Monitoring Corynespora cassiicola Resistance to Boscalid, Trifloxystrobin, and Carbendazim in China

Cucumber leaf spot (CLS), caused by Corynespora cassiicola, is a serious disease of greenhouse cucumbers. With frequent use of existing fungicides, C. cassiicola has developed resistance to some of them, with serious implications for the control of CLS in the field. With a lack of new fungicides, it is necessary to use existing fungicides for effective control. Therefore, this study monitored the resistance of C. cassiicola to three commonly used and effective fungicides, boscalid, trifloxystrobin, and carbendazim, from 2017 to 2021. The frequency of resistance to boscalid showed an increasing trend, and the highest frequency was 85.85% in 2020. The frequency of resistance to trifloxystrobin was greater than 85%, and resistance to carbendazim was maintained at 100%. Among these fungicides, strains with multiple resistance to boscalid, trifloxystrobin, and carbendazim were found, accounting for 32.00, 25.25, 33.33, 43.06, and 37.24%, respectively. Of the strains that were resistant to boscalid, 87% had CcSdh mutations, including seven genotypes: B-H278L/Y, B-I280V, C-N75S, C-S73P, D-D95E, and D-G109V. Also, six mutation patterns of the Ccβ-tubulin gene were detected: E198A, F167Y, E198A&M163I, E198A&F167Y, M163I&F167Y, and E198A&F200C. Detection of mutations of the CcCytb gene in resistant strains showed that 98.8% were found to have only the G143A mutation. A total of 27 mutation combinations were found and divided into 14 groups for analysis. The resistance levels differed according to genotype. The development of genotypes showed a complex trend, increasing from 4 in 2017 to 13 in 2021 and varying by region. Multiple fungicide resistance is gradually increasing. Therefore, it is necessary to understand the types of mutations and the trend of resistance to guide the use of fungicides to achieve disease control.

Susceptibility and diagnostic concentration for Bacillus thuringiensis toxins and newer chemical insecticides in Spodoptera frugiperda (Lepidoptera: Noctuidae) from China

The fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), is a cosmopolitan pest that exploits more than 350 host plants, including economically important crops such as corn, cotton and rice. Control of S. frugiperda largely relies on transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) and spraying synthetic insecticides. Here, we established the susceptibility and diagnostic concentration for 2 Bt toxins and 5 newer insecticides in invasive populations of S. frugiperda from southeastern China. Concentrations causing 50% mortality (LC50) in ten field populations sampled in 2022 ranged from 2.13 to 19.29 and 22.43 to 71.12 ng/cm2 for Cry1Fa and Vip3Aa, and 0.83 to 5.30, 2.83 to 9.94, 0.04 to 0.23, 4.59 to 8.40, and 1.49 to 6.79 mg/liter for chlorantraniliprole, chlorfenapyr, emamectin benzoate, indoxacarb, and spinosad, respectively. Relative to the susceptible strain YJ-19, the largest resistance ratio in the field populations was 5.1, 1.6, 6.2, 3.9, 4.6, 2.2, and 3.6 for Cry1Fa, Vip3Aa, chlorantraniliprole, chlorfenapyr, emamectin benzoate, indoxacarb, and spinosad, respectively, indicating that the field populations were generally susceptible to these Bt toxins and insecticides. Based on the pooled response of the field populations, the diagnostic concentration for resistance monitoring, estimated as ca. twice the LC99, was 400 and 1,500 ng/cm2 for Cry1Fa and Vip3Aa, and 2, 40, 60, 60, and 100 mg/liter for emamectin benzoate, chlorantraniliprole, chlorfenapyr, spinosad, and indoxacarb, respectively. These results provide useful information for monitoring resistance to key Bt toxins and insecticides for the control of S. frugiperda in China.

Baseline Susceptibility of the Field Populations of Ostrinia furnacalis in Indonesia to the Proteins Cry1A.105 and Cry2Ab2 of Bacillus thuringiensis

Genetically modified MON 89034 corn (Zea mays L.) expressing Bacillus thuringiensis (Bt) insecticidal proteins, viz. Cry1A.105 and Cry2Ab2, is a biotechnological option being considered for the management of the major corn pest in Indonesia, the Asian corn borer (Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae)). As a part of a proactive resistance-management program for MON 89034 corn in Indonesia, we assessed the baseline susceptibility of field-collected populations of O. furnacalis to Cry1A.105 and Cry2Ab2 proteins. Dose-response bioassays using the diet-dipping method indicated that the lethal concentration (LC50) values of Cry1A.105 and Cry2Ab2 in 24 different field populations of O. furnacalis ranged from 0.006 to 0.401 µg/mL and from 0.044 to 4.490 µg/mL, respectively, while the LC95 values ranged from 0.069 to 15.233 µg/mL for Cry1A.105 and from 3.320 to 277.584 µg/mL for Cry2Ab2. The relative resistance ratios comparing the most tolerant field populations and an unselected laboratory population were 6.0 for Cry1A.105 and 2.0 for Cry2Ab2 based on their LC50 values. Some field populations were more susceptible to both proteins than the unselected laboratory population. The LC99 and its 95% fiducial limits across the field populations were calculated and proposed as candidate diagnostic concentrations. These data provide a basis for resistance monitoring in Bt Corn and further support building resistance-management strategies in Indonesia.

Rapid test to detect insecticide resistance in field populations of Spodoptera frugiperda (Lepidoptera: Noctuidae)

Introduction: The development of insecticide resistance in Spodoptera frugiperda populations is a serious threat to the crop industry. Given the spread of invasive resistant populations, prospective monitoring should be accelerated, and the development of diagnostic tools for rapid and accurate assessments of insecticide resistance is essential. Methods: First, the discriminating dose and diagnostic time of the kit were determined by the glass vial method based on a susceptible strain. Then, pests that were collected from field populations were used to determine their susceptibility to seven insecticides by using the diagnostic kit. Finally, the accuracy of the kit was verified based on correlation analyses and the likelihood of insecticide control failure was assessed. Results: Here, we describe a diagnostic kit that enables the rapid detection of resistance to chlorpyrifos, bifenthrin, deltamethrin, lambda-cyhalothrin, phoxim, chlorantraniliprole and chlorfenapyr within 1-2 h in S. frugiperda at diagnostic doses of 0.98, 0.84, 0.38, 1.64, 0.0082, 1.75 and 0.65 μg/cm2, respectively. The linear equation between mortalities under diagnostic doses and actual resistance ratios measured by the diet-overlay bioassay was determined. The high correlation indicates that the insecticide resistance levels diagnosed by the kit were consistent with the results of the diet-overlay bioassay. Moreover, we found a significant negative correlation between diagnostic mortality and the likelihood of control failure for bifenthrin (r = -0.899, p = 0.001), deltamethrin (r = -0.737, p = 0.024) and lambda-cyhalothrin (r = -0.871, p = 0.002). Discussion: The insecticide resistance diagnostic kit for S. frugiperda is a user-friendly tool (portable, short detection time). Its excellent performance qualifies the kit as a reliable screening tool for identifying effective insecticides in sustainable resistance management.

Extended Sentinel Monitoring of Helicoverpa zea Resistance to Cry and Vip3Aa Toxins in Bt Sweet Corn: Assessing Changes in Phenotypic and Allele Frequencies of Resistance

Transgenic corn and cotton that produce Cry and Vip3Aa toxins derived from Bacillus thuringiensis (Bt) are widely planted in the United States to control lepidopteran pests. The sustainability of these Bt crops is threatened because the corn earworm/bollworm, Helicoverpa zea (Boddie), is evolving a resistance to these toxins. Using Bt sweet corn as a sentinel plant to monitor the evolution of resistance, collaborators established 146 trials in twenty-five states and five Canadian provinces during 2020-2022. The study evaluated overall changes in the phenotypic frequency of resistance (the ratio of larval densities in Bt ears relative to densities in non-Bt ears) in H. zea populations and the range of resistance allele frequencies for Cry1Ab and Vip3Aa. The results revealed a widespread resistance to Cry1Ab, Cry2Ab2, and Cry1A.105 Cry toxins, with higher numbers of larvae surviving in Bt ears than in non-Bt ears at many trial locations. Depending on assumptions about the inheritance of resistance, allele frequencies for Cry1Ab ranged from 0.465 (dominant resistance) to 0.995 (recessive resistance). Although Vip3Aa provided high control efficacy against H. zea, the results show a notable increase in ear damage and a number of surviving older larvae, particularly at southern locations. Assuming recessive resistance, the estimated resistance allele frequencies for Vip3Aa ranged from 0.115 in the Gulf states to 0.032 at more northern locations. These findings indicate that better resistance management practices are urgently needed to sustain efficacy the of corn and cotton that produce Vip3Aa.

Using Sentinel Plots to Monitor for Changes in Thrips Susceptibility to MON 88702 Cotton Containing the Cry51Aa2.834_16 Bt Protein

Transgenic Bt crops are important tools for growers to manage insect pests, but their durability is threatened by the evolution of insect resistance. Implementing a resistance monitoring program is essential to detect and mitigate resistance. For non-high-dose Bt crops, resistance monitoring is challenging, because insect control is not complete, so targeted insects and insect damage will be present even without resistance. Given these challenges, sentinel plots have been used to monitor for insect resistance to non-high-dose crops by assessing changes in the efficacy of a Bt crop over time relative to a non-Bt control. We optimized a sentinel plot resistance monitoring approach for MON 88702 ThryvOn™ cotton, a new non-high-dose Bt product targeting two sucking pest taxa-Lygus (L. lineolaris and L. hesperus) and thrips (Frankliniella fusca and F. occidentalis)-and report here on the thrips monitoring methods and results. Quantifying thrips immatures was the best metric to characterize the impact of the trait, with at least a 40-60% average reduction of thrips immatures on ThryvOn relative to the control cotton at all sites with higher thrips densities. These data can be used within a ThryvOn resistance monitoring program and represent a case study for establishing a resistance monitoring approach for a non-high-dose trait product.

Fungicide Resistance: Progress in Understanding Mechanism, Monitoring, and Management

Fungicide treatments are often essential for maintaining healthy crops and to achieve reliable and high-quality yields. However, continued use of fungicides with the same modes of action can lead to development of fungicide resistance, which has emerged in various plant pathogens and is a serious threat to effective crop protection. Exploration of resistance mechanisms is critical for resistance monitoring and management. This brief review summarizes advances during the past five decades in understanding the molecular resistance mechanisms of plant pathogenic fungi and oomycetes to major classes of fungicides, including benzimidazoles, myosin inhibitors, sterol demethylation inhibitors, quinone outside inhibitors, succinate dehydrogenase inhibitors, anilinopyrimidines, carboxylic acid amides, and oxysterol-binding protein homolog inhibitors. Based on known resistance mechanisms, PCR- and loop-mediated isothermal amplification-based approaches have been developed to allow high-throughput monitoring and early/rapid detection of emerging resistance. Classical principles in fungicide resistance management are also summarized, including using different modes of action of fungicides, limiting the number of applications of the chemicals with site-specific modes of action, and avoidance of their eradicant use. Future studies on novel strategies of disease management, including development of epigenetics- and RNA-based fungicides, will provide valuable knowledge for management of fungicide resistance.

Resistance Monitoring of Nilaparvata lugens to Pymetrozine Based on Reproductive Behavior

On the basis of the inhibition effects of pymetrozine on the reproductive behavior of N. lugens, we established a bioassay method to accurately evaluate the toxicity of pymetrozine in N. lugens and clarified the level of pymetrozine resistance of N. lugens in the field. In this study, pymetrozine's effects on the fecundity of N. lugens were evaluated using the topical application method and rice-seedling-dipping method. Moreover, the resistance of N. lugens to pymetrozine in a pymetrozine-resistant strain (Pym-R) and two field populations (YZ21 and QS21) was determined using the rice-seedling-dipping method and fecundity assay methods. The results showed that treatment of N. lugens third-instar nymphs with LC₁₅, LC₅₀, and LC₈₅ doses of pymetrozine resulted in a significantly reduced fecundity of N. lugens. In addition, N. lugens adults treated with pymetrozine, using the rice-seedling-dipping and topical application method, also exhibited a significantly inhibited fecundity. Using the rice-stem-dipping method, pymetrozine resistance levels were shown to be high in Pym-R (194.6-fold), YZ21 (205.9-fold), and QS21 (212.8-fold), with LC₅₀ values of 522.520 mg/L (Pym-R), 552.962 mg/L (YZ21), and 571.315 (QS21) mg/L. However, when using the rice-seedling-dipping or topical application fecundity assay method, Pym-R (EC₅₀: 14.370 mg/L, RR = 12.4-fold; ED₅₀: 0.560 ng/adult, RR = 10.8-fold), YZ21 (EC₅₀: 12.890 mg/L, RR = 11.2-fold; ED₅₀: 0.280 ng/adult; RR = 5.4-fold), and QS21 (EC₅₀: 13.700 mg/L, RR = 11.9-fold) exhibited moderate or low levels of resistance to pymetrozine. Our studies show that pymetrozine can significantly inhibit the fecundity of N. lugens. The fecundity assay results showed that N. lugens only developed low to moderate levels of resistance to pymetrozine, indicating that pymetrozine can still achieve effective control on the next generation of N. lugens populations.

Molecular diagnostics for monitoring insecticide resistance in the western flower thrips Frankliniella occidentalis

BACKGROUND

Insecticide resistance has emerged in various western flower thrips (WFT) populations across the world, threatening the efficiency of chemical control applications. Elucidation of insecticide resistance mechanisms at the molecular level provides markers for the development of diagnostics to monitor the trait and support evidence-based resistance management.

RESULTS

TaqMan and Droplet Digital polymerase chain reaction (ddPCR) diagnostics were developed and validated, against Sanger sequencing, in individual and pooled WFT samples respectively, for the G275E mutation (nicotinic acetylcholine receptor α6 gene, nAChR α6) associated with resistance to nAChR allosteric modulators, site I (spinosyns); L1014F, T929I, T929C and T292V mutations (voltage-gated sodium channel gene, vgsc) linked with pyrethroid resistance; and I1017M (chitin synthase 1 gene, chs1) conferring resistance to growth inhibitors affecting CHS1 (benzoylureas). The detection limits of ddPCR assays for mutant allelic frequencies (MAF) were in the range of 0.1%-0.2%. The assays were applied in nine WFT field populations from Crete, Greece. The G275E (MAF = 29.66%-100.0%), T929I and T929V (combined MAF = 100%), L1014F (MAF = 11.01%-37.29%), and I1017M (MAF = 17.74%-51.07%) mutations were present in all populations.

CONCLUSION

The molecular diagnostics panel that was developed in this study can facilitate the quick and sensitive resistance monitoring of WFT populations at the molecular level, to support evidence-based insecticide resistance management strategies. © 2022 Society of Chemical Industry.

Resistance of Panonychus citri (McGregor) (Acari: Tetranychidae) to pyridaben in China: monitoring and fitness costs

BACKGROUND

Panonychus citri is a major citrus pest worldwide. The short life cycle and high reproductive potential of P. citri, combined with heavy acaricide use, have led to high levels of resistance to acaricides, posing a threat to global resistance management programs. Here, resistance monitoring was established to determine the pyridaben resistance status of ten P. citri populations in China from 2014 to 2021 using a leaf-dipping assay. Four characterized strains-the susceptible strain (Lab_S), the resistant strain (Pyr_R), as well as the segregated resistant strain (Pyr_Rs) and the segregated susceptible strain (Pyr_Control) derived from the crossing of the Lab_S and Pyr_R strains, were used to evaluate the life-history characteristics using age-stage, two-sex life tables.

RESULTS

Most P. citri populations developed high resistance to pyridaben. Resistance levels exceeded 1000-fold in Yuxi, Anyue, Nanning, and Ganzhou populations compared with the Lab_S strain. Compared with Pyr_Control, two key fitness cost criteria, developmental period and fecundity, showed significant differences in Pyr_Rs under consistent conditions. The intrinsic rate of increase, net reproductive rate and gross reproductive rate were lower in the resistant strain compared with the Pyr_Control strain. The Pyr_Rs strain had a lower relative fitness of 0.934 compared with the Pyr_Control. Moreover, the life-history traits and population parameters of the Pyr_R strain also showed significant differences compared with the Lab_S strain.

CONCLUSION

The resistance levels to pyridaben varied greatly among the different P. citri populations and showed regional differences. Substantial fitness costs are associated with pyridaben resistance. This study provides potential implications for developing strategies for resistance management in P. citri. © 2022 Society of Chemical Industry.

Characterization of Italian Plasmopara viticola populations for resistance to oxathiapiprolin

BACKGROUND

Oxathiapiprolin is a novel fungicide and the first of the piperidinyl-thiazole-isoxazoline class to be discovered. This fungicide has been reported to have high activity against Plasmopara viticola, the grapevine downy mildew agent, and other plant-pathogenic oomycetes. In this study, the baseline sensitivity of Italian P. viticola populations towards oxathiapiprolin was established on 29 samples collected in 10 different viticultural areas. Two insensitive strains were characterized for their mechanism of resistance.

RESULTS

Oxathiapiprolin exhibited substantial inhibitory activity against 27 of the 29 populations tested, with EC₅₀ values ranging from a minimum of under 4 × 10^-5  mg L^-1 to over 4 × 10^-1  mg L^-1 , with an average value of 3.2 × 10^-2  mg L^-1 . Two stable suspected oxathiapiprolin-resistant mutants were isolated from population exhibiting reduced sensitivity, and sequenced for the oxathiapiprolin target gene PvORP1. The comparison with wild-type isolates revealed that the resistant isolates possessed a heterozygous mutation causing the amino acid substitution N837I, recently reported in the literature.

CONCLUSION

The results obtained indicate a risk for Italian P. viticola populations to develop resistance to oxathiapiprolin in association with the N837I mutation at PvORP1. Anti-resistance strategies should be carefully implemented and the sensitivity levels to this molecule should be monitored accurately in future to preserve its effectiveness. © 2022 Society of Chemical Industry.

Managing Practical Resistance of Lepidopteran Pests to Bt Cotton in China

China is one of the major cotton producers globally with small farmers. Lepidopteran pests have always been the main factor affecting cotton production. To reduce the occurrence of and damage caused by lepidopteran pests, China has employed a pest control method focused on planting Bt (Cry1Ac) cotton since 1997. Chinese resistance management tactics for the main target pests, the cotton bollworm and pink bollworm, were also implemented. For polyphagous (multiple hosts) and migratory pests such as the cotton bollworm (Helicoverpa armigera), the "natural refuge" strategy, consisting of non-Bt crops such as corn, soybean, vegetables, peanuts, and other host crops, was adopted in the Yellow River Region (YRR) and Northwest Region (NR). For a single host and weak migration ability pest, such as the pink bollworm (Pectinophora gossypiella), the seed mix refuge strategy yields a random mixture within fields of 25% non-Bt cotton by sowing second-generation (F2) seeds. According to field monitoring results for more than 20 years in China, practical resistance (Bt cotton failure) of target pests was avoided, and there were no cases of Bt (Cry1Ac) failure of pest control in cotton production. This indicated that this Chinese resistance management strategy was very successful. The Chinese government has decided to commercialize Bt corn, which will inevitably reduce the role of natural refuges; therefore, this paper also discusses adjustments and future directions of cotton pest resistance management strategies.

Resistance to dinotefuran in Bemisia tabaci in China: status and characteristics

BACKGROUND

Bemisia tabaci (Gennadius) is a serious agricultural pest worldwide. Neonicotinoids are the most important new class of synthetic insecticides used in the management of B. tabaci. However, B. tabaci populations have developed resistance to various active ingredients in neonicotinoids following long-term and widespread application.

RESULTS

Dinotefuran exhibited high toxicity against most B. tabaci field populations. One population (Din-R) with a high level of resistance to dinotefuran (255.6-fold) was first identified in the field. The Din-R population exhibited medium to high levels of resistance to all the tested neonicotinoid insecticides and a high level of resistance to spinetoram. Genetic inheritance analysis revealed that resistance to dinotefuran was incompletely recessive and polygenic. The synergist piperonyl butoxide significantly increased the toxicity of dinotefuran to Din-R. P450 activity in the Din-R population was 2.19-fold higher than in the susceptible population. RNA-sequencing analysis showed that 12 P450 genes were significantly upregulated in the Din-R population, of which CYP6DW5, CYP6JM1 and CYP306A1 were found to exhibit more than 3.00-fold higher expression in Din-R when using a reverse transcription quantitative real-time polymerase chain reaction. Expression of eight P450 genes was obviously induced by dinotefuran, and CYP6DW5 showed the highest expression level. After knockdown of CYP6DW5 in Din-R, the toxicity of dinotefuran increased significantly.

CONCLUSION

P450 had a crucial role in dinotefuran resistance in B. tabaci, and CYP6DW5 was involved in the resistance. These results provide important information for the management of resistance in B. tabaci and improve our understanding of the resistance mechanism of dinotefuran. © 2022 Society of Chemical Industry.

Establishment of Toxicity and Susceptibility Baseline of Broflanilide for Aphis gossypii Glove

The Aphis gossypii is an important pest that can damage cotton plants and can cause a huge economic loss worldwide. Chemical control is a main method to manage this pest, but the cotton aphid resistance to insecticides has become a severe problem in the management of the cotton aphid. It is important to introduce a novel insecticide for rotational application with other insecticides. Broflanilide, as a meta-diamide insecticide with a special mode of action, showed high efficiency against lepidopterous larvae. However, we found that broflanilide possessed high insecticidal activity against the sap-sucking pest A. gossypii. The susceptibility of A. gossypii to broflanilide from 20 field populations in main cotton planting areas of China in 2021 was determined by the leaf-dipping method. LC₅₀ values of broflanilide to A. gossypii ranged from 0.20 μg mL^-1 to 1.48 μg mL^-1. The susceptible baseline of A. gossypii to broflanilide was established with the LC₅₀ value of 0.41 μg mL^-1 and might be used to calculate the resistance ratio (RR) of cotton aphid population in broflanilide resistance monitoring. The RR value of field populations in China was from 0.49 to 3.61 in 2021. It suggested that the broflanilide may be a potential agent in the resistance management of A. gossypii to insecticides. These results are significantly useful for the rational chemical control of cotton aphids.

Baseline susceptibility of Helicoverpa armigera, Plutella xylostella, and Spodoptera frugiperda to the meta-diamide insecticide broflanilide

Broflanilide is a novel meta-diamide insecticide that acts as a γ-aminobutyric acid-gated chloride channel allosteric modulator. With its unique mode of action, broflanilide has no known cross-resistance with existing insecticides and is expected to be an effective tool for the management of insecticide resistance. Establishing the baseline susceptibility to this insecticide is an essential step for developing and implementing effective resistance management strategies. Here we evaluated the baseline susceptibility to broflanilide for 3 cosmopolitan lepidopteran pest species, Helicoverpa armigera, Plutella xylostella, and Spodoptera frugiperda. Broflanilide exhibited high activity against populations sampled in the major distribution range of these pests in China, with median lethal concentrations (LC₅₀ ) ranging between 0.209 and 0.684, 0.076 and 0.336, and 0.075 and 0.219 mg/L for H. armigera, P. xylostella, and S. frugiperda, respectively. Among-population variability in susceptibility to broflanilide was moderate for H. armigera (3.3-fold), P. xylostella (4.4-fold), and S. frugiperda (2.9-fold). The recommended diagnostic concentrations for H. armigera, P. xylostella, and S. frugiperda were 8, 4, and 2 mg/L, respectively. Little or no cross-resistance to broflanilide was detected in 3 diamide-resistant strains of P. xylostella and 1 spinosyns-resistant strain of S. frugiperda. Our results provide critical information for the development of effective resistance management programs to sustain efficacy of broflanilide against these key lepidopteran pests.

Status of the Resistance of Aphis gossypii Glover, 1877 (Hemiptera: Aphididae) to Afidopyropen Originating from Microbial Secondary Metabolites in China

The resistance of cotton aphids to various forms of commonly used pesticides has seriously threatened the safety of the cotton production. Afidopyropen is a derivative of microbial metabolites with pyropene insecticide, which has been shown to be effective in the management of Aphis gossypii. Several field populations of Aphis gossypii were collected from the major cotton-producing regions of China from 2019 to 2021. The resistance of these populations to afidopyropen was estimated using the leaf-dipping method. The LC₅₀ values of these field populations ranged from 0.005 to 0.591 mg a.i. L^-1 in 2019, from 0.174 to 4.963 mg a.i. L^-1 in 2020 and from 0.517 to 14.16 mg a.i. L^-1 in 2021. The resistance ratios for all A. gossypii populations ranged from 0.03 to 3.97 in 2019, from 1.17 to 33.3 in 2020 and from 3.47 to 95.06 in 2021. The afidopyropen resistance exhibited an increasing trend in the field populations of Cangzhou, Binzhou, Yuncheng, Kuerle, Kuitun, Changji and Shawan from 2019 to 2021. This suggests that the resistance development of the cotton aphid to afidopyropen is inevitable. Therefore, it is necessary to rotate or mix afidopyropen with other insecticides in order to inhibit the development of afidopyropen resistance in field populations.

Resistance to Beta-cypermethrin, Azadirachtin, and Matrine, and Biochemical Characterization of Field Populations of Oedaleus asiaticus (Bey-Bienko) in Inner Mongolia, Northern China

Oedaleus asiaticus (Bey-Bienko) is an economically devastating locust species found in grassland and pastoral areas of the Inner Mongolia region of northern China. In this study, resistance to three frequently used insecticides (beta-cypermethrin, matrine, and azadirachtin) was investigated in six field populations of O. asiaticus using the leaf-dip bioassay method. The inhibitory effects of synergists and the activities of detoxification enzyme activities in the different populations were determined to explore potential biochemical resistance mechanisms. The results showed that the field populations SB (resistance ratio [RR] = 7.85), ZB (RR = 5.64), and DB (RR = 6.75) had developed low levels of resistance to beta-cypermethrin compared with a susceptible control strain. Both the SB (RR = 5.92) and XC (RR = 6.38) populations had also developed low levels of resistance against matrine, with the other populations remaining susceptible to both beta-cypermethrin and matrine. All field populations were susceptible to azadirachtin. Synergism analysis showed that triphenyl phosphate (TPP) and diethyl-maleate (DEM) increased the toxicity of beta-cypermethrin significantly in the SB population, while the synergistic effects of TPP, piperonyl butoxide (PBO), and DEM on the toxicity of matrine were higher in SB (SR 3.86, 4.18, and 3.07, respectively) than in SS (SR 2.24, 2.86, and 2.29, respectively), but no synergistic effects of TPP, PBO, and DEM on azadirachtin were found. Biochemical assays showed that the activities of carboxylesterases (CarEs) and glutathione-S-transferases (GSTs) were significantly raised in all field populations of O. asiaticus, with a significant positive correlation observed between beta-cypermethrin resistance and CarE activity. The activities of cytochrome P450 monooxygenases (P450) and multi-function oxidases (MFO) were elevated in all six field populations, and P450 activity displayed strong positive correlations with the three insecticides. Our findings suggest that resistance to beta-cypermethrin in O. asiaticus may be mainly attributed to elevated CarE and GST activities, while P450 plays an important role in metabolizing matrine and azadirachtin. Our study provides insights that will help improve insecticide resistance management strategies.

Next-generation molecular diagnostics (TaqMan qPCR and ddPCR) for monitoring insecticide resistance in Bemisia tabaci

BACKGROUND

Insecticide resistance has developed in several populations of the whitefly Bemisia tabaci worldwide and threatens to compromise the efficacy of chemical control. The molecular mechanisms underpinning resistance have been characterized and markers associated with the trait have been identified, allowing the development of diagnostics for individual insects.

RESULTS

TaqMan and Droplet Digital PCR (ddPCR) assays were developed and validated, in individual and pooled whitefly samples, respectively, for the following target-site mutations: the acetylcholinesterase (ace1) F331W mutation conferring organophosphate-resistance; the voltage-gated sodium channel (vgsc) mutations L925I and T929V conferring pyrethroid-resistance; and the acetyl-CoA carboxylase (acc) A2083V mutation conferring ketoenol-resistance. The ddPCR's limit of detection (LoD) was <0.2% (i.e. detection of one heterozygote whitefly in a pool of 249 wild-type individuals). The assays were applied in 11 B. tabaci field populations from four locations in Crete, Greece. The F331W mutation was detected to be fixed or close to fixation in eight of 11 B. tabaci populations, and at lower frequency in the remaining ones. The pyrethroid-resistance mutations were detected at very high frequencies. The A2083V spiromesifen resistance mutation was detected in eight of 11 populations (frequencies = 6.16-89.56%). Spiromesifen phenotypic resistance monitoring showed that the populations tested had variable levels of resistance, ranging from full susceptibility to high resistance. A strong spiromesifen-resistance phenotype-genotype (A2083V) correlation (r_s  = -0.839, P = 0.002) was observed confirming the ddPCR diagnostic value.

CONCLUSION

The ddPCR diagnostics developed in this study are a valuable tool to support evidence-based rational use of insecticides and resistance management strategies. © 2022 Society of Chemical Industry.

Monitoring, cross-resistance, inheritance, and fitness costs of brown planthoppers, Nilaparvata lugens, resistance to pymetrozine in China

BACKGROUND

The brown planthopper, Nilaparvata lugens, is considered the most destructive pest of rice in many Asian countries including China. Use of pymetrozine in insect resistance management (IRM) has been one strategy to control this pest. In this study, we reported the status of pymetrozine resistance in Nilaparvata lugens (Stål) collected from China over the period 2017-2021 and selected a strain of N. lugens resistant to pymetrozine and evaluated the cross-resistance, inheritance and fitness costs of the resistance.

RESULTS

Monitoring data (2017-2021) showed that field populations of N. lugens in China developed moderate- to high-level pymetrozine resistance during these 5 years. By continuous selection with pymetrozine in the lab, the pymetrozine selected N. lugens strain (Pym-R₉₈ ) developed a 225.2-fold resistance compared to a susceptible strain. The Pym-R₉₈ strain showed high cross-resistance to dinotefuran (66.6-fold) and low cross-resistance to nitenpyram (5.2-fold) and sulfoxaflor (5.8-fold). Inheritance pattern analysis of Pym-R₉₃ revealed that resistance to pymetrozine was polygenic, autosomal and incompletely dominant. Fitness costs of pymetrozine resistance were present in Pym-R₉₀ and WA2020 strains with a relative fitness of 0.72 and 0.60, respectively. The developmental duration of Pym-R₉₀ and WA2020 was significantly longer and hatchability was significantly lower compared to pymetrozine-susceptible strain (Pym-S).

CONCLUSIONS

N. lugens has developed high level of resistance to pymetrozine. Pymetrozine-resistance brown planthopper had cross-resistance with some of neonicotinoids such as dinotefuran, nitenpyram and sulfoxaflor. The autosomal, incompletely dominant and polygenic resistance to pymetrozine in N. lugens and the fitness costs associated with this resistance can be exploited in IRM strategies to preserve the lifetime of pymetrozine for control of N. lugens in China. © 2022 Society of Chemical Industry.

Reviewing the WHO Tube Bioassay Methodology: Accurate Method Reporting and Numbers of Mosquitoes Are Key to Producing Robust Results

Simple Summary

The “WHO susceptibility bioassay” is a method from the World Health Organization used to monitor the resistance to insecticides in mosquito populations. This method was first developed in the 1960s and has undergone multiple changes since then. While these changes may appear minor, the numerous iterations of the test procedures leave some parameters open to interpretation, and changes to methodology may affect results. To address this, we reviewed the published test procedures for this method and the published literature which cited this method to see where the method could be optimized and exactly how people were reporting their use of this method. This revealed that the method is not being carried out consistently, and that the most up to date iterations of the test procedures are not always referenced. To address this, recommendations on the referencing and reporting of this method were developed. Alongside this literature review, we detail experimental work that explored whether altering parameters with room for interpretation in the test procedures could impact bioassay results. From the results, suggestions have been made to tighten certain parameters to avoid inaccurate measures of insecticide resistance. Closer adherence to the method and tightened parameters should lead to the generation of more robust data from the bioassay.

Abstract

Accurately monitoring insecticide resistance in target mosquito populations is important for combating malaria and other vector-borne diseases, and robust methods are key. The “WHO susceptibility bioassay” has been available from the World Health Organization for 60+ years: mosquitoes of known physiological status are exposed to a discriminating concentration of insecticide. Several changes to the test procedures have been made historically, which may seem minor but could impact bioassay results. The published test procedures and literature for this method were reviewed for methodological details. Areas where there was room for interpretation in the test procedures or where the test procedures were not being followed were assessed experimentally for their impact on bioassay results: covering or uncovering of the tube end during exposure; the number of mosquitoes per test unit; and mosquito age. Many publications do not cite the most recent test procedures; methodological details are reported which contradict the test procedures referenced, or methodological details are not fully reported. As a result, the precise methodology is unclear. Experimental testing showed that using fewer than the recommended 15–30 mosquitoes per test unit significantly reduced mortality, covering the exposure tube had no significant effect, and using mosquitoes older than 2–5 days old increased mortality, particularly in the resistant strain. Recommendations are made for improved reporting of experimental parameters

Inheritance and fitness cost of buprofezin resistance in a near-isogenic, field-derived strain and insecticide resistance monitoring of Laodelphax striatellus in China

BACKGROUND

Laodelphax striatellus is one of the most destructive pests of rice and other cereal crops. Chemical control is still the most efficient way to control this pest, but insecticide resistance always threatens this approach.

RESULTS

Monitoring data (2003-2020) showed that Chinese field populations of L. striatellus developed high-level buprofezin resistance within the first four years. This high-level resistance to buprofezin was stable for about ten years and persisted even when buprofezin selection pressure was absent. An established near-isogenic strain (YN-NIS) with 90.8-fold resistance to buprofezin had resistance inheritance of autosomal and incomplete dominance, and the resistance was controlled by multiple genes with no obvious fitness costs (relative fitness of 0.8707). Furthermore, the susceptibility of 29 field populations to another seven insecticides (2014-2020) showed that: (i) low-level resistance to pymetrozine, dinotefuran, sulfoxaflor and thiamethoxam was first detected in 2014 (eight years after introduction), 2016 (three years after), 2017 (four years after) and 2019 (19 years after), respectively, (ii) moderate resistance levels to chlorpyrifos were found for all populations across multiple years, and (iii) no resistance was detected for nitenpyram and triflumezopyrim.

CONCLUSION

The fast buprofezin resistance development in L. striatellus would be caused by incomplete dominant resistance with almost no fitness cost in the resistant strain. Nitenpyram and triflumezopyrim showed no resistance and can be used as the main insecticide for the control of L. striatellus. These findings provide key fundamental information for controlling L. striatellus.

Current situation and forecasting of resistance evolution to lambda-cyhalothrin in Spanish medfly populations

BACKGROUND

The control of the Mediterranean fruit fly Ceratitis capitata (Wiedemann) in Spanish field populations mainly relies on the insecticides lambda-cyhalothrin and spinosad as bait sprays. However, their sustainable used is compromised by the development of lambda-cyhalothrin resistance and the detection of spinosad resistant alleles. In addition, the use of lure-and-kill traps covered with deltamethrin has increased in the last years. It is thus urgent to predict the impact that the combination of both pyrethroids will have in the evolution of lambda-cyhalothrin resistance and how they could be combined with spinosad so as to establish proper resistance management programs.

RESULTS

Toxicity bioassays were performed to analyze the current levels of lambda-cyhalothrin resistance in field populations, proving that it has remained stable in the last decade. An evolutionary model was established to explore the weight of selected parameters in the evolution of lambda-cyhalothrin resistance in C. capitata and to forecast resistance development under different resistance management scenarios. Our results highlight the importance of fitness cost and inheritance to fit the experimental results. The analyses predicted that the rotation of lambda-cyhalothrin and spinosad, when deltamethrin traps are also deployed in the field, will slow down the evolution of resistance, especially when cross-resistance between both pyrethroids is considered.

CONCLUSION

Lambda-cyhalothrin resistance has not increased in the last decade, probably due to the alternation of this insecticide with spinosad. Our modelling results indicate that the best option to avoid an increase in lambda-cyhalothrin resistant alleles, considering that deltamethrin use is growing, would be to continue combining their use with spinosad. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Slow resistance evolution to neonicotinoids in field populations of wheat aphids revealed by insecticide resistance monitoring in China

BACKGROUND

The wheat aphids, Rhopalosiphum padi and Sitobion miscanthi, are serious agricultural insect pests of many crops. Neonicotinoid insecticides are commonly used as alternatives to organophosphate and pyrethroid insecticides for controlling wheat aphids.

RESULTS

Long-term monitoring of resistance to imidacloprid and acetamiprid in R. padi and S. miscanthi was carried out between 2007 and 2019. For this study, 344 specimens of the two wheat aphids were collected from field populations found in the main wheat production areas in China, from 2007 to 2019. In R. padi, the fluctuation in resistance was 14.7 times for imidacloprid and 1.4 times for acetamiprid; in S. miscanthi, it was 9.7 times for imidacloprid and 6.5 times for acetamiprid.

CONCLUSION

Our results demonstrated that both R. padi and S. miscanthi tended to have higher resistance to imidacloprid compared with acetamiprid. However, it is difficult for wheat aphids to develop a high level of neonicotinoid resistance given the pest control practices used in China. These results should be useful for the biorational application and resistance management of neonicotinoid insecticides. © 2021 Society of Chemical Industry.

A Practical Insecticide Resistance Monitoring Bioassay for Orally Ingested Dinotefuran in Anopheles Malaria Vectors

Attractive Toxic Sugar Baits (ATSB) deployed outdoors are likely to be particularly effective against outdoor biting mosquitoes and, if they contain insecticides with a different mode of action, mosquitoes resistant to pyrethroids. One such ATSB based on the neonicotinoid dinotefuran is currently under evaluation in Africa. As with any insecticide-based intervention, it will be important to monitor for the possible emergence of vector resistance. While methods for detecting resistance to insecticides via tarsal contact are recommended by the World Health Organization (WHO), these may not be applicable for orally ingested insecticides. Here, a new ingestion assay, appropriate for a controlled laboratory setting, is described using fluorescein sodium salt (uranine) as a feeding marker. Conventional topical application bioassays, more appropriate for routine deployment, have also been used to apply dinotefuran to the thorax of adult Anopheles mosquitoes with an organic carrier to bypass lipid cuticle barriers. The two methods were compared by establishing lethal doses (LD) in several Anopheles strains. The similarity of the ratios of susceptibility to dinotefuran between pairs of pyrethroid susceptible and resistant strains validates topical application as a suitable, more practical and field applicable method for monitoring for the emergence of resistance to orally ingested dinotefuran. A discriminating dose is proposed, which will be further validated against field populations and used to routinely monitor for the emergence of resistance alongside ATSB trials.

Nutritional status significantly affects toxicological endpoints in the CDC bottle bioassay

BACKGROUND

The CDC Bottle Bioassay serves as an inexpensive and effective way to screen field-caught mosquitoes against a wide variety of insecticidal active ingredients and commercial formulations, with the goal of detecting resistant individuals. For this study, we assessed how sucrose-water (10% w/v) feeding status impacted the response of Aedes aegypti mosquitoes to select insecticides.

RESULTS

Starvation for 24 or 48 h decreased permethrin and malathion mean survival time by about 40%, with little difference in the two starvation times. Similar findings were also observed in a pyrethroid-resistant Puerto Rico strain challenged with permethrin, but these effects were less pronounced. To test the impact of mosquito weight, we measured weight under different 48-h nutritional conditions and found that sugar-water-fed and sugar-only-fed individuals were approximately the same weight (ANOVA, Bonferroni post-test P value >0.999) and that individuals fed water only were considerably lighter than either sugar-water-fed (ANOVA, Bonferroni post-test P value = 0.034) or sugar-only-fed individuals (ANOVA, Bonferroni post-test P value = 0.027) but equal in weight to starved mosquitoes (ANOVA, Bonferroni post-test P value >0.99). Of the nutritional challenges, water-only-fed mosquitoes were the most insecticide tolerant (ANOVA, Bonferroni post-test P values to all other treatments <0.01).

CONCLUSIONS

The results indicate insect nutritional status is an important experimental variable, particularly the hydration status of mosquitoes shortly before insecticide exposure. Moreover, as significant differences were observed between starved and component-fed (water, sugar, or sugar and water) mosquitoes in a pyrethroid-resistant strain, some caution is appropriate when interpreting resistance/susceptibility diagnoses with this bioassay. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Field Evolved Resistance in Earias vittella (Lepidoptera: Noctuidae) From Punjab, Pakistan Against Commercial Formulations of Bacillus thuringiensis kurstaki

Transgenic commercial cotton expressing Bacillus thuringiensis (Bt) Cry endotoxins or vegetative Vip toxins provide protection to cotton against bollworm attack. Continuous exposure of these targeted pests to cry toxins and to Bt commercial spray formulations has resulted in the development of resistance through natural selection. Spotted bollworm Earias vittella (Noctuidae: Lepidoptera) is considered to be one of the most destructive pests of cotton and okra crops in South Asia including Pakistan and has developed resistance to various synthetic insecticides. In the present study, the level of resistance in field populations of the spotted bollworm E. vittella against Bt Cry toxins has been evaluated for the first time. We collected twelve populations of E. vittella from three districts of Punjab, Pakistan for testing against four commercial Bt formulations containing different strains of B. thuringiensis subspecies kurstaki (Btk) with a range of Cry toxins. Low to high levels of resistance were found in the field populations compared with a laboratory-reared susceptible population of E. vittella (resistance ratios 6 to 111-fold). These results suggest that E. vittella has developed resistance against different Cry toxins after continuous exposure to Bt cotton in field. In order to prevent field control failures, regular insecticide resistance monitoring programs are required together with the use of integrated management approaches, including the use of Bt cotton varieties expressing two or more toxins to delay the development of resistance against Bt toxins in E. vittella.

Successful development and implementation of a practical proactive resistance management plan for Bt cotton in Australia

This article describes the design and > 20 years of effective implementation of a proactive resistance-management plan for transgenic Bacillus thuringiensis (Bt) cotton that targets Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren) in Australia, considering pest biology and ecology, insights from resistance-evolution modelling, and the importance of the human component to effective implementation. This is placed in the context of processes associated with adaptive resource management. Bt cotton has provided Australian cotton growers with technology to manage Helicoverpa species that previously challenged the industry's viability, while at the same time resulting in no detectable changes in the resistance allele frequency in field populations of either Helicoverpa species in eastern Australia. This is the most long-lived and successful global example of a proactive resistance management plan for an insect pest. Six key learnings important to the successful development and implementation of a proactive transgenic-crop resistance management plan are: the programme has to have a strong science base; there has to be broad stakeholder support at all levels; there has to be a strong implementation programme; the plan needs to be supported by auditing and enforced remediation of deviations from the mandated resistance management plan; A programme of rigorous and on-going resistance allele monitoring; an attitude of continuous improvement for all aspects of the resistance management plan. The lessons learnt from the deployment of Bt cotton in Australia are relevant globally and provide important guidelines for the deployment of transgenic crops for insect control wherever they are grown. © 2021 Society of Chemical Industry.

Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea

Evolution of resistance by pests can reduce the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Because of the widespread resistance of Helicoverpa zea to crystalline (Cry) Bt toxins in the United States, the vegetative insecticidal protein Vip3Aa is the only Bt toxin produced by Bt corn and cotton that remains effective against some populations of this polyphagous lepidopteran pest. Here we evaluated H. zea resistance to Vip3Aa using diet bioassays to test 42,218 larvae from three lab strains and 71 strains derived from the field during 2016 to 2020 in Arkansas, Louisiana, Mississippi, Tennessee, and Texas. Relative to the least susceptible of the three lab strains tested (BZ), susceptibility to Vip3Aa of the field-derived strains decreased significantly from 2016 to 2020. Relative to another lab strain (TM), 7 of 16 strains derived from the field in 2019 were significantly resistant to Vip3Aa, with up to 13-fold resistance. Susceptibility to Vip3Aa was significantly lower for strains derived from Vip3Aa plants than non-Vip3Aa plants, providing direct evidence of resistance evolving in response to selection by Vip3Aa plants in the field. Together with previously reported data, the results here convey an early warning of field-evolved resistance to Vip3Aa in H. zea that supports calls for urgent action to preserve the efficacy of this toxin.

Insecticide Resistance Monitoring of the Diamondback Moth (Lepidoptera: Plutellidae) Populations in China

The diamondback moth, Plutella xylostella L., is a worldwide crop pest that is difficult to control because of its ability to develop resistance to many insecticides. To provide a reference for resistance management of P. xylostella in China, the present study used a leaf-dip bioassay to monitor the resistance of P. xylostella to nine insecticides in eight regions of China. The results showed that P. xylostella had developed a high level of resistance to beta-cypermethrin (resistance ratio [RR] > 112), and moderate (RR < 40) to high levels of resistance to indoxacarb, abamectin, and chlorfluazuron. For chlorantraniliprole, RRs > 100 were found in Midu (Yunnan Province) and Jinghai (Tianjin). In most regions, the resistance to spinetoram and chlorfenapyr and Bacillus thuringiensis (Bt) was low. No resistance was detected to diafenthiuron. Overall, P. xylostella resistance to insecticides was higher in Midu than in other regions. The data in this study should help guide the selection of insecticides for management of P. xylostella in China.

Baseline Susceptibility and Laboratory Selection of Resistance to Bt Cry1Ab Protein of Chinese Populations of Yellow Peach Moth, Conogethes punctiferalis (Guenée)

Yellow Peach Moth (YPM), Conogethes punctiferalis (Guenée), is one of the most destructive maize pests in the Huang-Huai-Hai summer maize region of China. Transgenic Bacillus thuringiensis (Bt) maize provides an effective means to control this insect pest in field trials. However, the establishment of Bt resistance to target pests is endangering the continued success of Bt crops. To use Bt maize against YPM, the baseline susceptibility of the local populations in the targeted areas needs to be verified. Diet-overlay bioassay results showed that all the fourteen YPM populations in China are highly susceptible to Cry1Ab. The LC₅₀ values ranged from 0.35 to 2.38 ng/cm² over the two years of the collection, and the difference between the most susceptible and most tolerant populations was sevenfold. The upper limit of the LC₉₉ estimates of six pooled populations produced >99% larval mortality for representative eight populations collected in 2020 and was designated as diagnostic concentrations for monitoring susceptibility in YPM populations in China. Hence, we evaluated the laboratory selection of resistance in YPM to Cry1Ab using the diet-overlay bioassay method. Although the resistant ratio was generally low, YPM potentially could evolve resistance to Cry1Ab. The potential developmentof resistance by target pests points out the necessity to implement resistance management strategies for delaying the establishment of pest resistance to Bt crops.

Monitoring the Resistance of the Beet Armyworm (Lepidoptera: Noctuidae) to Four Insecticides in Southern China from 2014 to 2018

The beet armyworm Spodoptera exigua (Hübner) is a serious polyphagous pest that infests vegetable crops worldwide and has rapidly developed resistance due to its long-term exposure to insecticides. The current resistance statuses to four insecticides exhibited by three field populations of beet armyworms collected in southern China from 2014 to 2018 were investigated. Monitoring data from five consecutive years demonstrated that all three tested S. exigua populations developed extremely high resistance to chlorantraniliprole in 2018 (220.58- to 2,597.39-fold). Two populations (Baiyun and Fengxian) developed low to moderate resistance to spinosad, whereas the Huangpi population remained susceptible (except in 2014, with RR of 6.11-fold). The RR of the Fengxian and Baiyun populations to indoxacarb steadily increased over the years, whereas that of the Huangpi population increased relatively slowly. The Baiyun and Fengxian populations developed moderate to high resistance to indoxacarb and methoxyfenozide, whereas the Huangpi population exhibited susceptibility to low resistance (1.06- to 6.45-fold) to indoxacarb and susceptibility to moderate resistance (1.53- to 14.22-fold) to methoxyfenozide. These results suggest that chlorantraniliprole should not be employed to control this pest in southern China. Reduced use of indoxacarb and methoxyfenozide or the use of alternating insecticides with low levels of resistance is recommended. Spinosad remains an effective insecticide for the management of S. exigua. To avoid the rapid development of insecticide resistance, rotations of insecticides with low levels of resistance and different modes of action based on the resistance patterns of S. exigua should be performed in southern China.

Populations of Helicoverpa zea (Boddie) in the Southeastern United States are Commonly Resistant to Cry1Ab, but Still Susceptible to Vip3Aa20 Expressed in MIR 162 Corn

The corn earworm, Helicoverpa zea (Boddie), is a major pest targeted by pyramided Bacillus thuringiensis (Bt) corn and cotton in the U.S. Cry1Ab is one of the first insecticidal toxins used in Bt crops, while Vip3A is a relatively new toxin that has recently been incorporated into Cry corn with event MIR 162 and Cry cotton varieties to generate pyramided Bt traits targeting lepidopteran pests including H. zea. The objectives of this study were to determine the current status and distribution of the Cry1Ab resistance, and evaluate the susceptibility to Vip3Aa20 expressed in MIR 162 corn in H. zea in the southeastern U.S. During 2018 and 2019, 32 H. zea populations were collected from non-Bt corn (19 populations), Cry corn (12), and Cry/Vip3A cotton (1) across major corn areas in seven southeastern states of the U.S. Susceptibility of these populations to Cry1Ab and Vip3Aa20 was determined using diet-overlay bioassays. Compared to a known susceptible insect strain, 80% of the field populations were 13- to >150-fold resistant to Cry1Ab, while their response to Vip3Aa20 ranged from >11-fold more susceptible to 9-fold more tolerant. Mean susceptibility to each Bt toxin was not significantly different between the two groups of the populations collected from non-Bt and Bt crops, as well as between the two groups of the populations collected during 2018 and 2019. The results show that resistance to Cry1Ab in H. zea is widely distributed across the region. However, the Cry1Ab-resistant populations are not cross-resistant to Vip3Aa20, and H. zea in the region is still susceptible to the Vip3Aa20 toxin. Vip3Aa20 concentrations between 5 and 10 µg/cm2 may be used as diagnostic concentrations for susceptibility monitoring in future. Additional studies are necessary to elucidate the impact of the selection with Bt corn on resistance evolution in H. zea to Vip3A cotton in the U.S.

Insecticide Rotation and Adaptive Fitness Cost Underlying Insecticide Resistance Management for Spodoptera frugiperda (Lepidoptera: Noctuidae)

Rotation of insecticide modes of action is recommended to delay selection for insecticide resistance. In this study, larvae of Spodoptera frugiperda (J.E. Smith) were exposed alternately to spinetoram and metaflumizone with insecticidal and biological response determined. Five generations (G) were evaluated with at least 200 larvae assayed per treatment. The experiment consisted of rearing field-collected and untreated larvae (M-Control), larvae with successive applications of either metaflumizone (M-MET) or spinetoram (M-SPI), and alternation of these insecticides (M-Rotation treatment) consisting of the following treatments: G₂ = exposure to SPI, G₃ = exposure to MET, G₄ = exposure to SPI, G₅ = exposure to MET, and G₆ = exposure to SPI. Four days after application, those surviving larvae were used to compose the following generations. In the G₇, evaluations were made on the selected populations M-MET, M-SPI, M-Rotation, and control larvae to determine biological characteristics used to calculate the fertility life table parameters and further comparisons across treatments. The frequency of resistance was increased at rate of 49.5% and 29.2% after five generations of selection pressure with MET and SPI, respectively. However, rotation of insecticides reduced the frequency of resistance about 50% over the generations. Individuals originated from successive exposures to the insecticides exhibited delayed egg incubation, greater percentage of pupae with deformation, longer larval and pupal periods, and reduction in emergence rate and longevity of adults, suggesting adaptive costs associated with resistance.

Insecticide residue longevity for on-site screening of Drosophila suzukii (Matsumura) resistance

BACKGROUND

Preventative application of insecticides reduces marketable yield losses caused by Drosophila suzukii females that selectively lay eggs into ripe and ripening fruits. However, repeated applications of insecticides increase the risk of resistance development. It is therefore critical to test field-collected flies on-site to assess the level of sensitivity of D. suzukii to insecticides to monitor resistance, before it becomes a widespread issue. This requires that insecticide-treated vials be readily available to conduct bioassays. Thus, bioassays were conducted using malathion-, methomyl-, zeta-cypermethrin-, phosmet-, spinetoram- and spinosad-treated scintillation vials at 1 to 28 days after treatment to assess how residue age affects insecticide toxicity in scintillation vials. The impact of temperature on residue longevity was also assessed.

RESULTS

Insecticide-treated vials stored for 28 days provided reliable estimates of susceptibility of D. suzukii to some of the tested insecticides. The toxicity of malathion remained consistently high throughout the experiment followed by methomyl. However, toxicities of zeta-cypermethrin, phosmet were variable whereas those of the spinosyns declined relatively quickly. Overall, storage temperature did not affect the residual toxicity of most of the tested insecticides except zeta-cypermethrin.

CONCLUSION

These findings suggest that the toxicity of insecticide residues in treated vials remains active for ≤28 d for malathion and ≤21 and 28 days in methomyl-treated vials stored at 4 °C in Georgia and Michigan, respectively. However, the toxicities of spinosad, zeta-cypermethrin and phosmet were less consistent. Hence, vials treated with these insecticides should be freshly made to be effective for screening D. suzukii field populations for resistance. © 2020 Society of Chemical Industry.

Monitoring the Resistance of the Citrus Red Mite (Acari: Tetranychidae) to Four Acaricides in Different Citrus Orchards in China

The citrus red mite, Panonychus citri (McGregor), is an important spider mite pest in citrus producing areas. Owing to long-term acaricide exposure, resistance has evolved rapidly in recent years. To evaluate the extent of resistance, seven field mite populations sampled from various geographical locations in China during 2015-2018 were tested using the leaf-dip bioassay method to determine their susceptibilities to four acaricides. In comparison with the susceptible strain maintained in the laboratory, low or moderate levels of fenpropathrin resistance, while no resistance to abamectin or cyflumetofen, were found among populations sampled from Liangping, Wanzhou, Daying, and Anyue in Southwestern China during the test period. High levels (>1,000-fold, with LC50 values that were greater than the recommended concentration) of resistance to fenpropathrin had evolved in field populations from Southern China, including Guilin, Nanning, and Yuxi, when compared with that of the susceptible strain. Populations from Guilin and Nanning also evolved high resistance levels to abamectin (1,088-fold and 1,401-fold) and cyflumetofen (2,112-fold and 9,093-fold). All the populations sampled in 2018 showed a moderate or high resistance to bifenazate. Generally, field populations of citrus red mites from Southwestern China were more sensitive to the tested acaricides than those of Southern China. The data provide a foundation for developing acaricide resistance management strategies in these regions.

Bt resistance alleles in field populations of pink bollworm from China: Similarities with the United States and decreased frequency from 2012 to 2015

BACKGROUND

Although most monitoring of pest resistance to widely cultivated transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) relies on bioassays, DNA screening for alleles associated with resistance has some advantages, particularly for rare, recessively inherited resistance. In China's Yangtze River Valley, where farmers first planted transgenic cotton producing Bt toxin Cry1Ac in 2000, bioassays have been used to monitor the recessive resistance of pink bollworm (Pectinophora gossypiella). Previous bioassay results show a small but significant increase in resistance to Cry1Ac during 2008-2010, followed by a significant decrease in resistance during 2011-2015 associated with extensive planting of second-generation hybrid cotton seeds that boosted the percentage of non-Bt cotton. Here we screened DNA from 19 748 pink bollworm collected during 2012-2015 from the Yangtze River Valley for seven alleles associated with resistance to Cry1Ac. These alleles were previously identified from lab-selected strains; three from the U.S. and four from China.

RESULTS

The most common resistance allele was first identified from the U.S. and accounted for over 71% of all resistance alleles detected. Resistance was rare, with the total frequency of the seven resistance alleles showing a significant, 2.3-fold decrease from 0.0105 (95% CI: 0.0084-0.0132) in 2012 to 0.0046 (0.0031-0.0067) in 2015.

CONCLUSIONS

The DNA screening data confirm results from bioassays showing pink bollworm resistance to Cry1Ac remained rare in the Yangtze River Valley from 2012-2015. The prevalence in China of the resistance allele identified from the U.S. implies a shared genetic basis of resistance that could facilitate molecular monitoring of resistance. © 2019 Society of Chemical Industry.

Dynamics of Insecticide Resistance in Different Geographical Populations of Chilo suppressalis (Lepidoptera: Crambidae) in China 2016-2018

In this study, the sensitivity of 20 field populations of Chilo suppressalis (Walker) from five provinces in China to seven insecticides was evaluated during 2016-2018. The results indicated that 20 field populations of C. suppressalis had evolved moderate to high levels of resistance to triazophos (RR 64.5-461.3) and chlorpyrifos (RR 10.1-125.0). Furthermore, C. suppressalis exhibited low to moderate levels of resistance to abamectin (RR 6.5-76.5) and decreased susceptibility to cyantraniliprole (RR 1.0-34.0). The population collected from Nanchang in Jiangxi Province (JXNC) showed high resistance to chlorantraniliprole (RR 148.3-294.3), and other geographical populations remained susceptible to moderate levels of resistance (RR 1.0-37.5). In contrast, C. suppressalis remained susceptible to low levels of resistance to spinetoram (RR 1.0-6.7) and spinosad (RR 1.0-4.6). Significant correlations were found between the Log LC50 values of chlorantraniliprole and cyantraniliprole, chlorpyrifos and triazophos, as well as cyantraniliprole and chlorpyrifos and triazophos. Similarly, significant correlations were found among abamectin, chlorpyrifos, and triazophos. In addition, a significant correlation was also observed between the activity of the detoxification enzymes and the log LC50 values of chlorantraniliprole, cyantraniliprole, abamectin, chlorpyrifos, and triazophos. The findings provide an important reference for implementing effective resistance management strategies and the development of new insecticides in insect pest control.

Monitoring and Mechanisms of Chlorantraniliprole Resistance in Chilo suppressalis (Lepidoptera: Crambidae) in China

Chlorantraniliprole, an anthranilic diamide insecticide, is widely used for controlling lepidopteran pests, because of its high insecticidal activity. However, overuse of chlorantraniliprole has led to the selection of resistance in many insect pests, including Chilo suppressalis (Lepidoptera:Crambidae), one of the most damaging rice pests in China. In this study, resistance levels to chlorantraniliprole for C. suppressalis was surveyed from eight populations of three provinces in China. The levels of resistance were ranged from 34.4-fold to 284.0-fold compared with a susceptible population. Then, a 15402 bp fragment of the full-length cDNA of ryanodine receptor gene (CsRyR) from the XS strain, the highest resistant population, and a 1992 bp fragment of CsRyR cDNA encoding the carboxyl-terminal of CsRyR gene from the other seven populations were sequenced. A common previously identified mutation that was associated with chlorantraniliprole resistance against C. suppressalis, G4910E, was not detected in any of the eight populations in this study. However, another mutation I4758M was found in all seven resistant populations. Furthermore, the relative mRNA expression levels of CsRyR gene in the seven resistant populations were all reduced compared with susceptible strain. Our study provides new insights into the basis of monitoring the development of resistance and the mechanism of resistance to chlorantraniliprole in C. suppressalis.

Field monitoring of Helicoverpa armigera (Lepidoptera: Noctuidae) Cry1Ac insecticidal protein resistance in China (2005-2017)

BACKGROUND

Bt cotton expressing Cry1Ac can efficiently control Helicoverpa armigera, but field-evolved Bt resistance reduces its efficacy. Susceptibility of H. armigera field populations to Cry1Ac protein has been monitored since Bt cotton was commercialized in 1997 in China. Here we report the results of our continued monitoring from 2005 to 2017 to provide important guidelines for sustainably growing cotton.

RESULTS

From 13 provinces and regions, 221 populations were sampled and measured. IC₅₀ values (concentration producing 50% inhibition of larval development to third instar) among different strains ranged from 0.004 to 0.212 µg mL^-1 , the percentage survival at a diagnostic concentration (IC₉₉ , 1.0 µg mL^-1 ) (SUR) ranged from 0% to 22.2%, and the percentage of field populations yielding survivors at diagnostic concentration (PSD) increased from 0 in 2006 and 2007 to 80% in 2015.

CONCLUSIONS

Compared to resistance level from 1998 to 2004 and the geographic baseline in different places of China in 1997, Bt resistance of H. armigera field populations to Cry1Ac protein has increased significantly, but failure of Bt cotton has not yet been detected. © 2018 Society of Chemical Industry.

Acute-Contact and Chronic-Systemic In Vivo Bioassays: Regional Monitoring of Susceptibility to Thiamethoxam in Soybean Aphid (Hemiptera: Aphididae) Populations From the North Central United States

The risks associated with soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), in the North Central soybean systems has fostered the adoption of prophylactic chemical control practices, such as planting seeds treated with neonicotinoid insecticides, especially thiamethoxam. Consequently, there is a concern that increased selection pressure imposed on the arthropod-pest complex by this insecticide may lead to resistance. Therefore, in vivo bioassays were conducted to determine the susceptibility of soybean aphid to thiamethoxam among North Central U.S.

POPULATIONS

Concentration-mortality data were collected using contact glass-vial and detached-leaf systemic bioassays. The results of these experiments indicate that both bioassays were reliable to detect shifts in susceptibility between different soybean aphid clones. The LC50s of field populations of soybean aphid were significantly different when mortality was recorded in contact and systemic exposure assays. Nevertheless, the magnitude of the resistance ratios was consistent in both methods. In addition, a significant increase in the LC50 and EC50 values was observed among field populations tested in detached-leaf systemic bioassays. These results represent the first extensive efforts to identify the variability in susceptibility of soybean aphid to thiamethoxam in the North Central United States Therefore, our results provide a baseline for future assessment and contribute to a better understanding of the applicability of in vivo bioassays for susceptibility monitoring and resistance detection of soybean aphid to thiamethoxam.

Resistance Monitoring of Four Insecticides and a Description of an Artificial Diet Incorporation Method for Chilo suppressalis (Lepidoptera: Crambidae)

Chilo suppressalis (Walker; Lepidoptera: Crambidae) is one of the most damaging rice pests in China. Insecticides play a major role in its management. We describe how we monitored the resistance of C. suppressalis to four insecticides in seven field populations from Jiangxi, Hubei, and Hunan Provinces, China, in 2014-2016. The topical application method for resistance monitoring was suitable for triazophos, monosultap, and abamectin. The conventional rice seedling dipping method proved ineffective for testing chlorantraniliprole so the new artificial diet incorporation method was substituted. This new method provided more consistent results than the other methods, once baseline toxicity data had been established. All populations had moderate to high resistance to triazophos from 2014 to 2016. Monosultap resistance in two populations increased from low in 2014 to moderate in 2016 and the other five populations showed moderate to high-level resistance throughout. Abamectin resistance in three populations increased from sensitive or low in 2014 to moderate in 2015-2016, and the other populations had moderate to high levels of resistance. Resistance to chlorantraniliprole increased from sensitive or low in 2014 to moderate to high in 2016. These results suggested that resistance management strategies should be developed according to the needs of a specific location. It was suggested that, in these localities, organophosphate insecticides should be prohibited, the application of nereistoxin, macrolide antibiotic, and diamide insecticides should be limited, and other insecticides, including spinetoram and methoxyfenozide, that exhibited no resistance should be used rationally and in rotation to delay resistance development.

A methodology based on insecticide impregnated filter paper for monitoring resistance to deltamethrin in Triatoma infestans field populations

The domiciliary presence of Triatoma infestans (Klug) (Hemiptera: Reduviidae) after control interventions was reported in recent years. Toxicological studies showed high levels of resistance to pyrethroids suggesting resistance as one of the main causes of deficient control. The aim of the present study was to develop a protocol to test resistance to deltamethrin in T. infestans collected from the field by discriminate concentration. To evaluate field insects, the effect of age (early vs. later) and nutritional state (starved vs. fed) on the deltamethrin susceptibility of each developmental stage was studied. Topical and insecticide impregnated paper bioassays were used. Using the impregnated paper, the susceptibility to deltamethrin was not affected by the age of the stadium and the nutritional states, and varied with the post-exposure time and with the different developmental stages. A discriminant concentration of deltamethrin (0.36% w/v) impregnated in filter paper was established for all developmental stages. Finally, the methodology and the discriminant concentration were evaluated in the laboratory showing high sensitivity in the discrimination of resistance. The present study developed a methodology of exposure to insecticide impregnated papers and proposes a protocol to test T. infestans in field populations with the aim to detect early evolution of resistance to deltamethrin.

Monitoring Trends in Insecticide Resistance of Field Populations of Sogatella furcifera (Hemiptera: Delphacidae) in Guizhou Province, China, 2012-2015

Sogatella furcifera (Horváth) is a migratory insect that is one of the most important pest species on rice in many Asian countries. Control of S. furcifera (Hemiptera: Delphacidae) primarily depends on the use of chemical insecticides, and with this extensive reliance on pesticides, determining the degree of resistance of S. furcifera populations to the chemicals used for its control is essential. In this study, the resistance level to six conventional insecticides in five populations of S. furcifera from Guizhou Province was monitored yearly using the rice-stem dipping method in 2012-2015 to precisely understand current resistance levels and to estimate trends in the development of insecticide resistance in S. furcifera in Guizhou. Overall, S. furcifera from five regions in Guizhou showed a trend toward decreased susceptibility to isoprocarb (resistance ratio [RR] 0.82-3.59), susceptibility to low resistance against thiamethoxam (RR 0.27-9.69), susceptibility to moderate resistance to imidacloprid (RR 0.71-26.06), and decreased susceptibility to moderate resistance to chlorpyrifos (RR 4.63-19.58). The resistance to pymetrozine (RR 10.48-84.65) was moderate to high, and that to buprofezin (RR 6.36-412.43) was low to very high. In conclusion, the use of buprofezin and pymetrozine to control S. furcifera should be reduced in Guizhou Province, whereas prudent use at a reasonable frequency of chlorpyrifos and imidacloprid can continue. Isoprocarb and thiamethoxam are the best choices for effective management of S. furcifera. Rotations using alternative insecticides with different modes of action are recommended for regions in which resistance is at a moderate level.

Liquid biopsy and NSCLC

In the era of high-throughput molecular screening and personalized medicine, difficulty in determining whether cancer mutations are truly 'actionable' remains a gray zone in NSCLC. The most important prerequisite to perform such investigations is the tumor tissue retrieval via biopsy at diagnosis and after occurrence of resistance. Blood-based liquid biopsy as circulating tumor cells, circulating tumor DNA and exosomes can offer a fast and non-invasive method to elucidate the genetic heterogeneity of patients, the screening and patient stratification and give a dynamic surveillance for tumor progression and monitor treatments response. Here we prospectively discuss the three main approaches in the blood-biopsy field of lung cancer patients and its clinical applications in patient management. We also outline some of the analytical challenges that remain for liquid biopsy techniques in demonstrating that it could represent a true and actionable picture in lung cancer management for the implementation into clinical routine.

Field-evolved resistance to insecticides in the invasive western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) in China

BACKGROUND

To understand the current status of insecticide resistance of the invasive western flower thrips, Frankliniella occidentalis, in China, the responses of six field populations to six commonly used insecticides, i.e. spinosad, spinetoram, cyantraniliprole, imidacloprid, acetamiprid and pyriproxyfen, were evaluated in comparison with a susceptible laboratory strain.

RESULTS

Field populations tended to be less susceptible than the laboratory strain. The population from Shouguang, Shandong Province, showed the lowest levels of susceptibility. A 15.64-fold and 17.29-fold resistance to spinosad and spinetoram was detected in the Shouguang population. A 11.74-fold and 13.64-fold resistance to cyantraniliprole was detected in populations from Daxing in the Beijing area and Shouguang. All populations showed a low level of resistance to imidacloprid, acetamiprid and pyriproxyfen, except for the Shouguang population, which was 127.58-fold more resistant to pyriproxyfen.

CONCLUSION

Variations in resistance to the tested insecticides were observed among the sampled population. Spinosad and spinetoram were the most efficient insecticides and are recommended for use in an integrated management programme. Resistance management strategies should be implemented to reduce the potential for resistance evolving. © 2015 Society of Chemical Industry.

Resistance Management for San Jose Scale (Hemiptera: Diaspididae)

The San Jose scale Diaspidiotus perniciosus Comstock is one of the most important pests of deciduous fruit trees. The major cause of recent outbreaks in apple orchards is thought to be the development of insecticide resistance, specifically organophosphates. The first report was given in North America, and now, in Chile. In the present study, San Jose scale populations collected from two central regions of Chile were checked for their susceptibility to different mode of action insecticides in order to establish alternatives to manage this pest. No evidence of cross resistance between organophosphates insecticides and acetamiprid, buprofezin, pyriproxyfen, spirotetramat, sulfoxaflor, or thiacloprid was found. Baselines of LC50-LC95 for different life stages of San Jose scale are given, as reference to future studies of resistance monitoring. The systemic activity of acetamiprid, spirotetramat, and thiacloprid was higher than the contact residue effect of these compounds. For sulfoxaflor, both values were similar. Program treatments including one or more of these compounds are compared in efficacy and impact on resistance ratio values. In order to preserve new insecticides as an important tool to control San Jose scale, resistance management programs should be implemented, considering insecticide mode of action classes alternated or mixed.

Baseline Susceptibility of Helicoverpa armigera (Lepidoptera: Noctuidae) to Indoxacarb, Emamectin Benzoate, and Chlorantraniliprole in Australia

Baseline susceptibility of Helicoverpa armigera (Hübner) to emamectin benzoate, chlorantraniliprole, and indoxacarb was determined in feeding assays on insecticide-incorporated artificial diet in the laboratory. The intraspecific variation of H. armigera was established from field populations collected between September 2012 and March 2013, primarily from commercial farms across eastern Australia. Emamectin benzoate had the highest toxicity with a median lethal concentration (LC50) of 0.01 µg/ml diet (n=20 strains). The LC50 for chlorantraniliprole was 0.03 µg/ml diet (n=21 strains), while indoxacarb had the lowest relative toxicity with an average LC50 of 0.3 µg/ml diet (n=22 strains). Variation in susceptibility amongst field strains was 2.3-fold for emamectin benzoate and 2.9-fold for chlorantraniliprole and indoxacarb. Discriminating concentrations of 0.2, 1, and 12 µg of insecticide per milliliter of diet for emamectin benzoate, chlorantraniliprole, and indoxacarb, respectively, were calculated from toxicological data from field H. armigera strains as a first step in resistance management of these classes of insecticide in Australia. The low intraspecific tolerance, high slope values, and goodness-of-fit to a probit binomial model obtained in this study suggest that a feeding assay using diet incorporated insecticide is an effective laboratory method for measuring the dose-responses of these classes of insecticides in H. armigera.

Current perspectives on HIV-1 antiretroviral drug resistance

Current advancements in antiretroviral therapy (ART) have turned HIV-1 infection into a chronic and manageable disease. However, treatment is only effective until HIV-1 develops resistance against the administered drugs. The most recent antiretroviral drugs have become superior at delaying the evolution of acquired drug resistance. In this review, the viral fitness and its correlation to HIV-1 mutation rates and drug resistance are discussed while emphasizing the concept of lethal mutagenesis as an alternative therapy. The development of resistance to the different classes of approved drugs and the importance of monitoring antiretroviral drug resistance are also summarized briefly.

Susceptibility of Sogatella furcifera and Laodelphax striatellus (Hemiptera: Delphacidae) to Six Insecticides in China

The whitebacked planthopper, Sogatella furcifera (Horváth), and small brown planthopper, Laodelphax striatellus (Fallén), both are important crop pests throughout China, especially in rice. Application of chemical insecticides is the major control practice. Consequently, insecticide resistance has become an urgent issue. In this study, resistance levels to six conventional insecticides were evaluated for these two species collected from major occurring areas of China. Additionally, imidacloprid- (resistance ratio [RR] = 10.4-fold) and buprofezin (RR = 15.1-fold)-resistant strains of whitebacked planthopper were obtained through laboratory selections for cross-resistance profiling and synergism assessment to understand resistance mechanisms. The results showed that all tested populations of both species exhibited low to high levels of resistance to chlorpyrifos, while remaining susceptible to thiamethoxam. Three of the 14 whitebacked planthopper populations showed low to moderate resistance to imidacloprid, while all small brown planthopper populations reminded susceptible. All small brown planthopper and whitebacked planthopper (except one) populations showed at least moderate resistance (RR = 10.1-271.1) to buprofezin. All small brown planthopper populations remained susceptible to pymetrozine and nitenpyram, and all whitebacked planthopper populations remained susceptible to isoprocarb. The imidacloprid-resistant whitebacked planthopper strain showed no significant cross-resistance to other tested insecticides. However, the buprofezin-resistant strain exhibited a low-level cross-resistance (CR = 3.1) to imidacloprid. Piperonyl butoxide, triphenyl phosphate, and diethylmaleate displayed no synergism effect on the resistant whitebacked planthopper strains.

Changes in malaria parasite drug resistance in an endemic population over a 25-year period with resulting genomic evidence of selection

Background. Analysis of genome-wide polymorphism in many organisms has potential to identify genes under recent selection. However, data on historical allele frequency changes are rarely available for direct confirmation.

Methods. We genotyped single nucleotide polymorphisms (SNPs) in 4 Plasmodium falciparum drug resistance genes in 668 archived parasite-positive blood samples of a Gambian population between 1984 and 2008. This covered a period before antimalarial resistance was detected locally, through subsequent failure of multiple drugs until introduction of artemisinin combination therapy. We separately performed genome-wide sequence analysis of 52 clinical isolates from 2008 to prospect for loci under recent directional selection.

Results. Resistance alleles increased from very low frequencies, peaking in 2000 for chloroquine resistance-associated crt and mdr1 genes and at the end of the survey period for dhfr and dhps genes respectively associated with pyrimethamine and sulfadoxine resistance. Temporal changes fit a model incorporating likely selection coefficients over the period. Three of the drug resistance loci were in the top 4 regions under strong selection implicated by the genome-wide analysis.

Conclusions. Genome-wide polymorphism analysis of an endemic population sample robustly identifies loci with detailed documentation of recent selection, demonstrating power to prospectively detect emerging drug resistance genes.