Fipronil resistance mechanisms in the rice stem borer, Chilo suppressalis Walker

Abstract

Fipronil resistance mechanisms were studied between the laboratory susceptible strain and the selective field population of rice stem borer, Chilo suppressalis Walker in the laboratory. The borer population was collected from Wenzhou county, Zhejiang province. After five generations of selection, fipronil resistance ratio was 45.3-fold compared to the susceptible strain. Synergism experiments showed that the synergistic ratios of PBO, TPP and DEF on fipronil in susceptible and resistant strains of C. suppressalis were 7.55-, 1.93- and 2.91-fold, respectively, and DEM showed no obvious synergistic action on fipronil. Activities of carboxylesterase and microsomal-O-demethylase in the resistant strain were 1.89- and 1.36-fold higher that in susceptible strain, and no significant difference of glutathione-S-transferase activity was found between the resistant and susceptible strains. The K_m and V_max experiments also demonstrated that fipronil resistance of C. suppressalis was closely relative to the enhanced activities of carboxylesterase and microsomal-O-demethylase. Moreover, cross-resistance between fipronil and other conventional insecticides and the multiple resistant properties of the original Wenzhou’s population were also discussed.

Introduction

The rice stem borer, Chilo suppressalis Walker is one of the most important insect pests of many subtropical and tropic paddyfields in Asia, North Africa and Southern Europe. In China, it is mainly found in the Yangtze drainage areas and hilly areas of southern provinces beyond Yangtze River. This insect feeds on rice causing significant economic losses to farmers. Besides, it also causes great damage to other crops including water bamboo, sugarcane, sorghum, corn, wheat, millet, horsebean and rape [1]. Since 1970s, there have been several outbreaks of pest attack in China because of the changes in the rice cultivation system and the use of hybrid varieties [2], [3]. The major control measure for the rice stem borer is the application of chemicals, which are known to cause resistance and control problems. Before 1980s, rice stem borer was mainly controlled by BHC, trichlorphon, parathion, parathionmethyl and chlordimeforn, however, these insecticides were banned for the bioaccumulation in food chain, chronic toxicity and/or high resistance at the end of 1980s. From the beginning of 1980s to the middle of 1990s, some organophosphate and nereistoxin insecticides such as methamidophos, triazophos, monosultap and dimehypo were mainly used to control the pest. However, because of long-term use of insecticides, some studies reported that rice stem borer is resistant to lots of conventional insecticides [4], [5], [6], [7], [8], [9], [10], [11]. At the end of 1990s, fipronil and abamectin were used for control and showed excellent effects. Recently, there are few reports about rice stem borer with low or middle level of resistance to fipronil [12], [13].

Fipronil [(±)-5-amino-1-(2,6-dichloro-α,α,α-trifluoro-p-tolyl)-4-trifluoromethylsulfinylpyr-azole-3-carbonitrile] is the first member of the phenylpyrazole insecticide. Its mode of action involves disruption of chloride ion flow by interacting at the GABA-gated chloride ionophore of the central nervous system [14], [15], or fipronil potently blocks glutamate-activated chloride channel and strongly inhibits glutamate-induced chloride ion flow [16]. Moreover, its oxidative sulfone is reported as the most effective inhibitor of glutamate-gated chloride channel than other metabolites [17]. Since 1997 the first use in Wentai region of Zhejiang province, fipronil has been used extensively in the whole Yangtze middle and downriver and Southeast coastal areas, and the dosage was increased year after year. Herein, the purpose of this study is to investigate the characterization and mechanisms of fipronil resistance in C. suppressalis, in order to offer some guidance for fipronil resistance management and reasonable use.