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Studies on Fipronil Resistance in Small Brown Planthopper, Laodelphax Striatellus (Fallen) and the Mechanisms

Author: WangYaoZhou
Tutor: HanZhaoJun
School: Nanjing Agricultural College
Course: Agricultural Entomology and Pest Control
Keywords: small brown planthopper fipronil insecticide resistance resistance mechanisms cross resistance fitness cost
CLC: S435.112.3
Type: Master's thesis
Year: 2009
Downloads: 36
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Abstract


The small brown planthopper (SBPH), Laodelphax striatellus (Fallen), belongs to Homoptera Delphacidae, and distributes an extensive geographic range in Asia and the temperate zone of Europe where from the Philippines to Siberia. It infests important crops including rice, maize and wheat not only by nymphs and adults, but also transmission of plant virus. This pest is usually controlled by chemical insecticides, and resistance has been developed to many insecticides commonly used. Fipronil is one of the most important insecticides recently introduced for rice pest control. However, previous work in our laboratory found that SBPH in Jiangsu province of China had lost its sensitivity to fipronil. In order to implementation of resistance management and efficient control, the resistant strain was selected in laboratory and efforts had been made to declare its resistance development, mechanism, realized heritability, fitness costs, and cross-resistance. Also the efficient substitute insecticides were selected in this work. As results, some interesting results were obtained which give a substantial foundation for further researches.Resistance development in SBPH was first studied by laboratory selection of fipronil resistance and analysis of the realized heritability. When the field population of SBPH was subjected to laboratory selection with fipronil for 24 generations, its resistance increased from 6.9 to 21.2 folds. This implied that continuous use of fipronil would result in resistance in field populations. However,3-4 times resistance developed from 24-generations selection also indicated that fipronil resistance in this pest developed slowly. It’s realized heritability is calculated as only 0.041 which is much lower than others reported. So, the risk of fipronil resistance in this pest is lower. If resistance management is implemented and different insecticides are used rotationally, the field SBPH would be delayed or even prevented to develop the resistance.According to the mode of action of fipronil, of the target GABA receptor was cloned from resistant and susceptible SBPH, Comparison of the gene sequences was also carried out. The result showed that the present resistant strain had not developed target resistance because no difference in target site was found between the resistance and susceptible strain. Analysis of detoxification enzyme activity found that the resistant and susceptible strain had also similar activity of esterase (EST) and glutathione-S-transferase (GST). However, the activity of cytochrome P450 monooxygenase in resistant strain was 1.58 folds higher. The enhanced activity of P450 might explain the resistance of SBPH to Fipronil. For confirmation, synergism tests were carried out with both resistant and susceptible SBPHs. The results demonstrated that TPP (1.07 folds) and DEM (1.10 folds) showed no synergism on fipronil in susceptible hoppers and only a little in resistant hoppers (1.39 and 1.35 folds, respectively). However, PBO exhibited synergism in both strains, and the synergism increased obviously with the resistance (from 1.44 to 13.68 folds). TPP and DEM are specific inhibitor of GST and EST, respectively. PBO can inhibit both MFO and EST. So, these synergism results certificate the conclusions deduced from detoxification enzyme activity analysis. Surely, the enhanced cytochrome P450 monooxygenase activity was the main mechanism for fipronil resistance in this pest.Fitness of resistant strain and susceptible strain on two rice varies was tested. The life table results revealed that the fitness of the resistant strain is significantly lower than that of the susceptible strain on both rice varies. Otherwise, hoppers fit better on W7 than on X63. The fitness order could be arranged as BJ-W7>BJ-X63>Fip-W7>Fip-X63. No obvious difference in female rate, copulation rate and eclosion rate was found between these experimental populations. But the oviposition and nymph survival rate varied significantly, and function as the key factors for the fitness change. This result demonstrated that not only the host varies can affect the breed of SPBH population, but also the fipronil resistance can load on SBPH with remarkable fitness cost. This also means that the resistant strain is inferior under the circumstance without fipronil selection. When the insecticide selection stress is lower, the ratio of resistant individuals in the population might decrease, which will have the susceptibility recover or the resistance development stop. This is also the basis of population genetics for pest resistance management.The cross-resistance between fipronil and other insecticides was studied with the susceptible and resistant strain of SBPH. It was found that the resistant strain selected by fipronil exhibited 21.24 folds resistance to fipronil, but showed no resistance to other 8 represent insecticides of 7 classes. Only the tolerance to avermectin increased a little (1.8 folds). These results revealed that fipronil had no obvious cross-resistance to the insecticides tested, which means that these insecticides can all be used alternatively with fipronil to delay resistance development. What is more important, the bioassay found that fipronil, avermectins, carbosulfan, indoxacarb, imidacloprid, and buprofezin is high toxic to SBPH with the LD50 ranged from 0.15ng to 0.38ng per pest. But the toxicity of ethofenprox, chlorpyrifos and phenthoate is much lower, especially for chlorpyrifos and phenthoate with LD50 value more than 10ng per pest. Furthermore, mixed use of avermectins or indoxacarb with other insecticides was evaluated. The results revealed that avermectins mixed with ethofenprox or phenthoate, and indoxacarb mixed with fipronil, phenthoate, or ethofenprox all showed additive effect. As that, those couple could be used in mixtures for SBPH control. But antagonism was found when avermectins was mixed with fipronil or indoxacarb. Thus, these kinds of mixtures should not used in practice.

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CLC: > Agricultural Sciences > Plant Protection > Pest and Disease Control > Crop pests and diseases and their prevention > Cereal crop pests and diseases > Rice pests and diseases > Insect pest > Planthopper
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