Jianya Su

Assessment of resistance risk in Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) to chlorantraniliprole.

BACKGROUND Beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), is a major pest of numerous cultivated crops. Chlorantraniliprole, the first commercialised ryanodine receptor insecticide from the anthranilic diamide class, has exceptional insecticidal activity on a range of lepidopteran pests. The aim of this study was to assess the resistance of S. exigua to chlorantraniliprole in the laboratory. RESULTS A field-collected population of S. exigua was selected after repeated exposure to chlorantraniliprole to determine the risk of resistance evolution. After 22 generations of selection, there was a 12.0-fold increase in LC(50) . The realised heritability (h(2)) of resistance was estimated as 0.1082 by using threshold trait analysis. The projected rate of resistance evolution indicated that, if h(2) = 0.1082 and 70% of the population was killed at each generation, then a tenfold increase in LC(50) would be expected in 21.7 generations for chlorantraniliprole. CONCLUSION These results show that the risk of resistance development to chlorantraniliprole exists in S. exigua after continuous application.

STATUS OF INSECTICIDE RESISTANCE OF THE WHITEBACKED PLANTHOPPER, SOGATELLA FURCIFERA (HEMIPTERA: DELPHACIDAE)

ABSTRACT Frequent outbreaks and widespread transmission of rice black-streaked dwarf virus by Sogatella furcifera (Horváth) (Hemiptera: Delphacidae) have aggravated yield losses of rice in eastern China. The use of insecticides for suppression of the vector has been a fundamental approach to prevent epidemics of the virus disease. However, the status of insecticide resistance in S. furcifera has not been examined recently in China. In this study, dose responses of S. furcifera to buprofezin, imidacloprid, thiamethoxam, chlorpyrifos and pymetrozine were evaluated. Most populations in eastern China have developed moderate resistance to buprofezin (up to 25-fold). Approximately 32% of field populations exhibited moderate resistance to imidacloprid, while other field populations showed minor changes (7.6-fold) in their susceptibility to this insecticide. Low variation of susceptibility to thiamethoxam (<6-fold) was observed among field populations, and no obvious resistance to this product was observed. Obvious variation (10.2-fold) of susceptibility to chlorpyrifos existed in field populations of which 8% displayed moderate resistance, and 32% exhibited low level resistance. Most populations (72%) were susceptible to pymetrozine, and relatively low variation of susceptibility to it was detected among the field populations of S. furcifera. Frequent and extensive use of buprofezin had driven the rapid development of resistance, and buprofezin resistance is widespread in the field populations of S. furcifera in China. To prevent further development of the resistance, use of buprofezin should be limited and rotated with alternative insecticides with different modes of action.

Biochemical mechanisms for metaflumizone resistance in beet armyworm, Spodoptera exigua.

The metaflumizone, which belongs to the class of voltage-dependent sodium channel blockers, was registered to control Spodoptera exigua on vegetables in China in 2009. The present study revealed S. exigua has developed high resistance to this novel chemistry insecticide shortly after 2-3 years application in Guangdong Province of China. The metabolic mechanisms for metaflumizone resistance in this insect were analysed. The inhibitor of esterases greatly potentiates the toxicity of this chemical against the field resistant populations. The synergism ratio is 5.7 and 3.4-fold for S. exigua collected from Huizhou, Guangdong Province in 2011 and 2012, respectively. The activity of esterases in field populations (HZ12) is also significantly greater than that in the susceptible strain, and further significantly increased by challenge with metaflumizone for 3 generations. However, the inhibitor of P450s or GSTs only has slight synergism on metaflumizone toxicity against resistant populations, and there are no obvious differences in activities of P450s or GSTs between resistant populations and the susceptible strain. These results suggest that esterases might take pivotal role in conferring metabolic resistance to metaflumizone in the field populations of S. exigua, and P450s or GSTs are not involved in this resistance. Moreover, flavin-dependent monooxygenases (FMOs) are discovered to involve in metaflumizone resistance in the field populations of S. exigua. The FMO inhibitor, methimazole, potentiates metaflumizone toxicity in resistant larva of this species substantially. The synergism ratios for methimazole in resistant populations HZ11 and HZ12 were 3.1 and 1.9, respectively. Enzymatic assays also revealed higher FMO activities in resistant populations than in the susceptible strain, and successive selection with metaflumizone further increased the FMO activity in the field resistant population, but not significantly. The higher FMO activities in the older larval stages and in the larval midgut signify the importance of FMO in the detoxification of xenobiotic from food sources. The synergism assay and FMO activity analysis suggest that FMO contributes to metaflumizone detoxification in resistant populations of S. exigua and conferred metaflumizone resistance in S. exigua. A novel mechanism for insecticide resistance by insect was proposed.

Differential Resistance and Cross-Resistance to Three Phenylpyrazole Insecticides in the Planthopper Nilaparvata lugens (Hemiptera: Delphacidae)

ABSTRACT Cross-resistance to two fipronil analogs, butene-fipronil and ethiprole, was detected in fipronil-resistant field populations and a resistant laboratory strain of the planthopper Nilaparvata lugens (Stål) (Hemiptera: Delphacidae), although the two analogs have not been used widely in rice-growing areas in China. The results showed that six field populations with 23.8–43.3-fold resistance to fipronil had reached a higher level of cross-resistance to ethiprole (resistance ratio [RR] = 47.1–100.9-fold) and had a minor level of cross-resistance (RR = 3.4–8.1-fold) to butene-fipronil. After 10 generations of selection, the RR to fipronil increased from 7.3-fold to 41.3-fold. At the same time, the insect increased cross-RR to ethiprole from 16.3-fold to 65.6-fold, whereas it had only minor increase in cross-resistance to butene-fipronil from 2.8-fold to 4.0-fold. These results confirmed that fipronil-resistant N. lugens could develop a higher level of cross-resistance to ethiprole, although it still maintained a lower level cross-resistance to butene-fipronil. Our data suggest that ethiprole is not a suitable alternative for controlling N. lugens, once the insect has developed a high level resistance to fipronil. Further investigation is necessary to understand the cross-resistance mechanisms in N. lugens.

Regression Analysis of Dynamics of Insecticide Resistance in Field Populations of Chilo suppressalis (Lepidoptera: Crambidae) During 2002–2011 in China

ABSTRACT To understand the evolution of insecticide resistance in the Asiatic rice borer, Chilo suppressalis (Walker) (Lepidoptera: Crambidae), in field, regression analysis based on a linear or nonlinear model was adopted for analyzing resistance dynamics to six insecticides of two field populations of the Lianyungang (LYG) and Ruian (RA) populations during 2002–2011. For the low-level resistance population, LYG population, sustained susceptibilities to abamectin and fipronil were seen for 10 yr; a polynomial curve regression model showed an increase in resistance to chlorpyrifos; exponential growth models fit to the resistance dynamics to triazophos and deltamethrin, and a sigmoidal growth curve for monosultap. For the high-level multiple resistance population, RA population, a slight increase from susceptible to a minor resistance to abamectin could be modeled by apolynomial cubic equation; an exponential growth model fit to the increase of resistance to fipronil from 8.7-fold to 33.6-fold; a sine waveform model fit to the vibrating tendency of resistance to chlorpyrifos; the dynamics of resistance to triazophos could be modeled by two combined curves, with a polynomial growth model and a sine waveform model; the high level of resistance to monosultap could be modeled with a sine waveform model; and a significant linear growth relationship of the resistance to deltamethrin of the RA population overyears was found. Then, the relationship between dynamics of resistance development to insecticides among the field populations of C. suppressalis and the application history of pesticides for controlling rice borers was discussed.

Geographic susceptibility of Chilo suppressalis Walker (Lepidoptera: Crambidae), to chlorantraniliprole in China

BACKGROUND Chilo suppressalis Walker, the rice striped stem borer (RSSB) is one of the most serious lepidopteron insect pests of rice in Asia. The registration of chlorantraniliprole added an option for the chemical control of RSSBs. RESULTS In this study, the geographic variability of susceptibility of field-collected RSSBs to chlorantraniliprole was assessed to establish a baseline for future comparisons of field population responses to the increased use of chlorantraniliprole products. Thirty-one RSSB colonies were established from 22 geographic areas in seven provinces of China, and were evaluated for chlorantraniliprole susceptibility. The range of mean lethal concentration (LC50 ) values in response to this chemical was between 0.821 and 17.720 mg (AI)/L across the populations. Susceptibility was significantly different (21.7-fold for LC50 and 55.6-fold for LC90 ) across the various geographic populations. No significant correlations were detected between the LC50 values of chlorantraniliprole and four conventional insecticides (monosultap, triazophos, chlorpyrifos and abamectin). CONCLUSION These results revealed great geographical variations in chlorantraniliprole susceptibilities among different geographic populations of RSSBs, which might indicate the potential for RSSBs to develop resistance to chlorantraniliprole. The effective execution of insecticide resistance monitoring programs and management strategies is urgently needed in China to sustain RSSB susceptibility to chlorantraniliprole.

Resistance in Cnaphalocrocis medinalis (Lepidoptera: Pyralidae) to new chemistry insecticides.

ABSTRACT The control of rice leaffolder, Cnaphalocrocis medinalis (Guenée), depended mainly on the insecticide application in China for a long time, and the resistance development impacted the effects of insecticide application. In this study, 13 conventional and new chemistry insecticides were assayed for the toxicities to the larvae of rice leaffolder collected from Nanning, Changsha, and Nanjing, China, with rice seedling dip method during 2011–2013. Among the tested chemicals, macrolide insecticides spinetoram, spinosad, abamectin, and emanectin benzoate have the highest toxicities, whereas monosultap and Bt have the least toxicities to this insect. Comparing with the baseline data established in 2010, the susceptibilities of rice leaffolder to chlorantraniliprole, metaflumizone, and tebufenozide are declining simultaneously and gradually in the three regions from 2011 to 2013, and C. medinalis are becoming resistance to chlorantraniliprole, metaflumizone, and tebufenozide. The synchronous decreases of susceptibility in three geographic populations were not observed for macrolide insecticides, indoxacarb, chlorpyrifos, monosultap, and Bt. The synchronous insecticide susceptibility declines in field populations of the migratory insect collected from different areas indicated resistance evolution, and the sequence application patterns of different insecticides should be scheduled to delay the further development of resistance along the migratory pathway of the rice leaffolder in China.

Changes in Insecticide Resistance of the Rice Striped Stem Borer (Lepidoptera: Crambidae)

ABSTRACT Application of insecticides is the most important method to control Chilo suppressalis (Walker) (Lepidoptera: Crambidae), and continuous use of individual insecticides has driven the rapid development of insecticide resistance in C. suppressalis during the past 30 yr. Monitoring insecticide resistance provides information essential for integrated pest management. Insecticide resistance of field populations to monosultap, triazophos, chlorpyrifos, and abamectin in China was examined in 2010 and 2011. The results indicated that the resistance levels of 14 field populations to four insecticides were significantly different. Four populations showed moderate resistance, and other populations possessed low-level resistance or were susceptible to monosultap. Nine populations displayed an extremely high or a high level of resistance to triazophos, whereas four populations were sensitive to this agent. Five populations exhibited a low level of resistance to abamectin, while the others remained sensitive. When compared with historical data, resistance to monosultap and triazophos decreased significantly, and the percentage of populations with high-level or extremely high-level resistance was obviously reduced. By contrast, the resistance to abamectin increased slightly. The increasing and decreasing resistance levels reported in this study highlight the different evolutionary patterns of insecticide resistance in C. suppressalis. An overreliance on one or two insecticides may promote rapid development of resistance. Slow development of resistance to abamectin, which was used mainly in mixtures with other insecticides, implies that the use of insecticide mixtures may be an effective method to delay the evolution of resistance to insecticides.

Susceptibility of the Rice Stem Borer, Chilo suppressalis (Lepidoptera: Crambidae), to Flubendiamide in China

ABSTRACT The rice stem borer, Chilo suppressalis (Walker), is an important rice pest in China, and has evolved resistance to several classes of insecticides. Flubendiamide is a phthalic diamide insecticide that shows selective insecticidal activity against lepidopterous insects. The susceptibility of 40 field populations of C. suppressalis, collected in 2011 and 2012 in seven provinces of south-eastern China, to flubendiamide was determined through rice seedling dipping bioassay method. Of these 40 populations, seven populations that were seldom exposed to flubendiamide were used to set up the baseline sensitivity, and the LC50 value was 0.092 mg/L. Variation in susceptibility among the 40 field populations was high (34-fold). The range of mean lethal concentration (LC50) values in response to this chemical was between 0.032 mg/L (FS11) and 1.090 mg/L (JH12) across the populations. Substantial variations of the susceptibility to flubendiamide were detected among different geographic populations. There was no significant difference observed between years for most populations, except for populations from Jinhua and Lujiang. Resistance ratios to the chemical ranged from 0.8 to 11.8, indicating that most colonies remained susceptible or showed certain decrease in susceptibility. It was found that 16 of the 40 populations had some level of resistance. However, moderate level of resistance was discovered in only one population from JH12 from Zhejiang province (11.8-fold). Other 15 populations showed low level of resistance (5.1–9.3-fold) to flubendiamide. These data are useful in future monitoring programs for detecting any changes in susceptibility as a result of using flubendiamide.

Molecular characterization and developmental expression of the gene encoding the prothoracicotropic hormone in the beet armyworm, Spodoptera exigua.

Prothoracicotropic hormone (PTTH), a neuropeptide hormone stimulating the prothoracic glands to synthesize ecdysone, plays an important role in regulating postembryonic development in insects. The cDNA encoding PTTH was isolated and sequenced from the beet armyworm, Spodoptera exigua (Spe). The deduced amino acid sequence is composed of a signal peptide, a peptide (65 amino acids) of unknown function, and a mature PTTH molecule (111 amino acids). The Spe-PTTH shows similarities (45.5%–70.3%) to other known PTTHs reported in Lepidoptera species, but 7 cysteine residues and the hydrophobic regions were conserved. Whole-mount immunocytochemistry by using an antiserum against recombinant Helicoverpa armigera PTTH showed that Spe-PTTH was synthesized in two pairs of neurosecretory cells in the S. exigua brain. Northern blot analysis demonstrates the presence of a 1.2-kb transcript in the brain. The Spe-PTTH mRNA is detectable at high levels at the wandering larval stage, early pupal stage, and pharate adult stage, suggesting that the Spe-PTTH gene might be correlated with molting, metamorphosis, and reproduction.