Congfen Gao

Evidence of field-evolved resistance to Cry1Ac-expressing Bt cotton in Helicoverpa armigera (Lepidoptera: Noctuidae) in northern China

BACKGROUND Evolution of resistance threatens the continued success of transgenic crops expressing insecticidal proteins. One of the key factors for successful resistance management is the timely implementation of monitoring programmes to detect early changes of resistance allele frequency in field populations. F(1)/F(2) screen, dose-response bioassays and field survey were used to monitor resistance to the Cry1Ac-expressing cotton in a field population of Helicoverpa armigera (Hübner), the primary target of transgenic Bt cotton in China. RESULTS Field survey showed an increased trend of egg populations of H. armigera on Bt cotton in the Qiuxian area from 2003 to 2007. By using the F(2) screening procedure, the resistance allele frequency in the Qiuxian (Hebei, China) population of H. armigera collected during 2007 was estimated to be 0.075 (95% CI: 0.053-0.100), which was 12 times greater than that estimated 9 years ago. Dose-response bioassay with the field population collected from the same area showed a significant resistance level (11-fold) to Cry1Ac toxin compared to a laboratory susceptible strain. CONCLUSION This study documented a case of field-evolved resistance in H. armigera after several years of intensive planting of Bt cotton. Proactive tactics must be adopted to prevent further increase of resistance gene frequency in the Qiuxian region.

Buprofezin susceptibility survey, resistance selection and preliminary determination of the resistance mechanism in Nilaparvata lugens (Homoptera: Delphacidae)

BACKGROUND Buprofezin has been used for many years to control Nilaparvata lugens (Stål). Assessment of susceptibility change in the insect is essential for maintaining control efficiency and resistance management. RESULTS Eleven-year surveys showed that most field populations were susceptible before 2004. However, substantially higher levels of resistance (up to 28-fold) were found in most of the rice fields in China after 2004. A field population was collected and periodically selected for buprofezin resistance in the laboratory. After 65 generations (56 were selected), the colony successfully obtained 3599-fold resistance to buprofezin. Synergism tests showed that O,O-diethyl-O-phenyl phosphorothioate (SV1), piperonyl butoxide (PBO) and diethyl maleate (DEM) increased buprofezin toxicity in the resistant strain by only 1.5-1.6 fold, suggesting that esterases, P450-monooxygenases and glutathione S-transferases had no substantial effect on buprofezin resistance development. CONCLUSION The results from this study indicate that N. lugens has the potential to develop high resistance to buprofezin. A resistance management program with rotation of buprofezin and other pesticides may efficiently delay or slow down resistance development in the insect. Further investigation is also necessary to understand the resistance mechanisms in N. lugens.

Inheritance, fitness cost and mechanism of resistance to tebufenozide in Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)

BACKGROUND Tebufenozide has been used as a key insecticide for controlling beet armyworm, Spodoptera exigua (Hübner). To evaluate the risk of resistance evolution and to develop a better resistance management strategy, a field-collected population was selected with tebufenozide in the laboratory. Thereafter, the inheritance and fitness cost of tebufenozide resistance in S. exigua were investigated. RESULTS After being selected with tebufenozide for 61 generations, S. exigua developed a 92-fold resistance to the chemical. The degrees of dominance for the reciprocal cross progeny were - 0.2698 and - 0.2785. The resistant strain had a relative fitness of 0.71, with substantially lower rates of larval survival, pupal weight, pupation and oviposition per female, and prolonged larval and pupal duration. PBO significantly increased the toxicity of tebufenozide (SR = 2) against resistant insects, and DEM and DEF also showed synergism with tebufenozide. CONCLUSION Tebufenozide resistance in S. exigua was inherited as autosomal, incompletely recessive and controlled by more than one gene. Development of the resistance may cost significant fitness for the resistant population. Mixed-function oxidases might play an important role in tebufenozide resistance in S. exigua. This study provided valuable information for further understanding tebufenozide resistance and for facilitating the development of resistance management strategies.

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 Monitoring and Assessment of Resistance Risk to Pymetrozine in Laodelphax striatellus (Hemiptera: Delphacidae)

ABSTRACT Laodelphax striatellus (Fallén) is a major pest of cultivated rice. Pymetrozine, a pyridine azomethine compound, represents a novel insecticide with a selective activity against sucking pests. The resistance of L. striatellus to thiamethoxam, nitenpyram, chlorpyrifos, and pymetrozine in five field populations, collected from Jiangsu, Anhui, and Zhejiang in China, was monitored from 2009 to 2011 in laboratory. All the populations kept susceptible to minor resistance to thiamethoxam and nitenpyram in the 3 yr (0.6- to 2.2-fold and 0.8- to 3.8-fold, respectively), while the insects developed low to high level resistance to chlorpyrifos (9.7- to 76.1-fold). Three populations were all susceptible to pymetrozine (1.1- to 2.1-fold) in 2009, but the Wuxi and Yancheng populations developed low level resistance to pymetrozine (5.5-fold and 5.3-fold, respectively) in 2011. Meanwhile, the resistance level of the selected strain reared in laboratory increased by 1.0-fold after 12-generation selection with pymetrozine. The realized heritability (h2 ) of resistance at different selection stages was estimated as 0.0470 (F1 to F13) and 0.2070 (F5 to F13) by using threshold trait analysis. It suggested that L. striatellus had the definite risk of resistance to pymetrozine.

Knockdown of the ionotropic γ-aminobutyric acid receptor (GABAR) RDL gene decreases fipronil susceptibility of the small brown planthopper, Laodelphax striatellus (Hemiptera: Delphacidae)

Insect γ-aminobutyric acid receptors (GABARs) are important molecular targets of cyclodiene and phenylpyrazole insecticides. Previously GABARs encoding rdl (resistant to dieldrin) genes responsible for dieldrin and fipronil resistance were identified in various economically important insect pests. In this study, we cloned the open reading frame cDNA sequence of rdl gene from fipronil-susceptible and fipronil-resistant strains of Laodelphax striatellus (Lsrdl). Sequence analysis confirmed the presence of a previously identified resistance-conferring mutation. Different alternative splicing variants of Lsrdl were noted. Injection of dsLsrdl reduced the mRNA abundance of Lsrdl by 27-82%, and greatly decreased fipronil-induced mortality of individuals from both susceptible and resistant strains. These data indicate that Lsrdl encodes a functional RDL subunit that mediates susceptibility to fipronil. Additionally, temporal and spatial expression analysis showed that Lsrdl was expressed at higher levels in eggs, fifth-instar nymphs, and female adults than in third-instar and fourth-instar nymphs. Lsrdl was predominantly expressed in the heads of 2-day-old female adults. All these results provide useful background knowledge for better understanding of fipronil resistance related ionotropic GABA receptor rdl gene expressed variants and potential functional differences in insects.

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.