuncheon-si, Hwaseong, and Busan strains from non-agricultural areas. Extremely high to low levels of susceptibility were measured: bifenthrin, susceptible ratio (SR) = 2.7–896.3; β-cyfluthrin, SR = 1.8–633.3; α-cypermethrin, SR = 1.2–1,051.9; deltamethrin, SR = 1.3–711.1; permethrin, SR = 1.5–1,053.4; etofenprox, SR = 2.2–29.3; chlorfenapyr, SR = 5.1–103.6; chlorpyrifos, SR = 2.3– 337.0; fenitrothion, SR = 2.0–142.3; and fenthion, SR = 1.4–186.2. Cx. tritaeniorhynchus populations from rice paddies had been under heavy selection pressure due to the agricultural insecticides, and that's why the mosquito species demonstrated high resistance to pyrethroids, which were used for a long time to control agricultural pests in the localities. These results indicate that careful selection and rotational use of these insecticides may result in continued satisfactory control against field populations of Japanese encephalitis vector mosquitoes.

Introduction

Japanese encephalitis (JE) is a vector-borne viral disease that occurs in South Asia, Southeast Asia, the Pacific, and East Asia, including the Republic of Korea (ROK) (Solomon 2006). As late as 1982, 1197 cases of JE were reported in the ROK, when the vaccination rate was 45.7% (Sohn 2000). However, government-mandated vaccination and vector-control programs decreased JE in the ROK to 6–7 cases reported annually over the last two decades, except for the recent outbreak of 26 cases in 2010 (Center for Disease Control and Prevention/Korea 2012). Culex tritaeniorhynchus Giles (Diptera: Culicidae) is the major vector of JE in the ROK (Burke and Leake 1988; Vaughn and Hoke 1992; Khan et al. 1996). Because Cx. tritaeniorhynchus breeds mainly in paddy fields, it is under heavy selection pressure with agricultural applications of insecticides and other pesticides sprayed on the rice fields. This selection pressure has resulted in the development of insecticide resistance (Shim et al. 1995a; Karunaratne and Hemingway 2000; Erlanger et al. 2009; Shin et al. 2011). Agricultural insecticide treatment can exert a selective pressure on both the larval and adult stages of vectors. Some of the breeding sites created by agricultural practices in rice fields and by irrigation schemes are sprayed directly. When agricultural breeding sites are treated, all the mosquito larvae are subjected to selective pressure, which is more likely to induce resistance than when houses are sprayed for mosquito control. Insecticides sprayed in crop fields can be carried by wind to nearby mosquito breeding sites. Finally, rains can wash pesticides applied on crops and drain them into ground pools or ditches where mosquito larvae breed. Adult mosquitoes have been observed resting on treated rice. In the ROK, rice-field-collected mosquito populations have developed locally unique insecticide resistances according to the long-term insecticides used in the region (Shim et al. 1982; Shim et al. 1995b; Chang et al. 2009; Shin et al. 2011).

In the ROK, the local public health centers have controlled mosquitoes, but they have not considered the insecticide resistance of vector mosquitoes and have used various insecticides. As vector mosquitoes obtain insecticide resistance, more insecticides may be applied, which can cause human and environmental health problems. The widespread use of commonly used less expensive insecticides has been a major obstacle in implementing cost-effective and safe integrated programs for mosquito management. In the ROK, 26 cases of JE occurred in 2010 (Center for Disease Control and Prevention/Korea 2012), but little monitoring of local insecticide resistance of Cx. tritaeniorhynchus has been performed for integrated vector mosquito control in the last 5 years.

Here, the resistance patterns of 10 currently used insecticides against six geospatially-distant field-collected populations of Cx. tritaeniorhynchus are reported. To estimate insecticide resistance of Cx. tritaeniorhynchus to agricultural pesticides, the resistance levels of Cx. tritaeniorhynchus collected from rice-field and non-rice-field areas were tested and compared.

Materials and Methods

Chemicals

The following ten insecticides were purchased from Sigma-Aldrich ( http://www.sigmaaldrich.com/) and were used in this study: bifenthrin (97.0% purity), β-cyfluthrin (98.0%), α-cypermethrin (97.5%), deltamethrin (99.5%), etofenprox (96.5%), permethrin (95.5%), chlorpyrifos (98.5%), fenthion (95.5%), fenitrothion (98.5%), and chlorfenapyr (99.0%). Triton X-100 was obtained from Shinyo Pure Chemicals (Osaka, Japan). All other chemicals used were analytical grade and available commercially.

Mosquitoes

Six different colonies of Cx. tritaeniorhynchus were established from larvae collected near rice paddy fields and cow sheds in Chuncheon (designated CC-CT; 37° 52′ 56.19″ N, 127° 46′ 12.16″ E), Hwaseong (HS-CT; 37° 12′ 18.40″ N, 126° 50′ 53.19″ E), Busan (BS-CT; 35° 12′ 21.73″ N, 129° 12′ 5.31″ E), Seosan (SS-CT; 36° 42′ 2